Built-up roof membranes (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. Often, a base sheet used as the bottommost ply is mechanically fastened. BUR 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 while 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.

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
  • Aluminum coatings or elastomeric coatings

A roof system composed of a BUR membrane with two or three plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as "hybrid" system.