Tar and Gravel
Built up roof membranes, referred in the industry by the acronym BUR, have been used in the United States for more than 100 years. These roof systems are commonly referred to as “hot tar” or “tar and gravel” roofs. BUR systems generally are composed of alternating layers of bitumen and fiberglass reinforced felts forming one membrane. The number of plies incorporated into the system denotes the number of plies on a roof. The reinforcing fabrics also are called roofing felts or ply sheets. Roofing felts are reinforced with either glass-fiber mats or organic mats. 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 propane fueled kettle 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. Common surfacings for BUR systems include aggregate (such as gravel, slag or mineral granules), glass-fiber or mineral surfaced cap sheets, hot asphalt mopped over the entire surface, elastomeric or aluminum coatings. BUR roofs are for dead level roof slopes up to a 2/12 roof pitch. There are literally hundreds of different configurations of base sheet, interply sheets and final surfacing materials used for BUR assemblies. Due to the labor intensity of these systems combined with the increasing cost of asphalt based products these systems are not as common as they once were. This is, however, one of the best low slope roofing systems available in the industry today.