Built-up roofing is a generic term for a roof system that is built by adding layers. It is most often made from alternating layers of roofing felt along with other various roofing materials, including coal tar pitch, asphalt, cold application adhesive, and modified asphalt. It is also sometimes called a "tar and gravel" roof because it can be finished off with a final layer of gravel, crushed rock, or other mineral substance, in order to minimize sun damage.