The mid-century-modern house that the TOH team is tackling in Brookline has a flat overhanging roof, an architectural element typical of the style, but one that can be prone to dry rot caused by standing water on the roof.
Flat roofs have long been a signature challenge on This Old House projects. As the magazine has noted, flat roofs aren’t architecturally logical, since rain and snow shed much more quickly off a sloped roof. Yet for historic Modern-style buildings, flat roofs are at the core of the architecture, meant to reflect the broad horizontal lines of the natural landscape. Traditional homes also frequently have sections of flat roof—over shed dormers, above porches and garages, and on balconies. Fortunately, modern materials for covering flat roofs have improved considerably over the past two decades, giving homeowners far more durable options than earlier generations had.
For the renovation, the homeowners and their architect chose a low-maintenance rubber roof. This Old House general contractor Tom Silva and home builder Charlie Silva made sure the PVC roofing was installed properly at every step of the way—including a drip edge and PVC soffits and fascia—to minimize the chance of water ever causing a problem.
“The biggest obvious challenge from a waterproof point of view with that style of roof is the low pitch,” Tom explained. “With a traditional, steeply sloped roof, the pitch can be 6:12 or more, meaning the slope rises 6 inches for every 12 inches of run, which makes the water run off more effectively. At the Brookline house, we have a 1:12 pitch, just one inch of rise.” One upside of a flat roof is the fact that installing it doesn’t involve the usual dangling at risky angles.
For this installation, Tom and Charlie laid down ISO insulation board first, a lightweight, high R-value cellulose board with an isocyanurate foam core. Then they glued the PVC roofing material in place and installed a drip edge and snow guards.
Tom explained the critical principle behind any flat roof installation: “The most important thing to know about a flat roof 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 ⅛ inch per foot. At the Brookline project, the team ordered tapered foam insulation that’s thicker on one end than the other, so when it lays on top of the flat roof, it forces water to go to the edge, causing no puddles. As Tom described on site, “We want to basically take the water from around the roof hatch and pitch it away.”
