Built Up Roofing Systems
Built up roof membranes (BUR), have been in use in the U.S. for more than 100 years. A BUR system is composed of alternating layers of reinforcing fabric and bitumen (asphalt) and is finished with a top layer of aggregate, such as stone or gravel. For this reason, BUR roof systems are commonly referred to as “tar and gravel” roofs.
This is the common roofing material used on low-slope roofs or “flat” roofs because it creates a continuous sealed surface. By contrast, roof shingles are not sealed and require a fairly steep roof slope to shed water effectively.
Built up roof membranes, referred to by the acronym BUR, have been in use in the U.S. for more than 100 years. These roof systems are commonly referred to as “tar and gravel” roofs. BUR systems generally are composed of alternating layers of bitumen and reinforcing fabrics that create a finished membrane. The number of plies in a cross section is the number of plies on a roof: The term “four plies” denotes a four ply roof membrane construction. Sometimes, a base sheet, used as the bottommost ply, is mechanically fastened. Built up roofs generally are considered to be fully adhered if applied directly to roof decks or insulation.
The reinforcing fabrics also are called roofing felts or ply sheets. Roofing felts are reinforced with either glass-fiber mats or organic mats. Felts are produced in a standard width of 36 inches and metric width of about one meter.
The bitumen typically used in BUR roof systems is asphalt, coal tar or cold-applied adhesive. The asphalt or coal tar is heated in a kettle or tanker and then applied by mop or mechanical spreader. Asphalt is a petroleum product refined from crude oil; coal tar is derived from the distillation of coal. Cold-applied adhesives typically are solvent-based asphalts that don’t have to be heated in a kettle or tanker.
Surfacing for built up roof systems include aggregate (such as gravel, slag or mineral granules), glass-fiber or mineral surfaced cap sheets, hot asphalt mopped over the entire surface, aluminum coatings or elastomeric coatings.