Built-Up roofing (Also known as: Hot mopped, tar and gravel) is one of the oldest roofing systems for low-slope. It is composed of several layers of bitumen, felt, and aggregate top layer. Each layer is mopped using melted tar and then covered with a flood coat and some sort of rock, pebbles, gravel or granules. They are know to be tough can last 10-30 years. Due to increases in cost of oil and labor they generally are no longer a cost effective solution especially where a cool roof is code requirement.