Built-up roofing uses multiple tar and fabric membranes to prevent leaks in flat top roofs. The roof base (can be wood, concrete, or metal) is covered with rigid roof insulation that is secured down using plastic cap roofing nails. Some roof bases will require a vapor barrier layer below the insulation. Next, a cover board is adhered to the roof insulation to provide a smoother top surface and insulate around fasteners. On top of the cover board, a series of asphalt and fibrous ply sheet layers are applied to create a seal; usually 2-4 asphalt/ply layers ending with a cap sheet containing gravel. Finally, a reflective roof coating is applied to reflect heat energy from the sun.