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.
The Importance of Building it Right
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.