More popularly known as the 2-ply modified bitumen system, this type of roof is an evolution of the original built-up roofing. This type of system utilizes 2 layers of the membrane to cover the insulation layers of a roof – the base sheet and a cap sheet. Depending on various situations, these membranes can either be self- adhered, asphalt mopped or torch-applied. This type of roofing eliminates the need for gravel on the roof. The cap sheet (exposed layer) comes in a limited number of colors, and this provides a clean aesthetic look for the building once the work is completed.