Carpentry Notes on Basic Roof & Ceiling Framing

Basic Roof and Ceiling Framing

CARP11

BASIC ROOF and CEILING FRAMING

Publishing details: These notes were prepared by Teachers of Carpentry TAFE NSW

2003 Edition  NSW TAFE Commission / DET CONSTRUCTION & TRANSPORT DIVISION WESTERN SYDNEY INSTITUTE OF TAFE

For Construction and Transport Division TAFE NSW Victoria Road Castle Hill NSW 2154 2154 Ph. (02) 9204 4600

First Published 1999 Second Edition 2003

ISBN 0 7348 1007 5

Construction and Transport Division TAFE NSW, 1999 Copyright of this material is reserved to Construction and Transport Division TAFE NSW Reproduction or transmittal in whole or part, other than for the purposes and subject to the provision of the Copyright Act, is prohibited without the written authority of Construction and Transport Division, TAFE NSW



Published by Construction and Transport Division

Printed and Distributed by Resource Distribution - TAFE Manufacturing and Engineering Division

©TAFE NSW Construction and Transport Division


Publishing details: These notes were prepared by Teachers of Carpentry TAFE NSW

2003 Edition  NSW TAFE Commission / DET CONSTRUCTION & TRANSPORT DIVISION WESTERN SYDNEY INSTITUTE OF TAFE

For Construction and Transport Division TAFE NSW Victoria Road Castle Hill NSW 2154 2154 Ph. (02) 9204 4600

First Published 1999 Second Edition 2003

ISBN 0 7348 1007 5

Construction and Transport Division TAFE NSW, 1999 Copyright of this material is reserved to Construction and Transport Division TAFE NSW Reproduction or transmittal in whole or part, other than for the purposes and subject to the provision of the Copyright Act, is prohibited without the written authority of Construction and Transport Division, TAFE NSW



Published by Construction and Transport Division

Printed and Distributed by Resource Distribution - TAFE Manufacturing and Engineering Division


CARPENTRY - HOUSING

Overview

1

Introduction to Pitched Roof Framing - definitions and roof types

2

PART 1:

Ceiling Framing - Introduction to ceiling framing

10

Ceiling trimmers

12

Hanging beams

14

Set Out and Erection of Ceiling Frames

15

Ceiling frame calculations

18

Alternative Ceiling Types

23

Skillion roof construction

25

Lean-to roof construction

26

PART 2:

27

Gable Roofs - parts, proportions and definitions

Structural Roof Members

29

Wind bracing

31

Purlins

32

Struts

33

Patent Type Strutting

38

Collar ties

39

Gable Ends

40

Calculating Drop-off

46

Eaves finishes

48

Erection Procedure for the Gable Roof

50

Roof Pitch

56

Setting Out and Cutting Rafters

60

The Steel Square

62

Calculating Frame Quantities

68 78

Glossary of Terms Further Reading

80 ©TAFE NSW Construction and Transport Division


Acknowledgments: Acknowledgment is due to the following for their permission to reproduce product materials and copyright materials or for development of this text; •

ACE Guttering Pty L td - for use of the fascia and gutter details contained in their sales brochure

•

Ivanka Susnjara - for desktop publishing and preparation work for printing.

•

Rob Young - for preparation and editing of these notes, including development of new graphics.

•

Special thanks - to Bob Bulkeley for the many years of dedication to re search, development and production of quality resources for use in the area of vocational education.


CARPENTRY - HOUSING

ISBN 0 7348 1007 5


CARPENTRY - HOUSING

This text introduces subject matter related to ceiling framing and basic roofing. Reference may be made to “Basic Building and Construction Skills”, produced by TAFE and Addison, Wesley, Longman Australia Pty Limited, to re-examine and reinforce these basic skills.

There are two parts to the text, PART 1 – Ceiling Framing and PART 2 – Gable Roofing, which address the following: Ceiling frames, roof types and terminology are explained, with special reference to gable roofs. Structural members are detailed and their purpose defined. Methods for determining lengths of members, setting out, cutting and erection processes are covered, including eaves construction for various situations and the various materials used. The text also covers calculation of members, their lengths, quantities and costs. Note: Only conventionally pitched roofs are dealt with in this text, as Trussed roofing will be dealt with in a separate text.

A comprehensive ‘Glossary of Terms’ is included at the end of the text, which provides a detailed description of trade terms, technical content and some trade jargon.


1


INTRODUCTION TO PITCHED ROOF FRAMING Definitions: Roof: “A roof is the weatherproofed upper covering of a building or structure, which is designed to protect the interior from atmospheric elements such as sun, rain, hail, snow, frost and wind.” Flat roof: Is a roof with a minimum of slope to allow water run-off, usually with a fall of 1 in 40 but a minimum of 1 in 60. These are usually confined to sheet roofs with the sheets being full length, no joins, for a slope less than 1 in 40. Pitched roof: This is a roof with a slope, or pitch, greater than 1 in 12 or 5 degrees. A roof may have a single pitch, like a skillion, a double pitch, like a gable, or an unequal pitch, where both sides of the roof have different angles. Coupled roof: This is where pairs of rafters are attached on opposite sides of a ridge and the feet are fixed to the wall plate. There is no tie between the feet, allowing the rafters to spread under load. It is restricted to small span gable roofs, which may be simply coupled. Close-coupled roof: This is the same as the coupled roof except there is a tie, such as a ceiling joist, placed between the feet of the rafters. This method is used for most roof construction, especially for gables with a wide span. Cut roof: Also known as a conventionally pitched roof, has all of its members cut and assembled individually. These roofs are made up of separate rafters, ridge, purlins, collar ties, struts, etc. Free roof: This refers to any roof, which does not have enclosed walls under it. It is typically used for a freestanding carport, portico, covered walk-way, lichgate, etc. Gablets: These are simply small versions of gables. They may be used on the ends of ridges for ventilation, over a dormer window or as an adornment to the main roof surface. Monoslope roof: Also known as a ‘Monopitch’ roof, it is any roof with a continuous slope, which has no ridge. Skillion and lean-to roofs are monoslope roofs. Open roof: Any roof, which is not enclosed underneath. Verandah, free roofs and garage roofs are typically not lined or framed with a ceiling and therefore classified as open roofs. Shell roof: Made from a thin self-supporting and curved structural membrane used over long spans. Precast, prestressed concrete is commonly used for this construction. Southlight roof: Generally refers to the vertical glass of a sawtooth roof, which faces south to allow glare-free light to enter the building. Umbrella roof: This is a roof placed over a structure, but does not form part of the structure.

2


CARPENTRY - HOUSING

THE ROOF The roof, and roof covering, makes up a large part of the external fabric of the building. It may be designed and constructed to create a picturesque roofscape using a variety of materials or they may be very plain and use only one type of material. The design of roofs has changed dramatically from the early part of the 20 th century where common features included ornate gables, gablets, turrets, spires, crested ridge capping, finials, vents, and complex chimneys decorated with terracotta pots. These steeply pitched and highly decorative roofs have given way to the modern low pitched, plainly coloured roofs of today commonly seen in most new housing developments. Roofing materials such as glazed and unglazed terracotta, slate and ‘ fibro’ have been generally superseded by concrete tiles and ‘Colorbond’ roof sheeting. As with most styles in building, the older t ypes of design eventually become incorporated into contemporary design, or make a comeback, therefore methods of development and construction of roof types should not be forgotten.

Fig. 1 Complex older style roofs


3


TYPES OF ROOFS There are many styles of roof, most of which are made up from variations on specific types. Some of these roof types are described below:

A-frame:

This is a steeply pitched roof, which forms a shape similar to the letter ‘A’. More commonly used in snow areas to allow the snow to slide off easily, rather than have it add excessive load to the roof frame. Wall of cottage A-Frame

Bellcast:

This is a roof, which changes its pitch to a lower pitch or angle near the eaves. It is commonly used where the main roof pitch meets the lower pitch of a covered balcony or verandah.

Bellcast

Clerestory:

This is a roof having two levels separated by a row of windows, which provide light and/or ventilation to the rooms below. It gets its name from the upper part of a church nave, which is the main source of light.

Clerestory Windows

Clerestory

Deck:

This roof type takes the form of a truncated or cut-off top pyramid with a flat or near flat section in the middle. This may occur where a hip roof has a deck or landing on top, with a handrail around it, used for entertaining or an observation deck.

Flat section with a handrail around perimeter

Deck

4


CARPENTRY - HOUSING

Gablet

Dutch gable:

This is a hip type roof with small gables or gablets at either end of the ridge. It may also be referred to as a ‘half-hipped roof’ or a ‘Gambrel’.

Dutch gable

Gable:

This is a roof with a double pitch and vertical ends. It may also be used as an add-on to a main roof in the form of gablets over entries or simply decorating the main roof surface in the form of a dummy gable.

Gable

Gablet

Gambrel roof: This is similar to the Dutch gable having gablets at either end of the ridge on a hip roof. In recent times the size of the gablet has increased providing a more distinctive style of roof surface.

Gambrel

Half-pitch :

This refers to a roof, which has a pitch angle where the rise is equal to the half span of the roof, i.e. forms a 45° angle.

Boxed gable end

Half-pitch ©TAFE NSW Construction and Transport Division

5


Helm :

Gables on Four sides

This is a pyramidal roof, having a square base, with four gables connected at the bottom horizontal position. The remaining roof surfaces are diamond-shaped. This type of roof was commonly used for spires on square towers.

Helm

Hip or Hipped: This is a roof with four sloping sides on a rectangular base. The ends are triangular in shape and the sides form a trapezoidal shape.

Hip or Hipped

Valley

Hip & valley:

This is basically a hip roof, which is ‘T’ or ‘L’ shaped on plan. The ridge lines are the same height for the main and extended roof sections.

Hip

Hip & Valley

(Broken) Hip & Again it is similar to the hip & valley: valley type except the ridge(s) of the extended sections are not at the same height as the main roof. This creates a shortened or broken hip used to link the minor ridge to the major ridge.

Broken hip

Broken Hip & Valley

6


CARPENTRY - HOUSING

Hyperbolic paraboloid:

Jerkin head:

This is a form of shell roof construction, which has raised diagonally opposite corners on a square base. This creates a convex curve between the low corners and a concave between the high ones. They have been used for small architecturally designed airport terminals and swimming centre shade roofs.

Hyperbolic paraboloid

This is a roof, which is hipped from the end of the ridge half way down to the eaves, and gabled from half way to the eaves. It is also sometimes called a ‘Hipped gable’ or a ‘Clipped gable’.

Hip end Gable end

Jerkin Head

Mansard:

This is similar to a hipped roof except all four sides have a double pitch. Each side has a steeply sloping section up from the eaves, then the top section flattens out up to the ridge. It was named after the French architect Francois Mansart, who died in 1666. It has also been referred to as a ‘Curb roof’ or a ‘French roof’. Mansard

Monitor:

Window area

This is a portion of a roof, which has been raised up above the main roof, usually flat, with continuous vertical glazing around the perimeter for natural lighting. Mainly used for industrial buildings.

Monitor


7


Monoslope:

Also known as a ‘Monopitch’roof, it is any roof with a continuous slope, which has no ridge. Skillion and lean-to roofs are monoslope roofs.

Monoslope

Pyramid:

This is a roof with square or other regular polygon shaped base, with all hips being equal and converging at a pointed apex.

