Built Up Roof (BUR) is a generic term for a roof system that is literally built by adding layers. This is by far the oldest of the "modern methods" of roofing dating back to the early 1800's. Formerly, the BUR membrane was the waterproofing agent and the bitumen was the adhesive. With the advent of synthetic fibers, the rolls of the two principle elements have been reversed. Now, the bitumen is the waterprrofing agent and the membrane is used to hold the bitumen in place. BURs can be installed using different waterproofing materials including coal tar pitch, asphalt, cold application adhesive, and modified asphalt. All systems will utilize either base sheet, ply sheets (felts), or cap sheets in some combinatioon. The BUR has a variety of surfacing options ranging from gravel, asphalt, coatings, to mineral surfaced.
Shingle of felts is a common installation of built up roofs. Overlapping the felts to develop an achieved minimal thickness characterized by 2-ply, 3-ply and 4-ply. It is said that each ply represents 5-7 years of service, so a 3-ply would indicate a 15-21year roof system and a 4-ply would signify a 20-28 year roof system. This is only if the roof system is properly installed and maintained over its life span.
Properly installed and maintained BUR systems should last 25 years.