Most commercial, industrial, and apartment buildings have flat or slightly sloping roofs. Most houses have pitched roofs. Most flat roofs are covered with several layers of materials. Roofers first put a layer of insulation on the roof deck. Over the insulation, they then spread a coat of molten bitumen, a tar-like substance.
Roofers use a mop to spread hot bitumen over the surface and under the next layer.
This seals the seams and makes the surface watertight. The top layer is either glazed to make a smooth finish or has gravel embedded in the hot bitumen for a rough surface. An increasing number of flat roofs are covered with a single-ply membrane of waterproof rubber or thermoplastic compounds. Roofers roll these sheets over the roof's insulation and seal the seams. Adhesive, mechanical fasteners, or stone ballasts hold the sheets in place.