Re-Building A Flat Roof Right

Here's how the pros here at Boyd Construction Co Inc, in Washington, DC 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."-TOH Yet for historic Modern-style buildings such as those found throughout the Washington DC area, 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. 

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.

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.

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.

3. Screw Down Underlayment
Makes sure to stagger the joints and to fit the pieces tightly against each other.

4. Create Beveled Corners
To keep water from pooling at thes inside corners, rip 2x4s lengthwise on a 45-degree bevel with a table saw. Then screw them into the joint between the roof and the wall to create a gradual transition.

5. Dry-Fit The Roofing
Before putting down the rubber, do 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. Cut a piece large enough to cover the roof plus an extra 9 inches all around. Take the piece and spread it into place then folds it back in half. To negotiate a vent stack cut a hole in the rubber slightly larger than the pipe, then slide the rubber down over it.

6. Glue Down the Rubber
Using a ¼-inch-nap paint roller on an extension pole, spread 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, get down on his knees and carefully spread the rubber over the iso board, pushing as much from the inside of the fold as you can to prevent wrinkles. Finish 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. 

7. Finish the Edges
At inside vertical corners, cut and overlap the EPDM, then use rubber adhesive to glue it down tightly. To keep the vertical sections from peeling off the walls, screw on metal brackets called termination stops, then cuts off the excess above the brackets. Run a bead of tri-polymer caulk along the top edge of the brackets to seal them. Later 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. 

Remember to get your roof ready for the fall and winter season. 

Contact Boyd Construction Co Inc your flat roof repair and rebuild specialist in Washington DC area for over 81 years.  
Call (202)223-ROOF today for a free estimate!