Plywood • • • • • • •
Introduction Uses Grades Sizes Quality Control Moisture Control Additional Information
Introduction Plywood is a panel product consisting of thin wood veneers (plies) glued together so that the grain direction of each layer of veneer is perpendicular to that of the adjacent layers. This cross-lamination provides excellent two-way strength and stiffness properties, resistance to impact damage, and dimensional stability when wet. Many kinds of plywood are manufactured in Canada. Two specie types (available in several grades) are assigned specified engineering strength values under the Canadian standard CS A O86-01 Engineering Design in Wood: unsanded Douglas Fir Plywood (DFP) and unsanded Canadian Softwood Plywood (CSP). Other common grades of DFP and CSP include sanded grades which are used primarily in concrete formwork or non-structural applications and Cleaned and Sized grades which are mainly utilized in floor underlayment applications where a smooth and solid surface is required. Plywood can also be manufactured from Poplar, including both unsanded and cleaned and sized type grades. During the manufacture of softwood plywood, veneers are united under high temperature and pressure with a phenol-formaldehyde resin glue that is completely waterproof, making the plywood suitable for use under conditions of extreme exposure to moisture. For a more in depth description of plywood see about CANPLY plywood.
Uses Floor Sheathing and Underlayment Plywood provides a superior floor beneath resilient flooring and carpeting because it is dimensionally stable. It remains flat to give a smooth, uniform surface that does not crack, cup or twist. T&G plywood panels reduce labour and m aterial costs by eliminating the need for blocking at panel edges. Select Tight Face grade is particularly suitable for use under resilient finish flooring. If Select grade is used, splits and other minor permitted characteristics in the grade should b e filled with a material which, on setting, will provide a hard, firm surface. Sheathing grade is suitable for subflooring, beneath such applications as hardwood flooring or concrete topped floors.
Wal l Sheathing Plywood panels used as wall sheathing provides excellent resistance to lateral forces such as wind and earthquake loads. Also, damage to finish materials in the structure due to uneven settling of the frame is minimized due to the extra rigidity plywood adds to the building. Plywood sheathing contributes to the over-all thermal performance of the wall because the large pan els have fewer joints through which heat can escape. A combined sheathing and siding system using one plywood layer as both a structural sheathing and a finish cladding is a sound and economical method of wall construction in which only plywood is applied over the studs. Specialty textured plywoods are manufactur ed to provide a good appearance for such applications.
Roof Sheathing Plywood is often used for roof sheathing. The panel size combines speed of erection with lightweight portability. The stiffness of the panels constitutes diaphragm action when prescribed framing and nailing patterns are used. Panels should be applied with the face grain at right angles to the supports to provide maximum strength. A 3mm gap should be left between panels to allow for expansion. H-clips, used only for roof sheathing, are an alternative to solid blocking. In conjunction with 9.5mm (3/8") plywood roof sheathing, spanning rafters placed 600mm (2') apart, H-clips are subject to the following conditions:
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One H-clip shall be placed midway in the joint of abutting sheets between each pair of rafters or joists. Rafter or joist spacing shall not exceed 600mm (2'). H-clips must fit snugly. Abutting plywood sheets shall be fitted as closely as clips permit. Occasional misfit of abutting sheets may be tolerated providing gaps at maximum opening do not exceed 6mm (1/4").
As an alternative to using H-clips, specialtly tongue and grooved plywood for roofing applications provide faster installation and better structural integrity. The T&G lock together to form a better rain barrier from rain during construction.
