Modified-bitumen options are even more complex than built-up roofing options. Not only are there two major types of modified-bitumens — those based on rubber modifiers (SBS and SEBS) and those based on plastic modifiers (APP) — but there are different installation methods that can be used with the different membranes.
Generally speaking, SBS membranes are applied with hot asphalt or cold adhesives, though they can be torch-applied. APP modifieds are generally torch-applied, but some can be adhesively applied. Almost all modified-bitumen membranes are surfaced with granules, making them less heavy than the traditional gravel-surfaced, built-up roof. Often, a modified-bitumen membrane will be used as a cap sheet on a traditional built-up roof. This gives the redundancy of a built-up roof with the flexibility of a modified membrane.
Modified-bitumens share most of the pros and cons of built-up roofing with one major difference - the modified bitumens were developed to perform better in cold weather by remaining flexible at low temperatures. They can stretch and relax with temperature changes better than built-up systems.
APP and torch-applied SBS systems should be used with care if there are flammable items on the roof, such as wood parapets. The membranes can smolder and erupt into flames long after being torched. They are the best choice of the modified-bitumen membranes if odor is a problem because there is no adhesive to flash off or asphalt to cause smells during installation. APPs and cold-adhesive-applied modifieds are equally good choices if delivery to the roof is limited, as both can be delivered via freight elevator and stairs.
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