Pyramid

Sawtooth:

This is a made up of a series of connected monoslope roofs, which appear to be sawtooth-shaped when viewed from the end. The shape is a series of right-angled triangles connected at the base or trough with a common box gutter. The vertical face is usually glass to allow natural light to enter. Commonly used for commercial and industrial work.

Glass along vertical faces

Sawtooth

Station:

This type of roof is typically used for train and bus stations where the roof is to cantilever past supports on both sides to provide shelter.

Station

8


CARPENTRY - HOUSING

Troughed:

This is a double-pitched roof with a valley between the two surfaces. This roof is also referred to as a "Valley Roof" or a "Butterfly roof".

Troughed

Tudor:

This is a steeply pitched roof, usually a gable style, with dormer windows on one or both sides.

Tudor

Fig. 2 Typical modern combination style hip and gambrel roof


9


PART 1 :

CEILING FRAMING

INTRODUCTION TO CEILING FRAMING The ceiling frame is the horizontal area between the top of walls and the roof, which is designed to enclose the room by providing a dust barrier, insulation and security. The frame consists of ceiling joists, ceiling trimmers, hangers and hanging beams. This system is designed to tie-in with a conventionally pitched skillion, gable or hip roof. Note: The ceiling frame of a trussed roof is made up of the bottom chords of the individual trusses and does not require additional ceiling joists, hangers or hanging beams.

Ceiling joists These are the horizontal members with ends that rest on top of the wall plates. They carry the ceiling sheets, and provide a lateral tie between the feet of opposing rafters to form a strong, coupled frame. They may be nailed or bolted to the rafters. They are spaced at maximum centres of 450 mm and 600 mm depending on their stress grade, section size and thickness of ceiling lining being used. They may be joined in length over a wall or under a hanger, where the join can be supported. Refer to AS 1684 for sizes and stress grades. Vertical deflection

X1 +X2 = lateral deflection

FAILURE UNDER LOAD

Ceiling Joist

STABILITY UNDER LOAD

Fig. 3 Position and purpose of ceiling joists

Ceiling joist fixed next to rafter position

CEILINGS JOIST FIXED TO TOP PLATE

RAFTER FIXED NEXT TO CEILING JOIST

Fig. 4 Placement and fixing of ceiling joists 10


CARPENTRY - HOUSING

CEILING TRIMMERS These are short lengths of ceiling framing material, fixed at right angles between ceiling joists, placed at the same maximum spacings as the joists. They are designed to: • Provide fixing for the ends of ceiling sheets and cornices; • Provide fixing for the top internal wall plates; and Provide continuous lateral stability for the ceiling frame once hangers are fixed. •

Ceiling Joists

Trimmers

Top plate to internal wall

Fig. 5 Placement and fixing of ceiling trimmers over an internal wall.


11


Patent type metal connectors may be used to provide a secure load-bearing connection between hangers and ceiling joists, which is particularly useful when work or an inspection is to be carried out inside the roof space in the future. The connectors are fixed on alternate sides, every second joist, to assist in preventing the hanger from overturning.

Hanger

Ceiling joist ‘JOIST STRAP’

‘TRIPLE GRIP’ (Trip-L-grip)

‘CEILING DOG’

Fig. 8 Final fixing of joists to hanger with typical patent connectors

When very deep, narrow hangers are used it may be necessary to fit a timber brace or hoop iron strap to the ends to prevent the hanger from twisting or rolling over. Alternatively, if the end of the deep hanger runs past the face of a hip end rafter or gable stud it may be bolted to it.

Hoop iron strap

Timber batten

Solid blocking

Solid blocking Top wall plate

Top wall plate

Fig. 9 Methods used to prevent rolling and twisting of hangers

12


CARPENTRY - HOUSING

Where hangers run across ceiling joists with the end protruding past the line of the rafters, as would occur at the end of a hip roof, the load is transferred to the wall plate via a ceiling trimmer. The top end of the hanger is bolted to the rafter and then the hanger is strapped to the ceiling trimmer.

Approved metal strap

Hanger

Hanger bolted to rafter Ends cut to roof pitch

Rafter

Ceiling dog

Note: The end of the hanger is cut to the pitch of the roof before being fixed into place.

Ceiling joist Trimmer Top plate

Trimmer support to hanger Approved metal connector Bolted joint

Fig. 10 Method of supporting end of hanger


13


Hanging beams A hanging beam, also known as a counter beam, runs at 90° to the line of hangers and supports them where their length exceeds the allowable span. The hangers are cut onto a bearing cleat on either side of the hanging beam to allow continuous support for the length of the room. The ends of the hanging beam are packed up slightly to allow for deflection. The section size and stress grade will be greater than the hanger. Refer to AS 1684 for stress grades and section sizes. Note: The roof frame must not be supported off hangers or hanging beams unless designed and specified by a structural engineer Hanger

Underside of ceiling joists adjusted to a common level

Hanger checked out over hanging beam

Ceiling dog or other approved fastener Ledger Hanging/Strutting beam packed at end support points to allow for deflection

SECTION

Hanger

Hanging beam

Ledger

Packing

Reinforcement blocking

Fig. 11 Method of supporting the ends of hangers onto a hanging beam

14


CARPENTRY - HOUSING

SET OUT AND ERECTION OF CEILING FRAMES The set out of the ceiling frame is based on the set out for the roof rafters. Whether the roof type is a gable or a broken hip and valley the set out of the rafter positions is carried out first so the joists may be fixed alongside.

Procedure STEP 1 Check the top wall plates for straight and that the wall frames are square overall. Gable: Mark the positions of the gable end rafters, at each end, then working from one end set out the positions of the common rafters, in-to-over, at the specified maximum spacing, i.e. 450 or 600 mm. Place a mark on one side, ‘R’ or ‘X’ to represent the position of the rafter and a ‘J’ on the other side to represent the position of the joist. Note: Ends are trimmed later at 90º to the ceiling joists to provide fixing for the ceiling sheets. Top wall plates

6 0 0 0 6 0 0 0 6

Top plate

Gable end rafter position for a flush gable Joist Common rafter

TYPICAL DETAIL Fig. 12 Setting out plates for a gable roof

Hip: Measure in the distance equal to the half span from both ends. This represents the centre line of the centring rafters. Measure half the rafter thickness on either side of this line and place an ‘R’ between the two outside marks. Place a ‘J’ on one side of the rafter position to show the joist position. Repeat this process for the other end of the roof, and then work from the centring rafter to the end of the walls marking rafters, in-to-over, at the maximum spacings. Mark rafter positions at both ends of the roof working from the centre to the outside. Finally, start from the centring rafter position at one end and mark rafter spacings to wards the other centring rafter, until the spacings run out. Mark these with an 'R' or 'X' and then place a 'J' beside them to show joist positions. ©TAFE NSW Construction and Transport Division

15


Centre line position of hip roof members C L

6 0 0 0 6 0 0 0 6 6 0 0 0 6 0 0 0 6 Top plate 6 0 0 6 0 0 6 0 0

Joist

½ s p a n

Centring Rafter

p a n s ½

L C

TYPICAL DETAIL Fig. 13 Setting out plates for a hip roof

STEP 2 Cut all ceiling joists to length and fix into position by double skew nailing the ends. Cut and fix ceiling trimmers to ends and above internal walls, which run parallel to the joists. Internal walls Ceiling joists fixed beside rafter positions

End trimmers may be laid flat to allow for rafter over Top wall plates

Ceiling trimmers fitted over internal walls

End of ceiling trimmed to take ceiling sheets and cornice Fig. 14 Fix ceiling joists and ceiling trimmers into position for a Gable roof

16


CARPENTRY - HOUSING

STEP 3 Fit hangers to the centre of the ceiling joist length over each room as required. Hangers are joined over internal walls and should be blocked off the top wall plate equal to the depth of the ceiling joists. Where ceiling trimmers are close to the required hanger position they may be used as the means of blocking. Note: Hangers may be run continuously over walls, but it may be more economical to treat each room separately and reduce unnecessary cost by using smaller sectioned members where possible. Cut the end to suit the pitch of the roof, for a hip roof. Smaller sectioned hangers over short spans

Ceiling dogs on alternate sides of hanger

End of deep hanger strapped with hoop iron and supported on a ceiling trimmer End of hanger bolted to gable stud to prevent twisting, for a gable roof Fig. 15 Fix hangers into position for the ceiling of a Gable or Hip roof

Note: Where hangers are over their allowable length, hanging beams may be required to support them at mid span.

Refer to AS 1684 - 1999 Part 2, for all member section sizes, spacings, spans and stress grades.


17


CEILING FRAME CALCULATIONS The basic procedures are similar to that used for wall framing. Once a system is adopted and formulae are identified it will be necessary to gather the following details for calculation purposes: Plan -

This will be required so the dimensions of rooms can be identified to allow a cutting list of material to be formed; and

Specification or Tables -

The specification or AS 1684 tables will provide section sizes and stress grades of framing members.

METHOD OF CALCULATING FRAME QUANTITIES

Joists -

Where possible the joists should run the short dimension of the room, but should always be placed to tie the feet of the rafters for the length of the roof. Calculate the ceiling joists for each room separately. Formula = (width of room) - 1 (as there is no 1st joist) Max. spacing Length of joists = internal room length + (2 x wall plate width) Ceiling trimmers are calculated separately.

Ceiling Trimmers -

Allow for ceiling trimmers where ever internal or external walls run parallel to the ceiling joists. Calculate each wall separately. Formula = (internal room length of wall) - 1 (as there is no 1st trimmer) Max. spacing Length of trimmers = maximum spacing of joists – (2 x joist thickness)

Hangers -

Allow one hanger at 2100 mm maximum centres, unless otherwise specified. Length of hangers = internal room width + (2 x wall plate width)

Hanging beam- Placed where the length of the hanger is greater than it’s maximum allowable span. (as per tables or specification) Length of hanging beam = between supporting walls + (2 x plate width)

Hanger / joist connectors -

18

Type as per specification. Allow one per ceiling joist for each row of hangers, per room.


CARPENTRY - HOUSING

WORKED EXAMPLE FOR CEILING FRAME QUANTITIES and COSTS The following worked example provides details of how the quantities are arrived at and how the individual materials are presented and costed. Details are as per plan and specification based on AS 1684 - 1999 Part 2.

SPECIFICATION Joists -

150 x 38 sawn Oregon F8 at 600 mm max. c/c (max. 3.6m continuous span) Joists are to be joined on a hanger where they exceed 3.6m in length. Where joists are joined on hangers the joins should be staggered, if possible.

Trimmers

100 x 38 sawn Oregon F5 at 600 mm max. c/c

Hangers -

Hangers to be spaced at max. 3600 mm c/c Max. span (mm)

Section size (mm)

Stress grade

Material

5400

240 x 45

F27

Seasoned hardwood

3000

190 x 35

F11

Seasoned hardwood

Hanging beams -

Not required.

Hanger / joist connectors -

Allow one for each joist, per hanger, per room. Fit ceiling dogs on alternate sides for the length of the hangers.

