Roofing felt or felt paper is a sheet material that consists of heavy felt impregnated with asphalt. The term felt comes from the base material felt. Typical uses for felt paper are as an underlayment beneath other building materials, such as roofing shingles, and as a component of asphalt built-up roofing (BUR) systems. They separate the roof covering from the roof deck, help to keep water away from the surface and provide overall weather protection.
Felt paper comes in Type 1 (lighter) and Type 2 (heavy) felt. Another designation is organic or inorganic. Organic felt paper has a base material made with wood or jute or hessian. While inorganic felt is made of polyester or fiberglass.
Felt paper is beneficial because it temporarily dries in a roof. It also increases the fire rating of the roof system and creates a moisture barrier between the wood decking and shingles. Many contractors will install it and many building codes require its use during new roof installations.