A roofer is a skilled trades person who installs a waterproof membrane on a building that has either a flat or sloped deck consisting of wood, steel or concrete. 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 that is applied by using a propane torch and melting the product over a flat surface. Most crews in the ICI roofing industry consist of five to eight workers with a foreman, two journeymen and the rest of the crew consisting of material handlers and apprentices. Roofers use many types of roofing equipment to perform their work, including overhead hoists, bobcats, asphalt kettles and conveyor belts. Strength, agility and a willingness to work in extreme weather conditions are the characteristics of the average roofer. Knowledge of the various types of roofing systems is also essential to a successful career in roofing.