Material costs -

Material 150 x 38 F8 sawn Oregon 100 x 38 F5 sawn Oregon 240 x 45 F27 Hardwood (seasoned) 190 x 35 Hardwood (seasoned) Ceiling dogs

Cost $ 4.20/m $ 3.10/m $19.20/m $12.45 /m $ 0.55/ each

Fig. 16 Typical plan of frame for a brick veneer cottage


19


Length = internal room length + (2 x wall thickness) + (100 mm for laps if required) Number = width of room – 1 Max. spacing Room A1 • •

=

5250 – 1 600

=

9–1

=

8

Max. span is 3600 mm, therefore join joists on the hanger in the centre of the room; Length = 6700 + 200 + 100 for join = 3.5m 2 Order - 16 / 3.6

Room A2

•

=

4200 – 1 600

=

7–1

=

6

=

5–1

=

4

=

7–1

=

6

=

5–1

=

4

Length = 3400 + 200 = 3.6m Order – 6 / 3.6

Room A3

•

=

2650 – 1 600

Length = 4200 + 200 = 4.4m

Order – 4 / 4.5 Room A4

•

=

4200 – 1 600

Length = 3200 + 200 = 3.4m

Order – 6 / 3.6

plus =

•

2650 – 1 600

Length = 2400 + 200 = 2.6m Order – 4 / 2.7 Order = 150 x 38 sawn Oregon F8 – 4/ 4.5, 28/ 3.6, 4/ 2.7

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CARPENTRY - HOUSING

Length = max. spacing – (2 x joist thickness) Number = length of wall – 1 Max. spacing Room A1

=

(6700 – 1) x 2 walls 600

=

(12 –1) x 2

=

22

Room A3

=

(4200 – 1) x 2 walls 600

=

(7 – 1) x 2

=

12

Room A4

=

=

(4 – 1)

=

3

•

∴

(2400 – 1) 600

Length = 600 – (2 x 38) = 524 mm 37 x 0.524m = 19.4m

Order = 100 x 38 sawn Oregon F5 – 1/ 6.0, 3/ 4.5

Required as per specification. Length = span of room + (2 x wall plate width) Room A1

= 5250 + ( 2 x 100)

=

5450

=

1/ 5.7

=

1/ 3.0

Order = 240 x 45 seasoned hardwood F27 – 1/ 5.7 Room A3

= 2650 + ( 2 x 100)

=

2850

Order = 190 x 35 seasoned hardwood F11 – 1/ 3.0

Allow one per joist, per hanger, per room Room A1

= 16

Room A3

= 4

Order = 20 ceiling dogs


21


Cost sheet

22


CARPENTRY - HOUSING

ALTERNATIVE CEILING TYPES Flat Roof Construction Generally, the construction for a flat roof combines the roof frame and ceiling frame to form one structure, which carries the load of the roof and the ceiling linings. A flat roof is one which is pitched at less than 10° or has a slope of less than approximately 1in 6. To allow for this extra load the rafters/ceiling joists are increased in section size and stress grade. They normally have a single span and are lined on-the-rake, either on top or under the rafters. The roof surface is covered with full-length sheets of corrugated iron, metal tray or decking sheets, clear or coloured fibreglass sheets and/or clear or coloured polycarbonate sheets. Batten

Sarking Trimmer

Solid blocking

Rafter Top plate

Stud FLAT ROOF CONSTRUCTION – SMALL SLOPE PROVIDED

Metal tray decking

Solid Blocking

Fascia Sarking CeilingTrimmer Barge board

Internal wall Rafter

Trimmer Batten

Top plate

CONSTRUCTION DETAIL Fig. 17 Flat roof construction

To provide a fall for the roof covering, where the ceiling frame is to be level, different thickness battens may be used. These are referred to as ‘grading battens’. The batten at the guttering end is the thinnest and the other battens gradually increase in thickness to the high end of the roof. This may require some battens to be laid on their flat, some on their edge and some may need to be checked-in slightly to achieve the correct height, at the nominated spacing. To prevent sideways movement of the ceiling frame members, solid blocking is provided where the joists span more than 2100 mm. Ceiling trimmers are fixed the same as for gable and hip roofs where the internal walls run parallel to the joists. ©TAFE NSW Construction and Transport Division

23


CONVENTIONALLY LINED CEILING:

Batten thickness graded to provide fall

Fall Metal tray decking

Vapour barrier sarking insulation Solid blocking where span exceeds 2100 ALTERNATE FINISH LAY-IN PANEL INFILLS TO CEILING:

Fall Metal tray decking

Batten thickness graded to provide fall to roof gutter Vapour barrier, sarking, insulation laid loosely over joists EXPOSED JOISTS CEILING:

Metal tray decking

Fall

Batten thickness graded to provide fall to roof gutter Sarking lapped and taped Sheets joined over a joist

Fig. 18 Various methods used to line ceilings

24


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Skillion Roof Construction This type of freestanding roof/ceiling frame is usually constructed by having the wall frame at one end of the building higher than the other end. Internal walls running parallel to the end walls would be built at different heights, depending on their location in the building. Walls running at 90° to the end walls would taper in height to fit under the sloping rafters/joists. This type of roofing system may also have its ceiling lined on-the-rake or be fitted as a false ceiling and placed level, as shown below:

RAKED CEILING

LEVEL CEILING Fig. 19 Basic design of the skillion roof/ceiling system

The simplest method of marking the skillion rafters/ceiling joists is to scribe them over the supporting plates in position. Once cut they are spaced at the maximum centres, to suit the battens and roof sheets, and then fixed into position by double skew nailing to the plates. They are also connected to the plates with patent metal connectors to prevent wind uplift forces. Rule (first position) Mark

Stud

Rafter rested on wall plates Rule (first position)

Rule (second position)

High wall

Mark Rule (second position)

Low wall

Notching at top Notching at foot Eave width

Eave width

Fig. 20 Practical method of marking rafters/ceiling joist ©TAFE NSW Construction and Transport Division

25


Lean-to Roof Construction This type of roof system is constructed against an existing wall or other roof structure. It is mainly used for simple extensions, carports, awnings, verandahs, etc. and provides an alternative to re-pitching the existing main roof to cover the extra room or space. The ceiling finish may be one of the following: i) The rafters and the covering may be left exposed for a carport, verandah, awning, etc;

Attached to wall under eaves

Top of rafters may be lined with fibrous cement sheeting to improve appearance

ii) The ceiling may be fixed to the underside of the rafters, making it a raking ceiling; or iii) A separate ceiling frame or false ceiling may be installed to give a level ceiling line.

Adjoining high wall

ii) ENCLOSED, LINED RAKED CEILING

i) OPEN, EXPOSED RAFTER TYPE

Abutting main roof

iii) ENCLOSED, LINED LEVEL CEILING

Fig. 21 Various methods used to line a lean-to roof/ceiling

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PART 2:

GABLE ROOFS

The gable roof is classified as being double-pitched and one of the simplest roof forms, due to the fact that all rafters in the roof are exactly the same length and have the same bevels. Gables are best suited for use on buildings or structures with a simple quadrilateral shape, which is typical of the freestanding car garage, most outbuildings, lichgates (or lychgate), portico’s, etc. They may also be used in a modified form, such as a gablet, to enhance the surface of any other roof type or may also be used as a means of providing light and ventilation to a room or roof space. Many modern roof designs use dummy gables to enhance plain designs or to break up large areas of straight roof surface. In a conventionally pitched gable roof there are many individual members, which have specific structural roles to perform. Each member is reliant on the next to form an unyielding structure and at the same time provide a framework for the roof covering.

PARTS, PROPORTIONS and DEDINITIONS

Span: Half span or Run of rafter: Centre line length of rafter: Hypotenuse:

Rise:

This is the horizontal width of the roof, measured overall the wall plates. This is the horizontal distance measured from the centre of the ridge to the outside of the wall plate. It is also the plan length of the rafter. This is measured along the top edge of the rafter taken from the centre of the ridge to plumb over the outside of the wall plate. It is equal to the length of the hypotenuse of the right-angled triangle formed by the rise and half span. This is the sloping length of a right-angled triangle.

This is the vertical distance between the ‘X-Y’ line and where the hypotenuse meets the centre of the ridge.

X-Y line:

This is an imaginary horizontal line, which passes through the position where the outside of the walls is plumbed up to meet the hypotenuse or top edge of the rafter. It is used to identify the centre line positions to calculate rafter set out length and the rise of the roof.

Plumb bevel:

This is the angle found at the top of the right-angled triangle, formed by the rise, half span and top of rafter edge. This bevel is used for the angled cut on the top end of the common rafters.

Level bevel:

This is the angle found at the bottom of the right-angled triangle, formed by the rise, half span and top of rafter edge. This bevel is used for the angled cut on the foot of the common rafters, where they rest on the wall plates. ©TAFE NSW Construction and Transport Division

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Eaves width:

This is the horizontal distance measured between the outside face of the wall frame, for a timber-framed cottage, or the outside face of the brickwork, for a brick veneer and cavity brick cottage, to the plumb cut on the rafter end .

Eaves This is the distance measured along the top edge of the rafter from the overhang: position plumb up from the outside of the wall frame, where the X-Y line passes through the hypotenuse, to the short edge of the plumb cut on the end of the rafter. Birdsmouth:

This is a right-angled notch taken out of the lower edge of the rafter, where it rests on the top wall plate. The purpose of the birdsmouth is to locate the bottom of the rafter over the wall plate and to provide an equal amount left-on so the top edges of the rafters will all be the same. This is only necessary when rough sawn timber is used. The depth of the notch should not be greater than ²/3 the width or depth of the rafter, to prevent it from being weakened.

Height of the roof:

This is the vertical distance taken from the top of the wall plates to the top of the rafters where they butt against the ridge. Note: this should not be confused with the ‘Rise’ of the roof .

STRUCTURAL ROOF MEMBERS COMMON RAFTERS These are the main sloping members, which all have the same length, running from the wall plate to either side of the ridge. They are spaced at 450 to 600 mm centres f or tiled roofs, and up to 900 mm centres for sheet roofs. They support the roof battens, which in turn support the roof covering. The rafters may be set out using a variety of methods, which include use of the steel square, full size set out and by calculating length. Since the rafters are all the same lengths, they are usually set out from a pattern. This pattern has the cutting length, plumb cuts and birdsmouth marked on it to allow for consistent accuracy during repetitive mark transfer. Note: Section size, timber species and stress grades for rafters may be obtained from AS 1684.

C e n t r e l i n T r u e e l e l e n n g t h T r ue g t h o o l o f f r a f f t er o v e e n g t h r a f t er r ha n g

X

Y

Fig. 23 Set out of the common rafter


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True length of reduction for ½ thickness or ridge Allowance for plumb cut

Plumb bevel

Level bevel

Birdsmouth notch

½ Thickness of ridge on plan PLUMB LEVEL

LEVEL BEVEL

Fig. 24 Bevels for the common rafter

RIDGE Usually a deep and narrow member, it is the highest member of the roof, which runs horizontally for the length of the roof. It must be level and parallel to wall plates, for the length of the roof with the rafters being nail-fixed onto it on opposite sides. Gable roofs may be very long, therefore the ridge may require one or more joins to create a continuous length, as shown below:

SCARF JOINTED

Scarf jointed at abutment of rafter pair

Ridge Rafters

CLOSE BUTT JOINTED

Joint spliced with full depth timber fish plates each side, 25 mm thick

Fig. 25 Methods of joining ridge boards

Before the roof is erected, the ridge is set out (usually on one side only) to suit the position of the rafters. The easiest way to set out the ridge is to lay the ridge on top of the completed ceiling frame, over the external wall plate, and transfer the rafter positions onto the ridge board. This ensures the rafters will be parallel and consistent with the positions marked along the wall plate.

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Rafter positions marked onto ridge Ceiling joists

Fig. 26 Setting out the rafter positions onto the ridge board

WIND BRACING Wind bracing is designed to prevent any movement of the roof, or racking out of plumb. Wind forces on the gable ends usually cause racking.

