RUBEROID HW 25 Smooth Membrane is a tough, resilient modified bitumen smooth-surface membrane manufactured to stringent GAF specifications. RUBEROID HW 25 Smooth Membrane is an ideal base or interply roofing membrane in modified bitumen systems. This Membrane is designed for new roofing and re-covering applications as well as in the construction of flashings. Its core is a strong, resilient non-woven glass mat that is coated with flexible polymer modified asphalt.