Flat roofs are fairly common where older row homes exist, like Washington D.C., Alexandria and Arlington and are a completely different animal than a sloped or pitched roof. The definition of a “flat roof” is any roof without sufficient slope to “shed” water, usually around 4/12 or less. The most popular of these materials in years past has been modified bitumen and build up, but much better options are gaining “ground” such as, TPO )Thermoplastic Polyolefin, and EPDM (rubber membranes).
Let’s go ahead and get the antiquated out of the way. Build up style roofs are all, but a thing of the past. The kettles required to keep the asphalt boiling are extremely expensive and insurance companies are very against use on a residential home (fires too common). Thus, the only places asking for them these days are school boards and other special groups.
Modified Bitumen has been a popular option for residential use because the materials are comparatively cheap and it doesn’t require as much skill to install. It’s a system composed of multiple layers in the case of self adhering and usually a singular layer in the case of torch down (again, fire danger in residential use…). The outer layer looks a lot like a shingle with the granuals to protect the asphalt and fiberglass from the UV rays, but that’s about where the similarities end. The detail work has to be very precise and corners become a work of art. You can usually expect to pay less than for the single ply membrane options, but the lifespan of a modified roof is usually about half of the competitors at 15-20 years.
EPDM or rubber membrane roofing was extremely popular over the last couple decades, especially on commercial buildings. The flexible rubber stands up to the sun much longer and is easy to spot problems to make repairs. These sheets are held together by special glue. Like modified roofs, the details are extremely important. Unlike the aforementioned, EPDM can actually hold ponding water. This isn’t ideal by any means and shouldn’t be any more than a half inch or so, but this is common on older roofs. Any good installer will try to persuade an owner into using tapered insulation board to create more slope and therefore, stop the ponding. One drawback with EPDM is that it’s black and attracts the maximum amount of sunlight. These roof decks can easily get to 180-190 degrees in the dead of summer, which can be very costly if you’re insulation isn’t quite up to par. These roofs generally last between 30-50 years if installed correctly.
TPO has somewhat emerged as the go-to roofing option of the large industrial buildings and commercial properties. Since more people are now learning how to install it, the material cost is weighing out the saved cost in labor so that it can compete directly price wise to EPDM in a residential use. This white reflective membrane is actually heat welded at the seams to for a “seamless” membrane. The huge benefit here is the ability to reflect most of the rays coming from the sun. Deck temps on a normal TPO roof are in the 110 degree range in the summer vs the 180 with EPDM. This helps cities have less “city effect” heating. It also saves utilities since the building can overall stay cooler. Like EPDM, minimal ponding is acceptable according to manufacturer specifications. The lastability has yet to really be tested as TPO hasn’t been around long enough to really have a lot of data. We can say that it beats EPDM in just about every engineering standard test, so the hopes are high that it can outlast the competition.