Effect of racking of the roof Inclined brace

Bracing of the roof frame may be done by having two opposite 45º timber braces from the ridge onto an internal load-bearing wall, normally using 75 mm square timber.

Ridge

45°

Alternatively, the roof frame may be permanently braced using metal speed bracing over the surface of the frame. Temporary bracing may also be inclined, as shown, or be diagonally fixed under the rafters from the ridge to the external wall plate.

Gable rafter

Chock

Top plate Stiffner Fig. 27 Inclined wind bracing


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PURLINS Purlins, also called underpurlins, are fixed to the underside of the rafters parallel to the ridge and wall plates. They provide continuous support under the rafters, similar to bearers under joists in a floor frame. They are normally spaced at 2100 mm centres, but this will depend on the section size and stress grade, including the section size and stress grade of the rafters.

2 1 0 0 m a x 2 1 0 0 m a x

Fig. 28 Spacing of purlins Halved scarfing

Joining purlins Purlins are supported by struts at 2100 mm centres, depending on the section size and stress grade, with an additional strut under any join. The most common method of joining is to half-lap and nail together. Joints may also be cleated to prevent spreading by using timber or metal connector plates.

Purlin Strut Rafter

Fig. 29 Joining purlins over a support

Positioning purlins Purlins are positioned by measuring up from the wall plate, the desired spacing, on the underside of the end rafters, and marking the top side of the purlin thickness. A string line or chalk line is run through and a temporary 75 mm nail driven into the underside of every third rafter. The purlin is lifted into position and pulled hard up against the temporary nail, clamped and double skew nailed. Note: Each rafter is sighted for straight before being nailed to the purlins.

Rafters sighted for straight prior to nailing off

75mm nail

Purlins cramped to rafter

Desired spacing up from wall to centre of purlin

Fig. 30 Joining purlins over a support 32


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STRUTS These members are placed under the purlins to transfer the roof load to the internal load-bearing walls, strutting beams or between other struts or supports. There are several strutting systems used for roofing, as follows:

Inclined and Flying or Fan struts These struts are cut around the purlin and run at 90º, or as near as possible, to the underside of the purlin to the top of a load-bearing wall or strutting beam. Chocks Chocks are placed behind the foot of the strut to prevent it sliding under load. Flying or Fan struts are chocked at the top as well by fixing blocks onto the face of the purlin, on the outside edge of the strut. Alternatively, they may be bolted through the purlin or have a sprea der bolted across the face fa ce to prevent them sliding apart under load. Solid timber struts are normally 75 x 75 mm, however this will depend on the stress grade, length of strut and imposed roof load. The following table provides a guide for imposed roof loads, which will assist in the selection of strutting material: TABLE 1 MASS OF ROOF MATERIALS TYPE Steel sheet

MATERIAL 0.76 mm thick 0.55 mm thick 1.2 mm thick Terra-cotta Concrete Pressed metal Corrugated sheet Flat shingles 38 x 75 battens at 900 mm c/c 25 x 50 battens at 330 mm c/c 35 x 35 battens at 330 mm c/c

Aluminium sheet sheet Roof tiles

Fibre cement (F.C.) Unseasoned hardwood Seasoned pine

Approx. MASS in kg/m² 10.0 6.0 5.0 58.0 54.0 7.5 16.0 15.0 3.2 3.8 2.0

Average mass of metal roof covering and battens is approx. 15 kg/m²; and Average mass of tiled roof covering and battens is approx. 60 kg/m². Note:

The size of struts should be based on AS 1684 - 1999 Part 2 Rafters Rafter

Chock Chock

Purlins

Alternative spreader bolted to struts

Inclined Strut Chock

Chock

Chock

Flying Struts Chock

Load-bearing wall ELEVATION

SECTION

SINGLE INCLINED STRUT

ELEVATION

Load-bearing wall

SECTION

FLYING OR FAN STRUTS

Fig. 31 Common strutting strutti ng methods ©TAFE NSW Construction and Transport Division

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Fitting struts to various angles It may not be possible to fit the struts at exactly 90° to the underside of the purlin, therefore the following adjustments may be made:

Rafter

Rafter 90° 'x' Small variations permitted (approx. or truly perpendicular)

'Y' Small variation permitted (approx or truly vertical)

D

B

Top of strut must reach at least to top edge of purlin

B

A Struts

C

A

'A' Not less than 44 mm 'B' Not less than 25 mm and not over 38 mm measured at bottom of purlin STRUT PERPENDICULAR TO RAFTER

C

Struts

C' Not less than 38 mm 'D' Not over 12 mm STRUT VERTICAL OR 'PLUMB' TO RAFTER

2/2.5 x75 mm nails Strut

Low angle flat strutting

Studs 'Z' Any angle from 0° upwards, provided that the strut is not flatter than 1 vertical to 2 horizontal units for a roof slope of 1:2, or 1 vertical to 1.5 horizontal units for a roof slope of 5:12

Not less than 5 nails at least twice length of chock thickness chock

Stiffener

NOTE: Long chock required to prevent slip caused by greater thrust LOW ANGLE FLAT STRUT

Strut Perpendicular or steep angle strutting

Not less than 3 nails at least twice length of chock thickness 2/2.7x75 mm nails

E

E

Top wall plate E' Not less than 38 mm, but not more than ½ width of strut.

FLAT STRUTTING

NOTE : Short chock required due to a more direct load to the wall STEEP ANGLE PERPENDICULAR STRUT

Fig. 32 Methods of fitting struts

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Supporting struts over internal walls It is preferable to position struts directly over studs, however as this is not always possible the load must be distributed in an alternative way. This may be achieved by strengthening the plate either between or over the studs with additional blocking. Also, where struts are placed at a low or flat angle, it will be necessary to fit a block, referred to as a chock, behind the foot of the strut to prevent it from sliding under load. Chock with not less than 3 nails at least twice length of chock thickness

Strut

Top wall plate reinforced over two studs. Stiffener 50mm thick, full width of wall plate

Top wall plate

Intermediate blocking to stiffen top plate NOTE : Avoid strutting between studs

ALTERNATIVE STIFFENING METHOD

STRUT BEARING BETWEEN STUDS Top plate locally reinforced to distribute load over two or more studs

Braces, 38mm thick, full stud width

NOTE : Avoid strutting over openings

STRUTTING OVER DOOR OPENINGS Lintel deemed to be a strutting beam. Refer to AS 1684

ALTERNATIVE STIFFENING METHOD Strut landing only if unavoidable

Preferred strut landing over a stud

STRUTTING OVER AN OPENING

Fig. 33 Methods of reinforcing top wall plates for struts ©TAFE NSW Construction and Transport Division

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Scissor struts These struts consist of deep-sectioned timber members supported over external walls and bolted where they cross in the centre of the roof space. They are designed to transfer the roof load to the external walls where there are no internal walls for support or the internal walls are non load-bearing. 20 Bolt, nut and washer

∅12

Bolt, nut and washer or well nailed

Spacer block

Underpurlin Scissor struts NOTE: Scissor struts must be kept clear of hangers Hanger

Hanger

20 Bolt, nut and washer to spliced joint of tie member

Fig. 34 Full scissor type strutting

If there is internal support available, but would cause an inclined strut to be too flat to be effective, then a half scissor may be used. 20 Bolt, nut and washer to top connection. Use ∅20 for all other connections. Half scissor strut Strut

Tie member required where ceiling joist can not be used as bottom chord of truss

Regular purlin strutting

Fig. 35 Half scissor type strutting

The foot of the scissor struts must be bolted to a rafter, and preferably a ceiling joist as well, and bolted together where they cross over in the centre of the roof. If ceiling joists are not full length they should also be bolted together where they lap over a wall, to prevent the external walls spreading under load. Section sizes and stress grades should be taken from AS 1684 - 1999 Part 2 Note:

Common rafter Scissor strut

15 min 20 bolt Ceiling joist (tie member)

Fig. 36 Bolt connection detail 36


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Strutting beams An alternative to using scissor struts over large room spans would be the use of large timber or steel strutting beams. They are usually placed parallel to the ceiling frame hangers, but must not rest on any part of the ceiling frame. To achieve this the ends must be packed up at least 25 mm above the ceiling joists, which allows for any deflection. In some situations the hanger and strutting beam may be the one member, but the member should comply with AS 1684 tables or be designed by a structural engineer to cater for the additional load. The following materials may be used for strutting beams: • Solid timber (hardwood or softwood with the correct stress grade); • Horizontally laminated timber (similar to Glulam beams); • Vertically laminated timber (similar to L.V.L. beams); • Boxed beams (made from plywood); and • Steel beams (either Universal channels, U.C., or Universal beams, U.B.) Note: Refer to AS 1684 for section sizes and stress grades of timber members. If deep solid timber is used, it should be seasoned to reduce the risk of shrinkage. Most timber species over 175 mm deep will shrink excessively. 12mm max

Rafter

Purlin

Purlin

Inclined Strut

38mm min

Chock

Plumb strut Deep strutting beam 25mm clearance to allow for deflection Ceiling joist STRUTTING BEAM RUNNING PARALLEL WITH HANGER

STRUTTING BEAM RUNNING PARALLEL WITH HANGER

Fig. 37 Positions of strutting beams

An alternative position for the strutting beam is directly under the rafters, which are notched over it as if it were another wall plate. The plumb struts under it would then rest on an internal load-bearing wall.

See adjacent detail

Beam

Strutting beam Birdsmouth to rafter provides secure seating to beam

Plumb strut

Fig. 38 Alternative strutting beam position ©TAFE NSW Construction and Transport Division

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PATENT TYPE STRUTTING Super Barap This is a patent type of strutting system used where conventional strutting methods cannot be used or it will be too expensive to use them. These patent struts consist of steel saddle brackets at either end connected by a 12 mm diameter steel rod and a centrally located adjustable fulcrum. They may be used under purlins, rafters, hips or valleys to provide the required support where sagging or excessive deflection of the member has or may occur. This system operates similar to a truss as it is made up of ties, which are in tension, and a strut, which is in compression. They may be purchased through hardware stores or direct from the manufacturer. Note: See manufacturers brochure and specification for further details and fitting instructions. Maximum span 2 fulcrums Purlin

6.700m

Saddle bracket

Adjustable fulcrums

Strut

Saddle bracket Tie rod Strut

Maximum span 1 fulcrum Purlin

5.500m

Saddle bracket

Adjustabl e fulcrum

Strut

Saddle bracket Tie rod Strut

Fig. 39 Patent type Super barap strut/brace

Cable truss This is another very effective patent type system, similar to the Super Barap, which uses two tensile steel cables instead of a solid steel rod. Each cable is made up of 7/ 1.6 mm Ø wire strands. The truss may also be fitted with an additional adjustable fulcrum or strut for use on longer members. The ends of the cables must be bolted within 200 mm Max. of the end supports. To calculate the length of the truss, measure the distance between the bolted ends and add 80 to 150 mm to allow for the cables to run over the single or double fulcrums. Note: See manufacturers brochure and specification for further details and fitting instructions. 'Tyloc' plates and bolt

Rafter

Purlin

Support block Adjustable fulcrum

Twin wire support system

Fig. 40 Patent type Cable truss/strutting system 38


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COLLAR TIES They are light sectioned horizontal members used for additional support, like spreaders, to prevent the rafters from sagging at the purlin position. These are fixed to alternative pairs of rafters, i.e. at 900 to 1200 mm spacings, and placed on top of the purlins running parallel to the ceiling joists. They may be half scarfed around the face and edge of the rafters and nail fixed with 2/75 mm nails.

