What is a Flat 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 flat roofing materials in years past have been modified bitumen and build up, but much better options are gaining ground, such as TPO (thermoplastic polyolefin) and EPDM (rubber membranes).
Build up roofs have become outdated as new options have been surfacing and are proven to be very dangerous. The kettles required to keep the asphalt boiling are expensive, and insurance companies often frown on their use on residential homes since build up roofs have been known to be flammable. Typically, the only groups asking for them these days are school boards and other institutional and commercial parties.
Modified bitumen has been a popular option for residential use because the materials are comparatively cheap. It’s a system composed of multiple layers (in the case of self-adhering modified bitumen) or usually a single layer in the case of torch down (again, fire danger in residential use).
The outer layer looks much like a shingle with granules that 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 competitor’s at 15-20 years.
EPDM or rubber membrane roofing has been very popular over the last couple decades, especially on commercial buildings. The flexible rubber stands up to the sun much longer and makes it easy to spot problems to make repairs. These sheets are held together by a special adhesive.
Like modified roofs, the details are extremely important when it comes to EPDM. EPDM can actually hold ponding water, which isn’t ideal by any means and the depth of the ponded water shouldn’t be any more than a half inch or so, but the problem is nevertheless common on older roofs. It is common for installers to try to convince property owners to use tapered insulation board to create more slope and therefore stop the ponding.
One drawback with EPDM is that it’s black, which makes it attract 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 your 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 for large industrial buildings and commercial properties. Since more people are now learning how to install it, the material cost is weighing out the cost saved in labor so that it can compete directly price-wise to EPDM in 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 versus 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 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.