Plyw ood Grades Types of Canadian plywoods commonly used in construction/industrial applications are Douglas Fir Plywood (DFP), Canadian Softwood Plywood (CSP), Poplar Plywood (Poplar). The two plywoods are named for the species of wood used for the front and back faces (the outer layers). DFP is produced to the manufacturing standard CSA 0121-M1978 Douglas Fir Plywood where front and back faces are, for most grades, Douglas Fir. Plywood containing other selected Canadian softwood species is labeled CSP and is manufactured to CSA 0151-M1978 Canadian Softwood Plywood. These standards specify minimum requirements for sizes, grades, specialty panels, manufacturing tolerances and glue bond quality. Both Douglas Fir and Canadian Softwood Plywoods are manufactured in several grades. The grades are dependent upon the appearance and the quality of the veneers used for the outer plies. The three qualities of veneer are designated by the letter A (best appearance), B, and C (the lowest appearance grade). The manufacturer, using these veneer grades in various combinations, can produce panels suitable for a variety of uses as shown in Table 1. Sheathing grades which are not specified for appearance usually carry the grade stamp on one of the faces, and the grades such as Good Two Sides carry the stamp on the edge so that it does not mar appearance.
Table 1: Pl ywo od - Standard Grades
Grade
Governing Canadian Standard
Good Two Sides (G2S)
CSA O121 (DFP)
Sanded
Poplar
Good One Side (G1S)
Individual Veneer Grades Face
Inner Plies
Back
Characteristics
Typical Applications Used where appearances of both sides is important. Furniture, cabinet doors, partitions, shelving, and concrete formwork.
A
C
A
Sanded. Best appearance both faces. May contain neat wood patches, inlays or synthetic patching material.
CSA O121 (DFP)
A
C
C
Sanded. Best appearance one Used where appearance on one side only. May contain neat side is important. Furniture, wood patches, inlays or cabinet doors, partitions, synthetic patching material. shelving, and concrete formwork.
SelectTight Face (SEL TF)
CSA O121 (DFP) or CSA O151 (CSP)
B+
C
C
Unsanded. Permissible face openings filled. May be Cleaned and Sized (C&S).
Underlayment, combined subfloor and underlayment, sheathing, and hoarding
Select (SEL)
CSA O121 (DFP) or CSA O151 (CSP) or Poplar
B
C
C
Unsanded. Uniform surface with minor open splits. May be Cleaned and Sized (C&S).
Underlayment, combined subfloor and underlayment, sheathing, hoarding and packaging.
Sheathing (SHG)
CSA O121 (DFP) or CSA O151 (CSP)
C
C
C
Unsanded. Face may contain limited size knots and other defects.
Roof, wall, and floor sheathing.
Medium Density Overlay (MDO)
CSA O121 (DFP) or CSA O151 (CSP) or Poplar
Smooth, resin-fibre overlaid surface. Best paint base.
Siding, soffits, paneling, builtin fitments, signs, or any use requiring a superior paint surface.
MDO 1 Side
C1
C
C
MDO 2 Side
C
1
C
C
1
Notes: 1. 2. 3.
Permissible openings filled with wood patches or putty. All grades are bonded with waterproof phenolic glue. Veneer grades: A: highest appearance grade; B: medium appearance grade; and C: low appearance grade.
Veneer Grade Characteristics Grade A
For further information Go to the CANPLY Website.
Grade B
Grade C
Sizes Canadian plywood is manufactured in several sizes and thicknesses as shown in Table 2. Table 2: Plywood Thicknesses and P anel Sizes Thicknesses Unsanded
Thicknesses Sanded
Select Tight Face, Select and Sheathing
Good Two Sides and Good One Side
Metric (mm)
Approximated Imperial ( in.)
Metric (mm)
Approximated Imperial (in.)
7.5
9/32
6
1/4
9.5
3/8
8
11/32
12.5
1/2
11
7/16
15.5
5/8
14
9/16
18.5
23/32
17
21/32
20.5
25/32
19
3/4
22.5
7/8
21
13/16
25.5
1
24
15/16
28.5
1-3/32
27
1-1/16
31.5
1-7/32
30
1-3/16
Panel sizes mm
in .
ft .
1220 x 2440 48 x 96
4x8
1220 x 2740 48 x 108
4x9
1220 x 3050 48 x 120
4 x 10
1220 x 2400 47.2 x 94.5 1220 x 2500 49.2 x 98.4 Notes: 1.