Common rafter

Collar-tie half scarfed or bolted to rafter Underpurlin

Alternatively, they may be run past the face of the rafters and be bolted to them at both ends using a single 10 mm Min. mild steel, cuphead bolt.

Strut

Fig. 41 Collar tie fitted to rafter over purlin

The size of collar ties depends on the stress grade and length of timber used. As a guide, they are normally 75 x 50 or 125 x 38 F5 to F7 up to 4200 mm long, and 100 x 50 or 125 x 38 F5 to F7 over 4200 mm long. Refer to AS 1684 for specific details. Note:

Placed every 2nd pair of rafters, 900 to 1200 mm apart

Fig. 42 Placement of collar ties in the roof frame


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GABLE ENDS There are three main methods used to finish the ends of gables: 1. Flush gable with no eaves; 2. Flush gable with raked eaves; and 3. Boxed gable.

Flush gables (no eaves) The end of the gable is flush or in-line with the outside face of the end wall. The end of the roof has no overhanging eaves, only a barge fixed flush with the outside of the end wall. This finish may be applied to timber framed cottages, where the walls are clad with boards or sheeting, or to brick veneer and cavity brick cottages, where the brickwork runs to the underside of the roof covering. The triangular section formed between the top of the standard wall frame and the underside of the rafters is framed with stud material, spaced at the same centres as the wall frames, fixed on flat or on edge. Fig. 43 Flush gable

Gable studs for cladding fixing or trying to brick work.

Fig. 44 Framed flush gable

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Flush gables with raked eaves The gable finish for this type is similar to that of a gable with no eaves. The main difference is the end of the roof frame is extended past the end wall to form an eaves overhang, which is lined on the rake. The ridge and top wall plates may be extended to provide support for the gable rafters. Where the overhang is particularly wide or the length of the gable rafters is excessive, the purlins may also be extended to provide additional support. Where the raked ends are required to adjoin level side eaves, the ends of the eaves are usually boxed to allow the raked section to terminate neatly.

Fig. 45 Flush gable with raked eaves

Rafters Gable stud

Trimmers Trimmer

Top plate Wall stud

Fig. 46 Framing for eaves lined on-the-rake ©TAFE NSW Construction and Transport Division

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Framing variations for raking eaves The top wall plates may be extended to take the gable rafters. It is only necessary where the ends of the side eaves are to be boxed, which will allow the raked gable eaves to terminate neatly. It is necessary to extend the ridge for this method as well.

Note:

Stiffener to support extended top plate

Fig. 47 Extended top plate

When the eaves width at the gable ends is excessive, i.e. say greater than 450 mm, or the unsupported length of the gable rafter is excessive for the section size of rafter, then it may be necessary to extend the purlins on both sides to support the mid length of the gable rafters. Also, with some roof design or when the eaves are lined on top of the rafters, it is desirable to expose the framing members. (this was a typical method used for the ‘Bungalow’ style of cottage during the Federation period)

Trimmers or outriggers

Fig. 48 Cantilevered gable framing

To provide continuous raked eaves with no framing members visible, it will be necessary to place cantilevered trimmers to support the gable rafters.

Gable rafter

Purlin

These trimmers, also known as outriggers, are either checked into the rafters on their flat or will be supported on-edge over the gable end wall frame, which has raking top plates. Short rafter trimmers are then cut between them to provide fixing for tile or roof sheet battens.

Fig. 49 Extended purlin

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Boxed gable A boxed gable occurs where it is desirable to have level eaves on both sides and ends of the roof. The face of the boxed gable may be clad with the same material as the end wall, but may also be featured by cladding with an alternative material finish. The fascia may be returned level around the corner or the barge may extend to the outside of the gutter and have a small timber ‘bellcast’ added to the top edge. The end of the boxed gable is framed up with gable studs, eaves trimmers and a full width bottom chord or tie, to allow for fixing of the cladding and eaves soffit lining.

Fig. 50 Boxed gable

Soffit lining Ridge

Fascia

Purlin

Ceiling joist

Top plate

Ceiling trimmers

Gable cladding Soffit bearer Batten for fixing

Gable stud Plates and purlins extended for support

Fig. 51 Framing for a boxed gable


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Verge finishes The verge is the section at the end of the gable roof where the roof surface meets the barge or verge board. The type of finish will depend on the roofing material used and the finish required. Tiled roofs may have a coloured mortar pointed verge, which is laid on a narrow fibre cement (F.C.) strip, it may be covered with purpose made barge cover tiles or the tiles may be cut against a pre-formed ‘barge soaker’. In recent times, the Colorbond barge soaker has become the preferred method of finishing the verge as it does not require any maintenance. Metal sheet roofs may also have a barge soaker or may be fitted with a covering pre-formed Colorbond barge capping. Screw Fixing

Pop rivet

Clip and/or bracket fixing

Clip

Vapour barrier

Fig. 52 Metal barge capping profiles Steep angle ridge

Standard ridge tiles

Tiles bedded and pointed Fibrous cement strip Fig. 53 Pointed verge Ridge capping

Barge cover tiles Reflective foil insulation/sarking

Fig. 54 Barge tiled verge End cap Vapour barrier

Barge capping Fixing clips NOTE: Metal barge capping may also be used on the verge of tiled roofs

Fig. 55 Metal barge capping to verge

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Gutter bracket Fig. 56 Matching barge and gutter


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‘Colorbond ®’ patent verge finishes In recent years the method of finishing the verge for gable roofs has changed. New metal fascia and barge profiles provide an alternative to using primed fascia boards. They are attached directly to the ends of the rafters or outriggers, using special brackets, and the gutter or barge soaker is attached to them. The benefits of using these Colorbond ® products is that they do not require painting, they are not susceptible to decay caused by leaking gutter joints, they do not cup or twist, they are available in a full range of popular colours and they are relatively simple to fit. The Colorbond ® metal barge is attached using barge rafter brackets and then the Colorbond ® metal barge soaker is placed over the top. The benefit of using the barge soaker removes the need to bed and point the verge, which eventually cracks and becomes loose over time. The following details were supplied courtesy of ACE GUTTERS PTY LTD.

Barge Soaker m m 2 6

180mm

Steel Fascia

10mm

70mm 28mm

97mm

16mm 95mm

m m 6 3

Barge Rafter Bracket

Straight Joiner

Barge Apex Cover

Barge Mould L.H. & R.H.

Fig. 57 Barge, soaker, and accessories ©TAFE NSW Construction and Transport Division

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CALCULATING ‘DROP-OFF’ The finished height of the brickwork is determined by the height of the timber frame and the drop-off needed to give the eaves width required. In turn the pitch of the roof and the head height of the windows influence the eaves width and drop-off. Where eaves soffit finishes above the head of the windows, the space may be infilled with brickwork or with timber framing and cladding material. The drop-off measurement is taken vertically from the top of the wall plate to finished height of the brickwork. The purpose of the drop-off measurement is to provide the bricklayer with a finished height, in relation to the height of the wall frame, to allow an even gauge to be set out on the storey rod . In a brick veneer cottage with a suspended timber floor, the brickwork is normally completed to the underside of the bearer and then the brick gauge is calculated to drop-off level.

Note:

Same Roof Pitch 30°

Same Roof Pitch

30°

Drop-off Drop-off

Smaller eaves width

Larger eaves width

SMALLER DROP-OFF (HIGHER WALL)

Larger Roof Pitch

LARGER DROP-OFF (LOWER WALL)

Smaller Roof Pitch

30°

22½°

Smaller eaves width

Larger eaves width

SAME WALL HEIGHT

SAME WALL HEIGHT

Fig. 58 Details showing how drop-off affects the width of the eaves 46


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Method used to calculate drop-off To calculate the amount of drop-off , it will be necessary to use the mathematical process known as trigonometry, which deals with the measurement of sides and angles of triangles, i.e. sine, cosine and tangent. The method required to calculate drop-off is tangent or simply Tan, when:

• Tan = opposite side

( Note: Use the Tan button on a scientific calculator)

adjacent side

• θ = the known angle of the triangle or the pitch of the roof, e.g. 30°, 22.5°, etc. Example 1: Find the drop-off measurement for a brick veneer cottage with a roof pitched at 30° and a required eaves width of 400 mm.

Formula = Tan θ = opposite side adjacent side Depth of birdsmouth

Tan 30° Tan 30°

=

opposite side 400 + 110 + 40

=

opposite side 550

To find the opposite side, transpose the formula by cross multiplying to allow the unknown measurement, i.e. opposite side, to be on its own:

∴

Tan 30° 1

0 2

x

f f o p o r D

30°θ

opposite side 0.550

= (opposite side x 1) = ( Tan 30° x 550) To find Tan 30°, insert 30 into the calculator and then press the Tan button. The answer will equal 0.577350269. Reduce this to 3 decimal places and use it for the remainder of the calculation, i.e. 0.577

400

110

40

Fig. 59 Detail of eaves

∴ opposite side = 0.577 x 0.550 0.317 m or 317 mm

∴The total drop-off as measured from the top of the wall plate to top of the brickwork will be: 317 + 20 (which is the depth of the birdsmouth) = 337 mm ©TAFE NSW Construction and Transport Division

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There are a number of ways to frame and finish eaves. Types include simple boxed level eaves, lined on-the-rake eaves, eaves lined on top of rafters or combinations of these. The level framing members, running between the wall and fascia, are referred to as soffit bearers or eaves sprockets. They are spaced at 450 to 600 mm centres to provide fixing for the eaves soffit lining, which may be timber boarding or more commonly 4.5 mm thick fibre cement sheeting joined with a PVC jointer.

Soffit bearers Fascia Lining to eaves soffit Fig. 60 Framing for level or boxed eaves

Barge board Fibrous cement strip Trimmer Trimmer Rafter Tile batten Anti-ponding strip

Fascia Soffit bearer

Eaves soffit Fig. 61 Framing for raking gable end meeting boxed eaves

Fig. 62 Raked side eaves

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Eaves soffit clearance Where timber eaves framing passes over face brick walls, as in brick veneer construction, an allowance for frame shrinkage must be provided. If no clearance is allowed, the top course/s of brickwork may be cracked or even dislodged when the timber shrinks causing the eaves framing to drop, which will allow the roof load to bear directly onto the brickwork. When unseasoned, or partly seasoned, timber is used it will continue to dry out causing a reduction in its section size. Timber shrinks in width, thickness and to a much lesser extent, its length. Unseasoned timber with a width of more than 175 mm will shrink excessively, i.e. up to 10 mm for every additional 25 mm of width in some cases. This shrinkage usually occurs in the width of bearers, joists, lintels and in the thickness of the top and bottom plates. (Stud length is relatively unaffected) Therefore, a clearance of 12 mm minimum is to be allowed between the underside of the eaves soffit bearer and the top of the brickwork. Note: No clearance is required for cavity-brick construction or where timber frame construction is used on its own or when the timber framing is fully seasoned. Shrinkage does not occur in steel framing or when manufactured products such as structural particleboard, ‘LVL’ or ‘Hyspan’ are used.

