Built Up Roofing (BUR) is the oldest technology, being the most common for well over one hundred years. Such systems consist of four plies of organic perforated felt paper, inter-mopped with coatings of hot asphalt. Asphalt is piped to the roof via kettles and tankers. Perimeter walls and roof appliances would commonly receive three additional plies of asphalt and felt flashings. More modern incarnations utilize modified bitumen membranes to reinforce walls and roof appliances such as drains and parapet walls.
A final flood coat of hot asphalt and pea gravel is applied at a prescribed rate to act as the finished surface and reflect UV rays and enhance longevity.