DuraWeld 4S APP is a durable smooth surfaced, premium APP (Atactic-Polypropylene) torch applied membrane. Manufactured with a strong non-woven polyester mat that is saturated and coated with a premium quality, nonblown asphaltic bitumen and APP resins for a durable, heat resistant, flexible and easily maintainable membrane. It is designed for use as a base sheet, interply sheet or cap sheet in modified bitumen roof systems.