Brick veneer

12 mm min to allow for shrinkage of framing

Fig. 63 Eaves soffit clearance

Material shrinkage causes frame to drop

Result of no clearance. Top course tilts under load

End of soffit bearer drops with frame

Fig. 64 Effect of frame shrinkage on brickwork ©TAFE NSW Construction and Transport Division

49


ERECTION PROCEDURE for the GABLE ROOF Ceiling dogs on alternate sites of hanger

After the ceiling frame is totally complete and the rafters have been set out and cut, follow the steps below: Smaller sectioned hangers over short spans

End of deep hanger strapped with loop iron and supported on a ceiling trimmer End of hanger bolted to gable stud to prevent twisting Fig. 65 Completed ceiling frame

STEP 1

Measure the length of the ridge and cut to length or join lengths together, as previously shown. Note: Allow extra length for gable overhang as required. Lay the ridge on flat and place the top edge flush with the ends of the ceiling joists. Using a square, transfer the rafter positions onto the edge of the ridge and square them down the face of one side. Ceiling joists

Fig. 66 Setting out rafter positions on the ridge board 50


Rafter positions marked onto ridge

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STEP 2

Erect a pair of rafters for each end of the roof. Nail the feet of each pair to the plate with the plumb cut ends butted together.

Temporary nail

Place a temporary nail at the top of each pair of rafters for stability.

Ridge Push ridge up between rafters flush with tops

Lift the ridge up between the rafters until it is flush with the top edge, or to a marked straight line, then nail through from one side into the end of one rafter with 2/ 75 mm nails.

Ridge Skew nails

Align the opposing rafter and skew nail from the opposite side using 2/ 75 mm nails.

This rafter is nailed first, from the other side

Fig. 67 Fixing the first pairs of rafters to the ridge

STEP 3

Plumb one end and attach a temporary brace, to prevent racking, and then attach a string line along the top of the ridge to ensure it remains straight while the remaining rafters are nailed into position. Note: Provided all rafters are exactly the same lengths and the side wall plates are straight, then the ridge should automatically finish straight. Block

Ridge String line Checking block Marked rafter positions Ceiling joists Block Rafter

Top plate Temporary brace Studs Fig. 68 Fix off the remaining rafters ©TAFE NSW Construction and Transport Division

51


STEP 4

Set out and fix purlins into position as required, then cut and fix the struts for the whole roof. Set out, cut and fix collar ties on top of purlins, bolting or nailing them as required. Rafter

Collar tie scarfed around rafter Purlin Inclined strut

Fig. 69 Complete the assembly of the structural frame

STEP 5

Fit permanent wind bracing. This may be in the form of opposing timber braces onto an internal wall or metal speed bracing over the surface of the rafters.

Inclined opposing wind braces

Speed bracing

Fig. 70 Types of permanent wind bracing

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STEP 6

Cut and fix gable studding into place to suit wall stud spacings and sheet cladding joins. A pair of rafters and a bottom chord or tie forms the boxed gable frame. The studs are cut around the rafters and tie. Raked eaves on a gable end have the addition of a raking plate on either side, fixed under the line of the rafters. Outriggers are supported on these raking plates with short trimming rafters cut between them, for rafter continuity.

Gable studs checked out around the rafter and tie

Bottom chord or tie

Eaves trimmers Gable studs placed at centres equal to wall framing or sheet cladding joins

Fig. 71 Boxed gable studding and framing complete

Rafter trimmers cut between outriggers Outriggers Gable studs cut onto top plate

Raking plate for fixing of cladding

Gable studs spaced at centres equal to wall framing or sheet cladding joins Fig. 72 Gable end studding and framing complete for raked eaves ©TAFE NSW Construction and Transport Division

53


STEP 7 To determine the eaves width, it will be necessary to calculate the drop-off position, unless these dimensions are given. Refer to previous details. Set out and mark the line of the overhang by measuring horizontally from the outside of the wall frame. Plumb a line down the face of the rafter ready to cut.

Mark the face of the rafter

550

Fig. 73 Marking the width of the eaves

STEP 8

Plumb a line down, the same distance out, at the other end of the roof. Drive in two temporary nails on the top edge of the end rafters and attach a string line. Work along the rafters marking plumb down from the string line with a spirit level.

String line plumbed down

Fig. 74 Marking the ends of rafters to a line

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STEP 9 After the ends of the rafters have been cut plumb to a straight line and the eaves soffit bearers fitted, cut and fit the timber or metal fascia ready to receive the gutter. The top of the groove should be in-line with the top side of the eaves soffit sheet. The top of the fascia will project above the top edge of the rafters to provide a bellcast. The bellcast ensures that the first course of tiles will have the same pitch as the remainder of the roof and the distance above the rafters should be equal to the thickness of a tile batten plus the thickness of one tile. Soffit bearers fitted from fascia to wall frame

Fascia double skew nailed to prevent it from being easily pulled off Top of groove flush with Fig. 75 Fitting the fascia ready to receive gutters

STEP 10 Once the fascias are fitted and the gable ends are clad, cut and fix timber barges or Colorbond metal barge soakers. The bottom edge of the timber barge is fixed flush with the bottom edge of the fascia and run past the fascia to enclose the end of the gutter. They will require a timber bellcast infill piece to be attached to the top edge. Barge attached to gable rafter

Quad gutter fitted to fascia Timber fillet placed on top to form a bellcast End of eaves boxed Fig. 76 Fitting the barge to enclose the end of the gutter ©TAFE NSW Construction and Transport Division

55


ROOF PITCH All pitched roofs are based on the same simple geometric shape, the right-angled triangle. In the case of the gable roof this shape is found on one side formed by the rise, run or half span and centre line length of common rafter top edge. The right-angled triangle shape contains one 90° angle and two complimentary angles, which make up another 90°. Therefore, the angles within a right-angled triangle will equal 180°.

h S E l e n g t U N T E r a f te r O P H Y e l i n e t r e n C o r Complimentary

60°

Same size, shape and proportions on both sides of the roof

f o o T r f H o G I e s E i H R r o

angles 90°

30° BASE or Run of rafter

Fig. 77 Proportions of the right-angled triangle in a gable roof

PITCH The pitch or slope of the roof surface may be calculated using one of four common methods: Rise 1 Unit 30°

Half span = 3 units Pitch is specified as a ratio, i.e. 1:3, or as the Rise per metre run, i.e. 333:1000

Pitch is measured at the foot of the rafter in degrees.

Fig. 78 Roof pitch or slope given as a ratio

Fig. 79 Roof pitch or slope given in degrees

Rise 1 Unit

Rise

Half Span Span = 3 units

Span The Specified as a fraction of the span, e.g. Rise = 2700 mm.

Fig. 80 Roof pitch or slope given as a fixed rise

56

rise

is

given

as

an

actual

measurement,

e.g. Pitch = 1/3

Fig. 81 Roof pitch or slope given as fractional pitch


CARPENTRY - HOUSING

CALCULATING ROOF PITCH The two main methods used to determine roof pitch ar e: 1. Degrees; and 2. Pitch ratio.

Degrees This method involves the same process as for calculating the drop-off, i.e.

• Tan = opposite side

( Note: Use the Tan button on a scientific calculator)

adjacent side

• θ = the known angle of the triangle or the pitch of the roof, e.g. 30°, 22.5°, etc. The calculated proportions are then used to determine the length of the rafter when used with the Pythagorean formula, i.e. a² = b² + c² Example 1: Find the length of the hypotenuse, or rafter centre line length, when the pitch of the roof is 30° and the plan length, half-span or run of the rafter is 2700 mm.

STEP 1

Formula = Tan

=

STEP 2

θ =

opposite side adjacent side

Tan 30°

=

opposite side 2700

To find the opposite side, transpose the formula by cross multiplying to allow the unknown measurement, i.e. opposite side, to be on its own:

∴ =

Tan 30° 1

x

opposite side 2700

(opposite side x 1) = ( Tan 30° x 2700) To find Tan 30°, insert 30 into the calculator and then press the Tan button. To find Tanwill 30°, insert 30 into the calculator and then press the Tan but The answer equal 0.577350269. The answer equal places, 0.577350269. Reduce thisit for to 3the decimal places an Reduce this towill 3 decimal i.e. 0.577, and use remainder of the calculation. it for the remainder of the calculation, i.e. 0.577

=

opposite side = 0.577 x 2.700

=

1.558m

∴ The length of the opposite side or the rise of the of the triangle = 1.558m


57


STEP 3

Use the Pythagorean formula to calculate the centre line length or hypotenuse of the triangle, i.e. a² = b² + c², when:

• a² = hypotenuse; • b² = run or plan length; and • c² = rise.

STEP 4

=

a² = 2.700² + 1.558²

=

a² = 7.29 + 2.427

=

a² = 9.717

To find ‘a’ on its own, it will be necessary to find the square root ( √ ) of 9.717. Therefore, enter 9.717 on the calculator and then press the √ button.

=

√ 9.717

=

3.117m

∴ The centre line length of the rafter or the hypotenuse of the of the triangle = 3.117m Note: The centre line length of the rafter is taken from the centre of the ridge to plumb over the birdsmouth. The length of the eaves overhang may be calculated in the same way and then added to 3.117 or the original run or plan length of the rafter may be increased to include the eaves width, which will allow the total cutting length to be calculated in one go.

Pitch ratio This method involves the use of a ratio, i.e. rise : metre run, to provide the proportions of the triangle or roof. The ratio is equal to the pitch of the roof, e.g. 30° = 1: 1.732, which means for every 1.0m of rise there will be 1.732m of run or plan length. This ratio, of 1 : 1.732, is converted to a rise in millimetres to a run of 1 metre, as follows:

=

1.000 1.732

=

0.577m or 577 mm. ) 0 0 0 . ) e 1 ( i s 7 R 7 ( 5 . 0

This means that for every 1.0m of run there will be 577 mm of rise. This is the same as a roof with a 30º pitch, as Tan 30º = 577 mm. 30° 1.000 (1.732) (run, plan length of rafter or half span)

Fig. 82 Proportions of the ratio shown on the triangle

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CARPENTRY - HOUSING

The benefit of using a pitch ratio, rather than working from degrees, is that a true length of rafter per metre run of rafter can be established and used as a constant for calculating the length of any rafter, with any half span or run, having the same pitch ratio: Example 2: Find the length of the hypotenuse, or rafter length, when the pitch of the roof is 1:1.732 and the plan length, half-span or run of the rafter is 2700 mm.

STEP 1

STEP 2

Rise per metre run

=

1.000 1.732

=

0.577m

=

a² = b² + c²

a²

=

1.000² + 0.577²

a²

=

Therefore, a

=

√ 1.333

=

1.155m

Length of hypotenuse

1.0

+ 0.333

Therefore, for every 1.0m of run or half span the hypotenuse or rafter length will be 1.155m

STEP 3

To find the centre line length of the rafter, simply multiply the run or half span by the constant, 1.155:

Centre line rafter length

=

2.700 x 1.155

=

3.118m

Note: The answer should be the same as for the method using degrees, within 1 or 2 mm depending on how the numbers were rounded off to 3 decimal places.

STEP 4

To find the cutting length of the rafter, i.e. including eaves overhang, simply add the eaves width to the run or half span, deduct half the ridge thickness, then multiply the answer by the constant 1.155. The ridge is 24 mm thick:

Cutting length of rafter

= =

[(2.700 + 0.400) - 24] x 1.155 2 3.088 x 1.155

=

3.567m

∴ Total cutting length of all the rafters will be 5.567m


59


SETTING OUT AND CUTTING RAFTERS The common rafters for a gable roof are all the same length, have only two bevels and may be set out from one pattern rafter. The length of this pattern rafter may set out by calculation, as previously mentioned in calculating roof pitch, or be set out using a steel roofing square, which will also have the two bevels required for the rafter.

