Installing a Low-Slope EPDM Roof
Done with care, a flat roof can provide 20 years of leak-free service.
Synopsis: Flat roofs are difficult to install properly, and as a result, many don’t last for anything close to the life span they’re capable of achieving. In this article, roofing specialist Dyami Plotke describes his process for getting the details right so that a low-slope EPDM roof can last for 20 years or longer of leak-free service. Plotke begins by making sure he’s working on a stable substrate, such as high-density gypsum board. He also lines up an assortment of appropriate sealants—urethane, water-block mastic, and lap caulk—and uses them in the correct locations. Seams between sheets of EPDM should be connected with a special 6-in.-wide seam tape; avoid the urge to use splice cement, which is seldom applied correctly and often fails prematurely. Pieces of edge metal, such as gravel stop and drip edge, are sealed in place. Also, sidewalls are a critical joint that requires counterflashing. If the roof has any pipe penetrations, they can be managed with boots, preformed pipe seals made from molded EPDM.
Editor’s note: If you want to learn how to build a deck on top of a flat roof, read the article Decking Over a Roof
Although we do a fair amount of residential roofing, my family’s bread-and-butter business is installing low-slope commercial roofing. A properly installed commercial roof can last for 20 or even 30 years. Unfortunately, many residential roofers are poorly trained in low-slope roofing techniques, so their flat roofs may last only half as long. When our company does residential low-slope roofing, we use commercial methods and materials, so our residential flat roofs last a long time with minimal maintenance.
The small sunroom on this 1930s stucco house is typical of what we find when we replace a residential flat roof. This old roof was hot-mopped asphalt, a once-reliable roofing system that is disappearing rapidly because of asphalt’s high price and diminishing quality. Nowadays for this type of roof, we use an ethylene propylene diene monomer (EPDM) membrane, which is the most UV-stable and therefore longest lasting low-slope roof.
Gypsum is a stable substrate
After the old roof is removed with a toothed roofing spade and any damaged areas of sheathing are patched with plywood, the roof deck is covered with 3/8-in.-thick, high-density (1000 psi) gypsum board. Gypsum doesn’t compress under drip edge and gravel stop like softer sheathings. It’s scored with a utility knife, then broken and fastened with 1-5/8-in. roofing screws and 3-in. plate washers. Longer screws (up to 8 in.) are available for thicker sheathing.
Spread the adhesive. EPDM is glued down using a special adhesive similar to contact cement. It’s rolled onto both the gypsum sheathing and the back of the EPDM membrane. When the solvents have flashed off (20 min. to 40 min. in warm weather) and the adhesive is tacky, the EPDM is laid in, and the surface is swept with a broom to ensure that it’s fully adhered.
Control expansion and contraction. Every roof membrane expands and contracts, which eventually causes it to pull away from the base of walls and other vertical transitions. This movement, called bridging, is prevented with a membrane-attachment strip. It’s made of 45-mil reinforced EPDM with a factory-applied tape and is secured every 12 in. with roofing screws driven through 2-in. plate washers.
Urethane sealant: The top edges of termination bars and metal flashing pieces are sealed with high-quality urethane sealant. Unlike other commercial roof sealants, urethane is available in many colors to match almost any siding or metal-flashing color.
Water-block mastic: In locations where water is likely to get beneath the EPDM membrane, non-curing, butyl-based water-block mastic prevents intrusion, but it works only in combination with pressure exerted by drip edge, gravel stop, or metal flashing.
Lap caulk: Cut edges of flashing tape and the line between edge metal and flashing should be wiped clean with weathered membrane cleaner and sealed with EPDM lap caulk. Lap caulk also should extend a few inches down any seams that intersect with flashing tape.
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