Built-up roofing ( BUR) is the term for the old standard tar and gravel roof. Because it's inexpensive, easy to apply, and easy to repair, it's among the most commonly used roofing systems for flat and low-sloped roofs. Built-up roofing involves layering a base sheet of asphalt or bitumen with fabric, followed by a protective layer of gravel, mineral granules, or some sort of aggregate rock.
Traditionally, the asphalt and surface gravel in a built-up roof have been black or dark gray, trapping much more light and heat than they release, but there are several ways to make a built-up roof cool. One is to replace the surface layer with white gravel or reflective marble chips to increase its solar reflectance. Another is to apply something called a mineral surfaced cap sheet, which incorporates reflective granules or a reflective coating into the roofing material itself. Finally, special coatings or pigments can sometimes be added to an existing asphalt surface to convert a standard roof to a cool roof. This typically requires one coat of a reflective material and another of an emissive material that releases heat [source: Department of Energy].