Proportions of common rafters Some of these proportions were dealt with earlier in this unit. The critical elements are shown below:

C e n t r e l i n e s T r u e e t -o l e n u t l T r g t h e n g u e o f t h o o l e v r a f t f r h a n g t h a f t e r er n g o f e r

True length of reduction for ½ thickness of ridge Allowance for plumb cut

Plumb bevel

Level bevel

½ Thickness of ridge on plan

Birdsmouth notch LEVEL BEVEL

PLUMB BEVEL Fig 83 Common rafter proportions

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CARPENTRY - HOUSING

Plumb and Level bevels The simplest method used to establish the plumb and level bevels is the ‘ pitch board method’ . Once the rise per metre run is known it is simply a matter of reducing the full measurements to a scaled size, say by dividing both measurements by 10. This means that a roof with a pitch of 577 mm to 1000 mm, or 30°, divided by 10 would be:

577 10 = 57.7mm

and

1000 10 = 100mm

These smaller measurements are then set out to look like half the roof on a piece of timber or board material. A sliding bevel is then laid against the edge and adjusted to suit the angle:

STEP 1

Set up the pitch board by drawing the half roof shape, using the scaled measurements.

Plumb bevel

Level bevel 5 7 . 7

0 1 0

Fig. 84 Pitch board method

STEP 2

Lay a sliding bevel against each edge and adjust the blade to suit the angles formed.

Sliding bevel set to the plumb bevel angle

Sliding bevel set to the level bevel angle

Fig. 85 Setting sliding bevels to the plumb and level bevels

Note: These bevels may be transferred to the rafter or any other roof member, which requires a plumb or level bevel. ©TAFE NSW Construction and Transport Division

61


THE STEEL SQUARE A common method used to set out the length of a rafter is to use the steel roofing square. This is a very versatile tool as it may be set up with the plumb and level bevels within the 90° triangle formed by the square and the adjustable fence or buttons. Again, the pitch of the roof is set up on the square using scaled measurements, which in this case are usually half the full size proportions. Example 1: If the pitch ratio is 1 : 1.732 or 30° it is firstly changed to a rise per metre run, which equals 577 to 1000 mm, then these measurements are halved to become 288.5 to 500 mm.

Setting up the square STEP 1

The proportion for the rise, i.e. 288.5 mm, is placed on the tongue of the square and the run or half span, i.e. 500 mm, is placed on the blade of the square.

40

0 0 4

Tongue Blade

600 Heel Fig. 86 The steel square

STEP 2

A timber fence may be used to link the measurements and form the right-angled triangle. The timber fence sits on the top edge of the rafter to be set out. This allows it to slide along the rafter edge travelling in increments of 500 mm, until the desired distance is reached. Fig. 87 Adjustable timber fence

STEP 3

An alternative to the timber fence is the use of patent type steel buttons or clips. They are attached to the tongue and blade measurements and then the square is used the same as for the timber fence. Note: When the square is turned upside down it may be used to set out risers and treads for stairs. Fig. 88 Alternative buttons, clips or guides

62


0 5

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Patent buttons, clips or guides 'Starrett' type

There are a number of brand types available for use with the steel square. One common type is the steel or brass button type produced by ‘Paulcall’ and manufactured in Australia. Another common type is the steel clip type produced by ‘Starrett’ and manufactured in the USA. 'Paulcall' type

Fig. 89 Common buttons and clips

Graduations Steel squares are marked off around the inside and outside edges of the tongue and blade in 2 mm graduations. Also, there are 10 mm and 100 mm graduations to allow for larger dimensions to be identified.

Fig. 90 Graduations on the steel square


63


Using the steel square to set up a pattern rafter Once the pitch of the roof has been determined, set the scaled dimensions on the square, select a straight length of timber to use as a pattern rafter and step along the required number of times until the run or half span distance has been travelled. Example 1: Set up a steel square with a pitch ratio of 1 : 1.732 or 30° and set out a pattern rafter, which has a run or half span of 2700 mm and an eaves width of 400 mm.

STEP 1

Set the fence or buttons on the steel square to suit the scaled proportions, which will provide the pitch and bevels for the roof, i.e:

• Pitch = 1 : 1.732 • Rise per metre run = 1.000 •

1.732 = 0.577m (Therefore 577 mm : 1000 mm) Divide both measurements by 2 = 577 = 288.5 mm, and 1000 = 500 m 2 2

1155 577

0 0 2

30° 1000

5 . 8 8 2

0 0 1

0 0 1

0 0 2

0 0 3

0 0 4

500 Fig. 91 Steel square ready for use

STEP 2

Place the square over the rafter with the fence on the top edge. Mark a plumb cut line across the face to represent the centre line of the ridge. Measure back half the thickness of the ridge and mark a firm plumb cut line on the face. This will be the cutting line for the top of the pattern rafter. Now move the steel square back to the original centre line ready to start stepping along the rafter.

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CARPENTRY - HOUSING

Plumb out

True length of reduction for ½ thickness of ridge Plumb bevel

Ridge position Plumb cut set back ½ thickness of ridge

½ Thickness of ridge on plan

Fig. 92 Detail of the reduction for the ridge

STEP 3 Divide the measurement on the blade, i.e. 500 mm, into the run or half span: = 2700

500 = 5.4 ∴ The square will be moved along 5 times plus an additional 200 mm to equal the run or half span of 2700 mm. Note: The overhang is marked by placing the edge of the tongue on the birdsmouth plumb cut and marking off 400 mm from the blade

CL

0 6 5 . 1 = f o o r f o e s i R

5 . 8 8 2

T o t al C e c u t n t r ti n g l e e l i n e n g t h s e t o f o u t c o 500 l e n m m g t h o n o f r a f c o 500 t er m m = 3 o n 5 8 0 r Centre a f t 500 e r = 3 line for 1 1 8 ridge 500

500

= 4 E a 6 2 O v v e s Y e r h a n g

200 Run or half span of rafter = 2700

Plumb cut for fascia

400 Eaves width

Run or half span of rafter plus eaves width = 3100 Fig. 93 Setting out the length plus overhang ©TAFE NSW Construction and Transport Division

65


Forming bevels on the square Once the scaled measurements are set on the square, with the fence or buttons fixed in place, the plumb and level bevels will automatically be formed in the complimentary angles of the square. When the square is laid over the edge of a rafter these bevels may be easily transferred as the square slides along the rafter.

Plumb bevel

Level bevel Rafter

Fig. 94 Rafter bevels on the square

Setting out with a pattern rafter A straight length of timber is selected for the pattern rafter, it is set out by measurement or using the steel square. It is then cut to form a finished rafter with a plumb cut at the top, plumb cut at the bottom and the birdsmouth checked out. A short length of batten is then nailed directly above the plumb cut at the top and the birdsmouth at the bottom. The pattern rafter is now ready for use. Lay the rafters to be cut on top of a pair of saw stools with the spring uppermost. The pattern rafter is then laid over each rafter, making sure the top edges are hard up under the short batten, and then the plumb cuts and birdsmouth positions are transferred by marking with a pencil. Note: The purpose of positioning each rafter hard up against the short battens is to ensure all the top edges will be in-line to maintain a straight roof surface.

Although the plumb cut at the bottom of the rafte r may be marked and cut at this time, it is usually better to leave the ends and cut them to a string line once the roof frame is complete. After the first pair of rafters is marked and cut, they should be tried in place to ensure the length and bevels are correct before proceeding with the remainder. Stop batten

Rafter to be marked out

Stop batten

Pattern must be straightest rafter possible Keep round edge of rafter to top always Fig. 95 Positioning the pattern rafter for marking 66


CARPENTRY - HOUSING

Fascia cut (Pattern only)

Stop batten

Rafter Bird's mouth

Rafter

Stop batten Plumb cut

Fig. 96 Transferring pattern set out to the rafters

Alternative pattern method A modified version of the pattern rafter is preferred by some tradespersons to mark out the common rafters. It is called a ‘rafter boat’ , which is a short template having a plumb cut at both ends, a birdsmouth and a cleat on top to act as a guide when the boat slides along the top edge of the rafter. All the rafters are laid across saw stools and placed together on edge so the length and position of the birdsmouth can be marked. These positions are squared across the top edge of all the rafters. The boat is moved along the top edge of each rafter to align with these marks and then the plumb cuts and birdsmouth shapes are traced onto the rafter, ready to be cut. Note: When metal fascias are used the ends of the rafters don’t have to be exactly in-line, therefore they may be pre-cut, as the fascia brackets are adjusted to a string line before they are fixed. Plumb cut for fascia Timber guide

g a n h r e o v s v e E a Birds mouth

Plumb cut for ridge Eaves overhang

Centre line length of rafter

Mark plumb cut for ridge

Mark birdsmouth

Position 1

Mark plumb cut for fascia Position 2

Fig. 97 Using the rafter boat to set out rafters ©TAFE NSW Construction and Transport Division

67


CALCULATING FRAME QUANTITIES The following example outlines a method used to calculate framing member lengths, quantities and costs. Example 1: Calculate the roof frame members for the tiled, boxed gable roof shown below. The pitch is 1 : 1.732 or 30°. The member sizes and costs are as follows:

TABLE 2 MEMBER

SECTION SIZE

MATERIAL

SPACING

STRESS GRADE

COST

Common rafters

90 x 35

Radiata pine

600 c/c

F8

$2.10/m

Ridge

150 x 32

Oregon

-

F5

$4.00/m

Purlins

125 x 75

Oregon

1800 c/c

F8

$9.00/m

Collar ties

70 x 35

Radiata pine

1200 c/c

F8

$1.90/m

Scissor struts

250 x 50

Hardwood

2400 c/c

F14

$15.00/m

Gable studs

70 x 35

Radiata pine

600 c/c

F8

$1.90/m

Gable tie

90 x 35

Radiata pine

-

F8

$2.10/m

Soffit bearers

70 x 35

Radiata pine

600 c/c

F8

$1.90/m

Fascia and barges

200 x 25

Primed Radiata pine 1800 x Fibre cement 1200 x 4.5 sheets

Eaves soffit sheets

-

$7.00/m

-

$15.00/ sheet

0 0 3

4 0 0

0 0 4 8

5 4 0 0 4 0 0

0 0 3

Fig. 98 Typical boxed gable roof

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CARPENTRY - HOUSING

RAFTERS Order length of rafters Calculate the ordering length of rafters for a pitch of 1 : 1.732. STEP 1

STEP 2

Rise per metre run

Length of hypotenuse

=

1.000 1.732

= =

0.577m a² = b² + c²

a²

=

1.000² + 0.577²

a²

=

Therefore, a

=

√ 1.333

=

1.155m

1.0

+ 0.333

Therefore, for every 1.0m of run or half span the hypotenuse or rafter length will be 1.155m

STEP 3

To find the centre line length of the rafter, simply multiply the run or half span by the constant, 1.155:

Centre line rafter length

STEP 4

2.700 x 1.155

=

3.118m

To find the cutting length of the rafter, i.e. including eaves overhang, simply add the eaves width to the run or half span, deduct half the ridge thickness, then multiply the answer by the constant 1.155. The ridge is 24 mm thick:

Cutting length of rafter

STEP 5

=

=

[(2.700 + 0.400) - 24] x 1.155 2

=

3.088 x 1.155

=

3.567m

To find the ordering length it will be necessa ry to add 100 mm to the cutting length to allow for the plumb cut at the foot of the rafter:

Order length of rafter

=

3.567 + 100

=

3.667

∴ Total order length, to the nearest 300 mm increment, of all the rafters will be 3.9m


69


Quantity of rafters The formula for the total number of rafters will be:

Number = [ (total length of ridge + 1 ) x 2 sides] Rafter spacing STEP 1 Ridge length

STEP 2

Number of rafters

=

length of roof + verge overhang for both ends

=

8400 + 300 + 300

=

9000 mm or 9.0m

=

[ (total length of ridge + 1 ) x 2 sides] Rafter spacing

=

[ ( 9.000 + 1 ) x 2 sides] 0.600

=

[ (15 + 1 ) x 2 sides]

=

[ 16 x 2 sides]

=

32

Order = 90 x 35 Radiata pine F8 – 32/ 3.9

RIDGE The length of the ridge will be equal to the total roof length plus 300 mm joint length, for ridges over 5.4m long. Note: Lengths greater than 5.4 become difficult to handle, therefore it is common practice to join the ridge as previously described on page 13 of this unit.

