Built-up Roofing (BUR)
Built-up Roof: “A continuous, semi-flexible roof membrane consisting of multiple plies of saturated felts, coated felts, fabrics or mats assembled in place with alternate layers of bitumen and surfaced with mineral aggregate, bituminous materials, a liquid-applied coating or a granule-surfaced cap sheet.”(NRCA Index)
Asphalt is an excellent waterproofing material, thus its use in shingles and Modified Bitumen. Asphalt breaks down very slowly in water and is a self-healer when struck by falling objects like hail. BUR asphalt roofs are very sustainable with cost-effective maintenance throughout its life cycle. The reflective properties of the BUR system is dependent on what type of media is used for surfacing- white gravel is most often used.
Like with any roofing system it is important that an annual inspection- usually per manufacturer warranty requirement- be conducted throughout its life cycle
Bitumen : “(1) A class of amorphous, black or dark-colored, (solid, semi-solid or viscous) cementitious substances, natural or manufactured, principally composed of high-molecular-weight hydrocarbons, soluble in carbon disulfide, and found in asphalts, tars, pitches and asphaltites; (2) a generic term used to denote any material composed principally of bitumen, typically asphalt or coal tar.”
Bitumen is used to weld all the different layers of BUR into one flexible, cohesive unit. They also act as a waterproofing compound. Coal tar previously was the standard but has been replaced by the safer alternative- asphalt. They are both thermoplastic; becoming fluid with heat and then reverting to a more solid material as they cool. This thermoplastic characteristic is what makes this type of roofing system last so long.
It is important to think of bitumen not as a glue but more of a process of welding or fusing together layers or plies.