Commercial roofing projects are completed with a variety of roofing materials. Built-up roof (BUR) membranes are made up of layers of asphalt bitumen that serve to waterproof the membrane. Roofing contractors install bituminous membranes in multiple-ply configurations. 3-6 layers of bitumen are applied between layers or piles of reinforcement. Fiberglass ply sheets compose the “built-up” membrane. The built-up membrane is often covered with a layer of pea gravel that is set in a flood coat of bitumen or coated with an approved coating.