A multi-ply roofing system composed of asphalt that has been modified with polymers like APP (atactic polypropylene) or SBS (styrene-butadiene-styrene). These modifiers give the brittle asphalt rubber-like properties, allowing it to stretch and recover as the building moves. The sheets are reinforced with fiberglass or polyester mats and applied using heat, cold adhesives, or self-adhering backs.
The spec calls for a two-ply modified bitumen system over the new cover board.
Also heard as
- mod-bit
Related terms
Cap sheet
The topmost layer of a multi-ply roofing system that protects the underlying membrane from weather and traffic.
Base sheet
The foundational first layer of felt or membrane in a multi-ply roof system.
Torch-down
A roofing method where modified bitumen sheets are melted onto the substrate using an open flame.