Flat roofs can't be really flat. Any roof must have some slope, or pitch, to allow water to run off. Any roof that has a slope of less than 3 inches per foot is considered flat. Even with some pitch, though, flat roofs pose a challenge and need different coverings than pitched roofs. The basic coating options for a flat roof are built-up, membrane or liquid.
A built-up roof is composed of layers of some kind of waterproof membrane, once tar paper but now more likely a plastic material, alternated with hot tar or roofing compound and gravel. It is the cheapest type of flat roof, but the most difficult and messiest to install and difficult for a homeowner to put on himself. A built-up roof is also heavy and often requires extra bracing on the roof framing. In addition, it can be hard to find leaks in a built-up roof.
Membranes embrace several types of material, usually some type of plastic or rubber compound, installed in rolls or sheets. Styles include thermo-plastic olefin (TPO), applied in large rolls with seams fused with heat; single-ply polyvinyl chloride (PVC), similar to TPO in appearance and installation; ethylene propylene diene terpolymer (EPDM), a rubber material applied with special contact cement; and modified bitumen, a material similar to the components of built-up roofs but installed from rolls in layers.
Liquid coatings are typically used in reroof situations, with a liquid roofing coating applied over some existing flat roof material. A liquid coating system involves cleaning and patching all seams and any obvious tears or damage to the existing material. A bonding resin is embedded in a polyester fabric that is rolled over the entire roof. A black tar-like liquid compound also is used to resurface or repair some membrane roofs.
Warranties on flat roof coverings vary widely. Some liquid coating roofs are warranted for only five to 15 years, and they may be subject to building codes governing the number of layers of roofing that can be applied. Built-up roofs usually have warranties of 10 to 20 years but can last longer. EPDM is the easiest material for a homeowner to install, although its contact cement requires careful handling, and it is the longest-lasting, with some types carrying 30-year warranties. It also is the easiest to patch.