STEP 1

Ridge length

=

length of roof + verge overhang for both ends + jointing 2

=

8400 + 300 + 300 + 300 2

=

9300 2

=

4650 mm or 4.650m

Order = 150 x 32 sawn Oregon F5 – 2/ 4.8

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CARPENTRY - HOUSING

PURLINS Purlins are spaced at 1800 mm c/c, therefore there will be only one row along each side. Each row will run the full length of the roof to the outside of the boxed gable ends. An allowance of 150 mm is added for every 5.4m of length for jointing.

STEP 1 Purlin length

STEP 2

Number of purlins

=

length of roof + verge overhang for both ends + jointing 2

=

8400 + 300 + 300 + 150 2

=

9150 mm

=

2/ total length 2

=

2/ 9150 2

=

2/ 4575 mm, say 2/ 4.800 per side

Order = 125 x 75 sawn Oregon F8 – 4/ 4.8 COLLAR TIES Collar ties are spaced every second pair of rafters, i.e. 1200 mm c/c, and it is assumed they will lie on top of the purlins at the centre of the roof rise. They will only be necessary for the roof frame within the length of the wall frames.

STEP 1 Collar tie

=

Width or span of the roof 2

=

5400 2

=

2700 mm or 2.7m

=

Length of wall frames + 1 spacing

=

8400 + 1 1200

=

7+1

=

8

length

STEP 2 Number of

collar ties

Order = 70 x 35 Radiata pine F8 – 8/ 2.7


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SCISSOR STRUTS The scissor struts will rest on the external wall plates, run under the purlins and cross over in the centre of the roof, where they will be bolted together. Allow the same length as the rafters, as they are set at a lower angle the additional length will be used for bolting. They are spaced, at maximum centres of 2400 mm, within the roof for the length of the wall frames only.

STEP 1

STEP 2

Length

Number of struts

=

Rafter length

=

3.9m each side

=

Length of walls +1 spacing

=

8400 +1 2400

=

3.5 + 1

=

4+1

=

5 per side

Order = 250 x 50 sawn hardwood F14 – 10/ 3.9 GABLE STUDS The length of each gable stud may be determined by reducing each one, in sequence, by multiplying the horizontal distance out from the f ascia line by the rise per metre run. The horizontal distance out is calculated by deducting the stud spacing for each stud.

7 7 5 h t g n e l n i n o i t c u d e r m r o f i n U

No 1

7 7 5

No 2

7 7 5

No 3 No 4

7 7 5 7 7 5

No 5

Equal stud spacing at 600 c/c Run of boxed gable rafter Fig. 99 Gable stud spacings

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CARPENTRY - HOUSING

When the rise per metre run = 1.000 = 0.577m 1.732 STUD No. 1 Distance out

=

Half span + eaves width

=

3.100

∴

Length of stud

=

3.100 x 0.577

=

1.789

STUD No. 2

Distance out

=

3.100 – 0.600

=

2.500

∴

Length of stud

=

2.500 x 0.577

=

1.443

STUD No. 3

Distance out

=

2.500 – 0.600

=

1.900

∴

Length of stud

=

1.900 x 0.577

=

1.096

STUD No. 4

Distance out

=

1.900 – 0.600

=

1.300

∴

Length of stud

=

1.300 x 0.577

=

0.750

STUD No. 5

Distance out

=

1.300 – 0.600

=

0.700

∴

Length of stud

=

0.700 x 0.577

=

0.404

Note: Use an off cut to make up the last short stud position in this case.

Add the total length of all the shortened studs together for each boxed gable end, i.e. No. 2 to No. 5, and then add the centre stud, i.e. No. 1.

Total length for each side of = gable end =

1.443 + 1.096 + 0.750 + 0.404

Total length for centre studs =

1.789 x 2

=

3.693m, allow 4/ 3.9m 3.578m, allow 1/ 3.6m

Order = 70 x 35 Radiata pine F8 – 4/ 3.9, 1/ 3.6

GABLE TIES These will be equal to the full span plus two times the eaves width, for each end.

Total length of tie

=

5.400 + (400 x 2)

=

6.200, allow 2/ 6.3m

Order = 90 x 35 Radiata pine F8 – 2/ 6.3 ©TAFE NSW Construction and Transport Division

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SOFFIT BEARERS Divide the length of each side by the spacing of the bearers. Treat the overhang of the boxed gables separately from the sides, as they have a smaller eaves width. The formula for the total number of soffit bearers for the sides will be:

Number = [( length of side ) + 1] x 2 sides bearer spacing =

[(8.400 )+ 1] x 2 0.600

=

[14 +1] x 2

=

15 x 2

=

30

∴30 x 0.500 = 15.0m, say 3/ 5.1 (allow 0.650 for brick veneer construction) The formula for the total number of soffit bearers for the boxed gables will be:

Number = [( length of gable end) + 1] x 2 ends bearer spacing =

[( 5.400 + 1] x 2 ends 0.600

=

[9 + 1] x 2

=

10 x 2

=

20

∴20 x 0.400 = 8.0m, say 2/ 4.2 (allow 0.550 for brick veneer construction)

Order = 70 x 35 Radiata pine F8 – 3/ 5.1, 2/ 4.2

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CARPENTRY - HOUSING

FASCIAS The fascia will run the full length of the roof, equal to the ridge length.

Fascia length

= Length of wall + gable end overhang + jointing

Fascia length

=

8.400 + (300 x 2) + 0.100

=

8.400 + 0.600 + 0.100

=

9.1m, allow 1/ 4.8, 1/ 4.5m per side

Order = 200 x 25 Primed Radiata pine – 2/ 4.8, 2/ 4.5

BARGES The barges will be the same length as the common rafters plus an additional 300 mm to allow for the thickness of the fascia, the plumb cut at the bottom end and the extended section past the end of the gutter. Allow for two lengths per gable end.

Barge length

=

Cutting length of rafter + 300 mm

=

3.569m + 0.300

=

3.869m, allow 3.9m

Order = 200 x 25 Primed Radiata pine – 4/ 3.9

EAVES SOFFIT SHEETS The eaves soffit sheet strips will be cut from full 1800 x 1200 mm wide sheets. Allow the gable end sheets to run the full width of the boxed gable and butt the sides into them. Note: The joint between the external wall cladding or brickwork and the soffit sheets will be covered with a 25 mm quad, or similar, during the Exterior cladding/ finishing stage.

Number of sheets for sides = (length of side ) ÷ 3 length of sheet Number of soffit strips per side

=

8.400 1.800

=

4.666

=

Say 5 strips


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Number of sheets (both sides)

=

(5 x 2) ÷ 3

=

10 ÷ 3

=

3.333, allow 4 sheets

Number of sheets for gable ends = (length of end ) ÷ 4 length of sheet Number of soffit strips per end

Number of sheets (both ends)

=

5.400 + (0.400 x 2) 1.800

=

5.400 + 0.800 1.800

=

6.200 1.800

=

3.444

=

Say 4 strips

=

(4 x 2) ÷ 4

=

8÷4

=

Allow 2 sheets

Order = 1800 x 1200 x 4.5 mm Fibre cement sheets – 6 off

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CARPENTRY - HOUSING


77


GLOSSARY OF TERMS Adjacent -Means it is placed next to or found beside something. Apex -This is the very top or point of something, like the apex of a roof, meaning where members come together at a c ommon top position. Bisect -This means to cut, separate or divide something exactly in half, such as when a 90° angle is bisected it becomes two 45° angles. Circa -This means around, approximately, round about, etc. Usually refers to dates when estimating the age of a building or structure such as circa 1854, also written as (c 1854). Cluster -This is a term, which refers to a group or gathering of a number of members in a frame, such as a roof cluster, which consists of the end of the ridge, two centering rafters, a crown end rafter and two hip rafters. Complimentary -In this case it means any two angles, which make up a right angle. Constant -In this case it means a number, quantity or amount, which is used as the basis for several calculations. For example the length of 1.414m is a constant, which may be used to calculate the 45° hypotenuse length of a right-angled triangle, once the length of one side is known. Corbelled -Refers to stepped out brickwork used to support other members, such as corbelled eaves. Dihedral -This is the angle formed between any two surfaces where they meet along a common length, such as a ridge in a roof. A dihedral angle is formed between the underside of the two roof surfaces or where a roof surface meets a parapet wall, etc. F.C. -This is an abbreviation for fibre cement, similar to the product ‘Hardiflex’ . Hypotenuse -This is the angled side of a right-angled triangle. Inclined -Means to be at an angle to something, such as an inclined strut or brace. In-situ -This is an abbreviated version meaning in situation or position, such as pouring concrete in-situ, meaning to pour in place into the formwork. In-to-over -This is a method used for marking the spacings of members. It literally means marking from the inside face or edge of one member to the outside face or edge of the next member. It is equal to working centre-tocentre but is more practical for fixing purposes as it allows one edge to be lined up with the mark, so it is easily seen, ready for fixing. Lined on-the-rake -This is a term used to describe the ceiling lining of a pitched roof, which is fixed to the underside of the rafters. There is no access to the roof structure as there is no roof space formed. Parapet -This is a vertical wall or gable, which extends past the line of the roof to enclose the roof from view. The parapet is usually constructed of brickwork or timber framing and clad with sheet material. Patent -This is a term used to describe a product which has had its design registered with the Patents office. It is the original idea of a person or persons, which cannot be copied without consent. Primed -This is a protective white or pink paint coating applied to timber before it is fixed into place. It seals the timber and provides a surface ready to take undercoat paint prior to the finishing coats. It helps to prevent timber decay from occurring.

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CARPENTRY - HOUSING

Scribe -This is where the shape of one piece is to be fitted to the surface shape of another. It may also refer to anything placed in-situ and marked to suit the final resting position of that member, such as scribing a hip in position. Slat -Refers to a thin narrow piece of timber or several thin arrow pieces put together to make something else, such as slats of timber used to make a sheet of lattice. Soffit -This is the horizontal under face of a structure or lining. The eaves soffit is the underside face of the eaves sheeting. Trapezoid -This is an irregular quadrilateral with only two parallel sides. The shape may be found on the side roof surface of a hip roof made up of the fascia, ridge and two hips.


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Carpentry Notes on Basic Roof & Ceiling Framing

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