Steps // Building a Flat Roof Right
1 ×

The Importance of Building it Right

 
Step One // Building a Flat Roof Right

The Importance of Building it Right

sheathing the flat roof at the Cambridge TV project 2005
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.

 
2 ×

Frame the Roof

 
Step Two // Building a Flat Roof Right

Frame the Roof

Tom Silva building a flat roof at the Cambridge TV project 2005, cutting framing to nail to the joists
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.

 
3 ×

Sheathe With Plywood

 
Step Three // Building a Flat Roof Right

Sheathe With Plywood

sheathing the flat roof at the Cambridge TV project 2005
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.

 
4 ×

Screw Down Underlayment

 
Step Four // Building a Flat Roof Right

Screw Down Underlayment

Tom Silva building a flat roof at the Cambridge TV project 2005, screwing down the 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.

 
5 ×

Create Beveled Corners

 
Step Five // Building a Flat Roof Right

Create Beveled Corners

Tom Silva building a flat roof at the Cambridge TV project 2005, creating the 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.

 
6 ×

Dry-Fit The Roofing

 
Step Six // Building a Flat Roof Right

Dry-Fit The Roofing

Tom Silva building a flat roof at the Cambridge TV project 2005, dry-fitting the EPDM layer
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.

 
7 ×

Glue Down the Rubber

 
Step Seven // Building a Flat Roof Right

Glue Down the Rubber

Tom Silva building a flat roof at the Cambridge TV project 2005, gluing down the EPDM layer
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.

 
8 ×

Finish the Edges

 
Step Eight // Building a Flat Roof Right

Finish the Edges

finished beveled edges on the flat roof at the Cambridge TV project 2005
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.

 
 

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