These types of roofs have special requirements. Unlike steep-sloped roofs, which rely on gravity to shed water, flat roofs depend on drains and rely on a waterproof membrane to protect the property. Actually, the term ‘flat’ is misleading here — no roof should be completely flat or you’d find yourself living under a small lake. “Low slope” is a more accurate description of what’s commonly called a flat roof. Low slope roofs should have at least a 1/4 inch of slope per foot (preferably more) so that water can flow downhill to drains and then to the ground.
Built-up roofing (aka BUR) is what many people call “hot tar roofing.” A century old, time-tested roofing method, BURs are made of three or four overlapping layers of asphalt felts fused together with hot asphalt.
Built up roofs must be protected from weather, sunlight and foot traffic by some kind of surfacing material such as gravel, a mineral granule covered top layer (a cap sheet), a smooth coat of hot asphalt or a special reflective coating.
Modified bitumen’s, like built-up roofing, are made of reinforced asphalt impregnated felts. However, unlike BURs, the asphalt in the felts is modified with plastic or rubber polymers (hence the name) to improve its elasticity, durability and overall performance. Because of the added polymers, modified bitumen is often applied in a single layer with overlapping seams. They can be “glued” to the roof with hot asphalt or an adhesive, or by using propane torches to melt the asphalt underside. Some modified bitumen require protective surfacing materials, usually mineral granules or a liquid coating.
Single ply membranes are made of rubber, plastic or a hybrid of the two. As the name implies, single ply membranes are applied in a single layer. Installation is simple: the membrane is rolled out, the seams between sheets are heat welded, chemically welded or glued together, and the membrane is attached to the roof with fasteners (usually screws/flat plates), ballast (like rocks) or adhesive.