Most houses with flat or nearly-flat roofs are topped with built-up roofing, made from layers of asphalt-coated roofing felt.
The top layer of asphalt on these roofs is usually covered with crushed rock or gravel, which serves to hold the roofing material down and protect it from damage by ultraviolet sunlight. Light-colored gravel also reflects the sun's heat. (Such roofs are also known as tar-and-gravel roofs.) Built-up roofs can last from 10 to 20 years, depending on the severity of the weather.
Similar to built-up roofing is roofing made from a single layer of asphalt-impregnated material, called asphalt roll roofing. This is used more often on garages and outbuildings than on houses because of its relatively short life span--from 5 to 15 years. It is generally the least expensive roofing material available.
