Flat roofs have absolutely no slope to them. The way you choose to cover your building's flat roof can have an enormous effect on how well it protects the contents inside from the elements outside. You can choose from several different types of material to cover your flat roof.
Asphalt-roll roofing, in use since the 1890s, consists of one layer of asphalt-saturated base felts made from either organic materials or fibreglass, applied over roofing felt. Nails and a cold asphalt cement hold the roof in place, along with a granular mineral surface at the top. Fill any seams in the roofing material with a roofing compound to prevent leaks.
Single-ply membrane flat roofing is comprised of an elastomeric membrane such as ethylene propylene diene monomer (EPDM), and polyvinyl chloride (PVC) sheets, as well as rubberised asphalt or modified bitumen. You can connect a single-ply membrane to a roof using a self-adhesive that comes with the material, or use hot asphalt for a more rugged bond.
Multiple-ply roofing, also called built-up roofing (BUR), consists of overlapping several rolls of coated felt or mats. Each mat is interspersed with layers of bitumen. Cover the surface of a multiple-ply flat roof with a granular roofing sheet, ballast or tile pavers made from crushed stone embedded in a coating of asphalt or coal tar. Multiple-ply roofs last between 20 and 30 years.
You can install a modified bitumen roof following the same method required for multiple-ply roofing (BUR) roofing; however, bitumen is a very different material than BUR roofing. The chemical composition of bitumen includes rubber and elastomers, making it more flexible as well as stronger than BUR roofing.
Flat-Seamed Metal Roofing
Flat-seamed metal roofing, in use since the 19th century, is made from flat sheets of metal soldered together so that the seams are flush. This type of flat roof can last several decades.