Built-up roofing, also called BUR, is the most common roofing material used on low-slope roofs. It is composed of alternating layers of reinforcing fabric and bitumen (asphalt) and is finished with a top layer of aggregate, such as stone or gravel. BUR is preferred for low-slope, or “flat,” roofs because it creates a continuous sealed surface. Built-up roofing is nothing new. The concept been around for over 100 years, although the material and its installation certainly have evolved during that time. Modern built-up products incorporate a rigid insulation layer for improved energy efficiency. Depending on the climate and the specific materials used, built-up roofing has an average life expectancy of 15 to 30 years, but some constructions can last up to 40 years. In general, built-up roofing tends to fare better in warmer climates than in cold regions.