Modified bitumen systems are created using standard roofing asphalt. In most cases, modifiers are added to the material to replace plasticizers. Then the bitumen, otherwise known as pitch, is reinforced with polyester fiber matting or fiberglass to increase both strength and durability.
From there, the manufacturing process can take two paths. On one path, styrene-butadiene-styrene, or SBS, is added to further increase the flexibility of the membrane, which also increases the ability of the material to respond well to constant expansion and contraction. On the other path, attactic polypropylene, or APP, is added instead. This helps the roof withstand ageing. However, SBS systems are often more popular because of the rubber-like membrane that it creates, which aids in both strength and flexibility. However, SBS systems do require an extra application of a UV protective coating to help ensure long life against the effects of the sun’s rays.
Because of the way both these systems are manufactured, they are considered single-ply. This occurs because all of the layers that make up the system are combined during the manufacturing process, resulting in a single layer that can be applied using several different methods, each of which can be ideal under varying circumstances. The Torch down method is the most popular of these choices.