What Is Modified Bitumen?

The asphalt is modified with either SBS, a rubber-like polymer that adds flexibility, or APP, a plastic-like polymer that adds heat and UV resistance. The sheet is reinforced with polyester or fiberglass and usually topped with a granulated cap sheet for sun protection. Rolls go down by torch, which heats the underside until it bonds, or with hot asphalt, cold adhesive, or a peel-and-stick backing. A system usually has two layers, a base sheet and a cap sheet, so the roof carries some built-in redundancy.

It turns up on low-slope commercial roofs, apartment buildings, and flat residential sections, especially where the existing roof is the same type. The first question to ask is how the work was or will be applied, because torch-applied installation involves open flame, and some owners, insurers, and jurisdictions restrict it or call for hot-work precautions and permits. The ply count and the condition of the cap sheet granules tell an estimator how much life is left.

Mod bit gets lumped in with built-up roofing, and the two are related, but modified bitumen arrives as prefabricated polymer-modified sheets while built-up roofing is assembled on site from layers of felt and bitumen. Failures tend to start at laps, flashings, and penetrations rather than in the middle of the sheet. Hail can bruise the mat under the granules, and granule loss exposes the asphalt to sunlight.