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 Modified bitumen sheets are composed primarily of polymer modified bitumen reinforced with one or more plies of fabric such as polyester, glass fiber or a combination of both. Assembled in factories using high quality control standards, modified bitumen sheets are made to have uniform thickness and are manufactured to have consistent physical properties throughout the sheet.
The finished roofing membrane may consist of one or more modified bitumen sheets, or may be comprised of a combination of built-up roofing (BUR) felts and one or more modified bitumen sheets. The type of substrate and the performance objectives influence the specification of the modified bitumen membrane system.
 Asphalt used in manufacturing modified bitumen systems is generally modified with one of two modifiers: Atactic Polypropylene (APP) or Styrene Butadiene Styrene (SBS). These modifiers create a uniform matrix that enhances the physical properties of the asphalt.
APP is a thermoplastic polymer that forms a uniform matrix within the bitumen. This matrix increases the bitumen's resistance to ultra-violet light, increases its flexibility at high and low temperatures, and increases its ability to resist water penetration.
SBS-modified bitumen membranes resist water penetration while exhibiting
superior elongation and recovery properties over a wide range of temperature extremes. This high-performance benefit makes them exceptionally durable and particularly applicable where there may be movement or deflection of the underlying deck.
To obtain the best roofing system possible, the designer, building owner and contractor should understand the dynamics of the roof as well as the roofing system when specifying either APP or SBS modified roofs.
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