This type of low slope roofing material is referred to as modified bitumen due to it's material makeup which consists of modifiers that are added to standard roofing asphalt to replace plasticizers removed during the distillation process. Bitumen is reinforced with fiberglass or polyester fiber matting in order to increase durability and strength.
The two most common types are SBS and APP. The use of SBS (styrene-butadiene-styrene) increases the flexibility of the membrane and allows it to have stronger expansion and contraction capabilities. A standard SBS modified bitumen roof system is a two-ply system applied in rolls that overlap the edge of the previous roll. The other type of modified bitumen roof is the APP (attactic polypropylene) which is used for its ability to increase the aging ability of the roof system. SBS is often the more popular system as the synthetic rubber additive increases the products flexibility.
There are a handful of methods for installing a modified bitumen roof system which consist of the most common method of torch applied, as well as with asphalts or cold adhesives.