Building a Flat Roof Right

The Process:

1. Frame the Roof "The most important thing to know about a flat roof," says Tom, "is that it's not flat." To prevent water from pooling and eventually invading the home, flat roofs are always built on a slight incline—at least 1/8 inch per foot. Many slope in several directions, like squashed hip roofs, toward scupper holes that connect to downspouts.

On large roofs, Tom achieves this incline with a foam underlayment that he has a commercial roofing company make to fit the roof's specs. However, on a small roof, like this 8 ½-by-13-foot one, he saves money by ripping 2x4s into long wedges that he nails to the joists—which, to handle snow load, are larger and closer together than on sloped roofs—to create the incline.
(see photo 2)

2. Sheathe With Plywood
On top of the framing goes 5/8-inch plywood sheathing with a 1/8-inch gap at all the joints to allow for expansion and contraction.
(see photo 3)

3. Screw Down Underlayment
The sheet-rubber roofing material that Tom is using requires a substrate called iso board—½-inch-thick rigid foam (made of polyisocyanurate) with a special fiberglass backing. The iso board (a flat version of the same material he orders custom-fitted for larger roofs) cuts easily with a utility knife and anchors to the plywood sheathing with screws and large galvanized steel washers. It provides a soft, protective base for the rubber. Tom makes sure to stagger the joints and to fit the pieces tightly against each other, as iso board doesn't expand and contract like plywood.
(see photo 4)

4. Create Beveled Corners
On two sides, the roof abuts a parapet (a short wall common around flat roofs), as well as a wall for the third floor; here the rubber roof will need to run up the walls and glue to the sheathing. To keep water from pooling at these inside corners, Tom rips 2x4s lengthwise on a 45-degree bevel with a table saw. Then he screws them into the joint between the roof and the wall to create a gradual transition.
(see photo 5)

5. Dry-Fit The Roofing
Before putting down the rubber, Tom does a quick sweep-down to remove any debris that could puncture the new roof. EPDM (which stands for ethylene propylene diene monomer) comes in 10-foot-wide rolls and cuts easily with shears or a utility knife. Tom cuts a piece large enough to cover the roof plus an extra 9 inches all around. He takes the piece and spreads it into place then folds it back in half. (This roof is small enough to require only a single sheet. On larger roofs, Tom overlaps sections about 6 inches, but doesn't glue the seams until the very end.) To negotiate a vent stack, Tom would cut a hole in the rubber slightly larger than the pipe, then slide the rubber down over it.
(see photo 6)

6. Glue Down the Rubber
Using a ¼-inch-nap paint roller on an extension pole, Tom spreads the glue over the exposed section of iso board and the corresponding folded-over rubber. Like ordinary contact cement, the glue goes on both surfaces and bonds instantly the moment the two meet. So after letting it dry to the touch, he gets down on his knees and carefully spreads the rubber over the iso board, pushing as much from the inside of the fold as he can to prevent wrinkles. He doesn't pick up the rubber or it will stretch from its own weight, then spring back, causing it to lay down unevenly. "You only get one shot," says Tom. Working in sections, he finishes the rest of the roof, smoothing out air bubbles with the now-dry roller before gluing the edges up the wall and parapet and 6 inches down over the front of the roof. Had there been a seam to contend with, Tom would first clean it with a special solvent, then apply a black rubber-to-rubber adhesive. Because seams are so vulnerable, he would also glue a 12-inch-wide strip of uncured rubber (which, unlike EPDM, has no "memory," meaning it can be stretched without springing back and wrinkling) onto the seam itself. At vent stacks, he would stretch a special rubber collar over the pipe and glue it to the EPDM, then seal it with glued strips of uncured rubber and a bead of tri-polymer caulk.
(see photo 7)

7. Finish the Edges
At inside vertical corners, Tom cuts and overlaps the EPDM, then uses rubber adhesive to glue it down tightly. He also seals this seam with uncured rubber. To keep the vertical sections from peeling off the walls, Tom screws on metal brackets called termination stops, then cuts off the excess above the brackets. He runs a bead of tri-polymer caulk along the top edge of the brackets to seal them. Later he will nail down a custom lead-coated copper flashing over the front edges of the roof, then glue down a 12-inch strip of uncured rubber over the flashing's top nail edge. The off-the-rack alternative to custom flashing is a galvanized drip edge specially made for flat roofs. Called a gravel stop, it works on any kind of flat roof and comes in many colors. Once the flashing is in place, the roof is ready to face the elements.
(see photo 8)

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