Flat, built-up roofing is the most popular material used on low-slope roofs. A built-up roofing is composed of several layers of bitumen surfaces, and finished up with an aggregate layer or coating. Recent built-up products incorporate a rigid insulation layer as part of the built-up roofing. The fabrics or layers used on built-up roofing are called ply sheets. These sheets are commonly reinforced with fiberglass mats or organic mats, depending on their application. Ply sheets are commonly produced in a standard size about 36 inches width.