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Building a Flat Roof Right

Here's how the pros construct one so it stays tight and tight

Flat roofs aren't architecturally logical, as rain and snow will shed much more quickly off a sloped roof. Yet for historic Modern-style buildings like This Old House’s Cambridge TV project, flat roofs are at the core of the architecture, meant to reflect the broad horizontal lines of the natural landscape.

Of course, you don't need to live in a modern house to deal with a flat roof. Traditional homes often have sections of flat roof—over shed dormers, above porches and garages, and on balconies. And their horizontal lines abound in the West and in urban areas.

But flat roofs take a pounding from harsh weather, which is why they rarely last as long as a good sloped roof. Fortunately, modern materials for covering flat roofs have improved considerably over the past two decades; some carry warranties of up to 20 years, approaching those for sloped roofing.

"If you build and flash one right—and keep it clean—a good flat roof won't leak," says TOH general contractor Tom Silva, who had seven different flat roofs to contend with on the Cambridge project house. He sealed them all with a glue-down EPDM rubber membrane system, a favorite of his because it's light, quick to install, and requires no special equipment.

It's so easy, in fact, that a handy homeowner could do it himself. So we watched Tom put down EPDM on a small section at the project house to learn how a pro makes a flat roof smooth, strong, and watertight.

Step 1

The Importance of Building it Right

Photo by Russell Kaye

Sheathing goes down quickly on a roof that isn’t sloped and thus provides some footing. But the horizontal lines of this 8 ½-by-13-foot flat roof section at the Cambridge TV project house, make it vulnerable to heavy snow and pooling rainwater, which is why it must be built to last.

Step 2

Frame the Roof

Photo by Russell Kaye

"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.

Step 3

Sheathe With Plywood

Photo by Russell Kaye

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.

Step 4

Screw Down Underlayment

Photo by Russell Kaye

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.

Step 5

Create Beveled Corners

Photo by Russell Kaye

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.

Step 6

Dry-Fit The Roofing

Photo by Russell Kaye

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.

Step 7

Glue Down the Rubber

Photo by Russell Kaye

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.

Step 8

Finish the Edges

Photo by Russell Kaye

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.