A flat roof is a horizontal or nearly horizontal roof that covers a building or structure. As opposed to sloped roofs, flat roofs let water flow off using a very slight inclination rather than a distinct slope. If a flat roof was totally flat it would result in damaging water build-up, which is also 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, so walking on flat roofs is ill-advised.
Single-Ply Flat Roofs
Single-ply flat roofs use a single layer of plastic or rubber compound to create a roof. Common single-ply flat roof materials include thermoplastic polyolefin and polyvinyl chloride, both of which reflect heat well, which in turn keeps cooling costs down. Ethylene propylene diene monomer, or EPDM, is also often used for single-ply flat roof construction because it is easy to install and also reflects heat well.
Single-ply roofing can be self-adhered, applied with hot asphalt, or installed by heating with a torch, depending on the material used.
Multiple-Ply Flat Roofs
Multiple-ply roofs, otherwise known as built-up roofs (BUR), are made by overlapping felt rolls or mats separated by layers of asphalt. This layering is then covered with a granular mineral surface, tiling or other ballast to protect the layers from weather-related damage.
Roll Asphalt Flat Roofs
Roll asphalt flat roofs are made from material that is rolled onto a roof over felt and then nailed down or covered in cold asphalt cement. Like multiple-ply flat roofs, roll asphalt flat roofs are then often covered in granular mineral surface much like gravel. Fibreglass felt saturated in asphalt is also used as the base felt material. Roll asphalt flat roofs are the cheapest to construct, but tend not to last as long as other flat roof construction methods.
Flat-Seamed Metal Flat Roofs
Flat-seamed metal roofs are constructed from tile- or shingle-like sheets of metal that are then soldered together. These sheets of metal are usually made from copper or steel and are lightweight and heat-reflecting. However, flat-seamed metal roofs are the costlier than other flat roofs to construct.
Modified Bitumen Flat Roofs
Modified bitumen flat roofs are similar to multiple-ply flat roofs; however, they are made with modified bituminous membranes, which are membranes made from a mix of asphalt and a polymer. This combination of materials causes the asphalt to take on polymer-like characteristics, making it more similar to a single-ply roof in substance. Modified bitumen roofs are also covered in mineral or gravel, or sometimes a liquid coating for added light reflectivity.