A roofer is a skilled tradesperson who installs a waterproof membrane on a building that has either a flat or sloped deck consisting of wood, steel or concrete. They may specialize in residential or industrial roofing. The most commonly used roofing product is a layer of asphalt paper embedded in hot asphalt up to four times over a layer or layers of rigid board insulation, which in turn is covered with a ballast called pea gravel that is also embedded in hot asphalt. It is also common to use a single-ply rubber membrane that is laid over rigid board insulation or to use rolls of modified bitumen paper applied by using a propane torch and melting the product over a flat surface.