Built-Up Roofing systems (BUR) are commonly known as “Hot Tar” roofs. This is the most durable flat roofing system available with life expectancy of 50 years and warranties up to 30 years. 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 are considered the most durable and longest lasting flat roof assemblies.
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.
Surfacings for built up roof systems include aggregate (such as gravel, slag or mineral granules), glass-fiber or mineral surfaced cap sheets, modified bitumen, hot asphalt mopped over the entire surface, aluminum coatings or elastomeric coatings.
Modified Bitumen (MB)
There are two types of MB roofing membranes:Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. These systems typically are installed as a two-ply system, although there are many other options as well.
- SBS polymer-modified bitumen membranes commonly are installed in hot moppings of asphalt (similar to BUR systems) or cold adhesive. Some SBS modified membranes are self adhering; that is, they contain an adhesive backing. Self adhering SBS membranes are safer applications with less fumes and no dripping hot asphalt.
- APP polymer-modified bitumen membranes typically are heat-welded or torch-applied
Consumers should be cautioned that the NRCA does not recommend torch-applying for a modified bitumen membrane sheet directly to a wood deck.
Generally, APP modifiers impart a "plasticized" quality to asphalt, and SBS modifiers impart a "rubberized" quality to asphalt. MB membranes and EPDM, a thermoset membrane, often are confused by consumers because of colloquialisms used by roofing contractors. MB and EPDM membranes are sometimes called "rubber roofs."
Surfacings for MB membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing.
Modified Bitumen membranes have warranties up to 20 years depending on the system chosen.