Tar paper was originally sheets of heavy paper impregnated with tar. These days you are more likely to use a glass fiber, polyester fleece, or recycled cellulose material infused with a tar-like substance (asphaltic bitumen). Because these forms are not made out of paper they are sometimes called roofing felt or felt paper.
Regardless of the exact material used, tar paper is used as a moisture barrier. The most common use it to waterproof roofs, but it can also be used for walls (as in the classic tar paper shack), to underlie tiles in showers, tubs, and tile floors, or as a moisture barrier between a structure the ground. When used for roofing, it goes between the sheathing (the supporting plywood, slats, or what have you) and the outer roof covering (shingle, gravel, etc.)
Tar paper is sold in hardware stores in three-foot wide rolls, usually 50 or 100 feet long. It comes in at least three different 'numbers', indicating its weight. (A #15 tar paper may also be referred to as a 15-weight tar paper.)
- #15 tar paper: The standard underlayment for most roofs.
- #30 tar paper: Twice as thick as #15, it is both more durable and more water resistant than #15.
- #75 tar paper: Commonly used for flat roofs and commercial jobs. It is often granulated and porous, and is intended to have hot tar spread over it after it is spread over the roof.