2.
3.
Canadian plywood has been made in metric thi cknesses since 1978. However, some, but not all thicknesses still approximate Imperial sizes: 6mm (1/4"), 9.5mm (3/8"), 12.5mm (1/2"), 15.5mm (5/8"), 19mm (3/4"), and 25.5mm (1"). Tongue and groove plywood is usually available in Sheathing, Select, and Select Tight Face Grades in thicknesses of 12.5mm and greater. Other thicknesses are available on special order.
Plywood is most commonly available in sheets 1220mm (4') wide by 2440mm (8') long. It is also available in a metric size of 1200 x 2400mm. Other sizes are manufactured to fill custom orders. Thicknesses of sheathing panels, although stated in metric terms, remain similar to those Imperial thicknesses established through experience and traditional usage. The thicknesses for sanded (Good Two Sides and Good One Side grades) and unsanded (Select Ti ght Face, Select, and Sheathing) grades for DFP and CSP plywoods are given in Table 2.
Quality Control Each plywood plant has resident quality control personnel to help ensure that the finished panel will meet the requirements stipulated in the manufacturing standard . Veneer thickness, moisture control for veneer drying, panel grading, and glue bonding are examples of stages of manufacture that are continuously monitored in quality control. * Indicates Species Designation CSP, DFP or POPLAR **Indicates the CSA standard governing manufacture: CSA O151M, CSA O121M or CSA O153. In Canada, approximately 80% of all construction/industrial grade plywood is manufactured under a quality system that i ncludes auditing and control of in-plant procedures by the Canadian Plywood Association (CANPLY). These products are typically identified by a CANPLY certification surface mark. This mark signifies that the plywood has been subject to an accredited third party quality assurance program, which entails continual monitoring of inner ply grade, surface ply grade, panel thickness, panel dimensions, panel marking, workmanship, and glue bond strength to ensure that the plywood will meet or exceed the requirements of the applicable manufacturing standard Other Canadian softwood/hardwood plywood plants may also employ voluntary quality auditing by independent certification agencies to ensure conformance to applicable requirements.
Moisture Control Plywood, like any other panel product, requires careful handling and storage. Despite its sturdy cross-laminated construction, face veneers, panel edges and panel corners are vulnerable to damage and should always be protected. Plywood is manufactured at a low moisture content and while small changes in moisture content will not appreciably affect its dimensions, large changes should be avoided since they may encourage checking of the face veneer with consequent impairment in its qualities as a paint base. It is good practice to store plywood, which is to be used for interior finish, under conditions that approximate those expected in service. When handling or storing plywood, practice the following: • • •
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Store plywood panels flat and level. Keep finish faced inward and cover stacks to protect from bumping and abrasion. Protect panel edges and corners. This is especially important with tongue and groove plywood. Carry panels on edge (always being careful not to damage faces, edges and corners). Protect panels from water.
General Guidelines •
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Indicate the type of plywood required. The specifications covering manufacture are: CSA 0121-M1978 Douglas Fir Plywood, CSA 0151-M1980 Canadian Softwood Plywood and CSA 0153-M1978 Poplar Plywood. Indicate the grade or grades acceptable and the minimum thickness. Indicate the support spacing and direction o f face grain orientation with relation to supports. Indicate the type, size and spacing of fasteners. Check the product for a quality certification grade mark as an assurance that the plywood conforms to a CSA specification. Plywood panels should be protected from rain and snow before and after installation to prevent an increase in moisture content.
Fire Safety Plywood products are used structurally in fire rated floor, wall, and roof assemblies. Plywood panels may also be used for fire stops in concealed spaces such as attics. The Fire Safety Design in Buildings book provides information on the uses of plywood in f ire-rated assemblies for all types of buildings.
Links • • • • •
Plywood MSDS sheet CANPLY tech notes Plywood for concrete forms Plywood manufacturing Technical Plywood publications CANPLY publications o CWC publications o
