C ONCRETE I DEAS Have you come up with an inventi i nventive ve way way to pl ace concrete concrete more efficiently ffi ciently or to solve ol ve an annoying annoyi ng concreting problem? If so, please share your idea. Send information to: Ideas Editor, Concrete Concrete Constructi on, 426 S. Westgate Westgate St., Addi A ddison, son, I L 60101 (fax: 630.543.3112; 630.543.3112; e-mai e-maill: ccedi ccedito tor@
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A system for building sloped concrete roofs B Y M.K. HURD
n spite of the growing acceptance of insulating i nsulating concrete forms (ICFs (IC Fs)) for res residential identi al construction, concrete concrete generally stops at the roofline. Even the roofs of most ICF I CF h omes omes are are conventional convention al wood-frame wood-f rame construcconstruction. tion . Th e few few concrete roofs that have been built usually are flat and have little aesthetic appeal for most homebuyers. However, However, the th e res residential roofing roofi ng market may soon change due to the inge in genui nuity ty of architect architect Jose Henriquez and engineer Francisco Bermudez. They have developed a method method of forming and and building buil ding sloping roofs of reinforced concrete and have applied for a patent for their system, which uses commonly available construction materials and methods. Because both men live and work in South Florida, Flori da, they’re es especially pecially conscious of the need for stronger roof structures, anchored to resist the ravages of nature. In fact, the name of the th eir company, company, Enviro-Sa Envi ro-Safe fe Concrete Roofing System Inc., was selected to reflect their concerns with homeowner safe safety ty and envienvi ronmental issues such as resource and energy conservation.
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Roof construction construction The sloping loping roofs roofs are suita uitable for for homes having masonry or concrete walls. Forming for the roof begins with post or scaffold-type shoring, erected to support wood jois joi sts and and stringe trin gers like li ke those used in i n conventional convention al sussuspended-slab construction. Workers place the form form sheathi sheathing—4x8-foot ng—4x8-foot sheets of polyisocyanurate insulation 7 8 inche board, 2 ⁄ in ches s thick—directly thick—directly on the
Wood 2x2s placed on the roughly finished first layer of roof concrete (right) retained the concrete for the final layer, creating a tiered slope. The gleaming white finish (above) was achieved with two applications of an elastomeric coating. joists joists,, which which are are typically spaced 16 inches in ches on cent cente er. A network of metal or plastic channels placed along the edges of the insulation board projects above it to support the roof reinforcement. The 5 3 ⁄ 8-inch-deep steel studs used in commercial/residential framing framin g are suitable uitable for the channels.
Conventional shoring supports formwork joists and and stringers for the roof. Form sheathing is made of polyisocyanurate insulation panels that remain in place in the finished structure.
. c n I m e t s y S g n i f o o R e t e r c n o C e f a S o r i v n E y s e t r u o c s o t o h p l l A
3 Embedded ⁄ 4 inch into the roof slab after concreting, these channels retain the insulation and serve as attachment points for interior ceiling finishes. Workers can assemble the formwork on the ground in large sections then use a crane to lift the sections into place. Reinforcement projecting from the concrete or masonry walls is bent to the slope of the roof and lapped with the roof’s rebar to provide secure anchorage. A stiff concrete mix, specified at 3000 psi, is pumped into place as workers advance up the slope to vibrate and finish the concrete to the desired thickness. Polypropylene fibers have been used in the concrete for the roofs constructed thus far. A corrosion inhibitor also is added if the roof will be exposed to salt-laden ocean air, and Bermudez says they plan to experiment with fly ash in future roof mixes. Curing practices must meet local requirements, with a minimum curing time of seven days. When the concrete has gained the required strength, workers can remove the shoring, joists and strin gers, leaving the insulation in place. Roofs can be finished with an elastomeric coating, conventional roofing or clay tiles. The roof shown in the photos has a distinctive stepped concrete surface, primed and coated with a white elastomeric material. Interior ceiling finishes such as drywall can readily be attached to the channels along the edges of the insulation board.
Runner to hold insulation down
Shoring has been removed, exposing the insulation panels and metal studs, which are ready to receive the interior finish.
Folded-plate design Structurally these roofs are folded plates, so they require an engineering design. But Bermudez says the design is relatively simple to create using finite-element software. The roof should have a slope of at least 3 on 12, with a practical maximum of 6 on 12. This permits concrete placement without the use of top forms. Hipped roofs are ideal for this method of construction, but gable roofs can be constructed by adding a tie beam across the gable ends. With a concrete thickness of 5 or 6 inches, the folded plates can span up to 38 feet. The roof shown in the photos, which was recently constructed in Ft. Lauderdale, Fla., is 5 inches thick and has a maximum span of 32 feet. Rebar
Concrete
Reinforcement consists of #3 bars spaced 9 inches on center.
Cost of construction The initial cost of the sloping concrete roof can be significantly higher than the cost of the conventional wood-frame alternative—as much as $5 more per square foot in some areas. But for this price premium, the homeowner gets a fireproof, lifetime roof that, in combination with masonry or concrete walls, will withstand hurricanes and tornadoes of great i ntensity. Calculations indicate the roof will resist uplift in winds as strong as 400 mph, a real plus in South Florida where the hurricane season is an annual event. The construction method will also be available for license in other states. M.K. Hurd is an engineer and writer specializing in concrete building methods. She is a former editor of Concrete Construction and author of Formwork for Concrete published by the American Concrete Institute.
Insulation
Publication #C99A144 C-shaped metal or plastic channels
Intermediate support for insulation
Cross-section view of a finished roof. Metal or plastic channels holding the insulation panels project just high enough above the panels to act as supports for the reinforcing steel. Concrete and insulation thicknesses are designed to meet local requirements.
Copyright© 1999, The Aberdeen Group a division of Hanley-Wood, Inc. All rights reserved
