Modified roofing systems provide a durable, long-lasting, weather resistant low slope roof option. Modifiers 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. APP (attactic polypropylene) increases the aging ability of the roof system. SBS is often the more popular system as the synthetic rubber additive increases the products flexibility.