Flat roofs are nearly horizontal roofs that cover a building. Unlike sloped roofs, flat roofs allow water to flow off at a very slight inclination rather than a distinct slope. If a flat roof was totally flat, it would result in water build-up, otherwise known as ponding. Flat roofs are very sensitive to human use, and any small crack or puncture in the surface can quickly lead to substantial damage and leaks. Flat roofs also don't stand up to heavy snowfall well and are more common in warmer climates that don't receive much snow.
Single-ply flat roofs are made from a single layer of plastic or rubber compound. Standard single-ply flat roof materials include thermoplastic polyolefin, TMO and polyvinyl chloride (PVC), both of which reflect heat well and in turn keep cooling expenses low. Ethylene propylene diene monomer (EPDM) is also often used for single-ply flat roofing. Depending on the material used, single-ply roofing can self-adhere, apply with hot asphalt, or install by heating with a torch.
Multiple-ply roofs, or built-up roofs (BUR), are constructed from overlapping felt rolls or mats separated by layers of asphalt. These layers are then covered with a granular mineral surface like gravel, tiling or other ballast to protect the layers from weather-related damage and exposure.
Modified bitumen flat roofs are similar to multiple-ply roofs except they are made using modified bituminous membranes, which are membranes made from a mix of asphalt and a polymer, rather than straight asphalt. This combination of materials allows the asphalt to take on polymer-like characteristics, making it similar to a single-ply roof in substance. Modified bitumen roofs are covered in mineral or gravel and can also have a liquid coating for added light reflectivity.
Flat-seamed metal roofs are made from tile- or shingle-like sheets of metal that are soldered together. The sheets of metal are usually composed of copper or steel and are lightweight and heat reflecting, important properties for a flat roof construction material to possess. However, flat-seamed metal roof materials are costlier than other flat roofs; therefore, flat-seamed metal roofs are more expensive to install.
Roll asphalt flat roofs are made from rolling asphalt-saturated or fibreglass-based felt material onto a roof over an initial felt layer and nailing it down or covering it in cold asphalt cement. Like multiple-ply flat roofs, roll asphalt flat roofs are often covered in granular mineral surface, such as gravel. Roll asphalt flat roofs are the cheapest to construct but don't last as long as other flat roofing.