Modified bitumen is thought of as the evolutionary cousin of the built-up roofing systems (BUR) that have been used on low-slope roofing for more than 100 years. It is made of asphalt combined with polymerized rubber or plastic, then reinforced with fiberglass to create a rugged-yet-flexible waterproof membrane.
Modified bitumen roofing is primarily used on flat or low-slope commercial roofs. Depending on the type and material, these membranes may be installed as part of a two-layer system or as part of a multi-ply system. Modified Bitumen typically falls into 2 varieties, APP and SBS. Commonly nicknamed “plastic asphalt,” atactic polypropylene (APP) membranes consist of asphalt blended with plasticized polymers that become elastic under high temperatures. APP is resistant to UV rays and is a good choice for structures seeking a “cool roof” effect. SBS stands for styrene-butadiene-styrene, but it helps to think of SBS roofing as “elastic” or “rubberized” asphalt. This membrane combines asphalt with polymerized rubber for natural flexibility and elasticity. SBS is a good choice for buildings whose roofs expand and contract with extreme temperature changes, as well as roofs subject to higher winds.