Tar paper is a heavy-duty paper used in construction. Tar paper is made by impregnating paper or fiberglass mat with tar, producing a waterproof material useful for roof construction. Tar paper is distinguished from roofing felt, which is impregnated with asphalt instead of tar, but these two products are used the same way, and their names are sometimes used informally as synonyms.
Tar paper has been in use for centuries. Originally, felt was made of recycled rags, but modern felt is made of recycled paper products, typically cardboard, and sawdust.
The most common product is #15 felt. Before the oil crisis,[which?] felt weighed about 15 pounds (6.8 kg) per square, hence the asphalt-impregnated felt was called "15-pound felt" (15#). Modern, inorganic mats are no longer the same weight, and to reflect this fact, the new felts are called "number 15 felt" (#15). Modern #15 mats can weigh from 7.5 to 12.5 pounds (3.4 to 5.7 kg) pounds per square, depending on the manufacturer and the standard to which felt is made (such as ASTM, CGSB, or none). A thicker and stronger variant, once known as 30-pound felt (30#), is number 30 felt (#30), which usually weighs 16 to 27 pounds (7.3 to 12.2 kg) per square.
Tar paper is more accurately a Grade D building paper—the Grade D designation is derived from a federal specification in the United States—and is widely used in the West.[which?] Building paper is manufactured from virgin kraft paper, unlike felts, and then impregnated with asphalt. The longer fibers in the kraft paper allow for a lighter-weight product with similar and often better mechanical properties than felt. Grade papers are rated in minutes: the amount of time it takes for a moisture-sensitive chemical indicator to change color when a small boat-like sample is floated on water. Common grades include 10-, 20-, 30-, and 60-minute. The higher the rating, the heavier and more moisture-resistant the paper. A typical 20-minute paper will weigh about 3.3 pounds (1.5 kg) per square, a 30-minute paper 3.75 pounds (1.70 kg) per square, and a 60-minute paper about 6 pounds (2.7 kg) per square. The smaller volume of material, however, does tend to make these papers less resistant to moisture than heavier felts.
Tar paper is used as underlayment with asphalt, wood, shake, and other shingles, or even gravel, since tar paper itself isn't particularly wind- or sun-resistant. It is sold in rolls of various widths, lengths, and thicknesses – 3-foot-wide (0.91 m) rolls, 50 or 100 feet (15 or 30 m) long and "15 lb" (7 kg) and "30 lb" (14 kg) weights are common in the U.S. – often marked with chalk lines at certain intervals to aid in laying it out straight on roofs with the proper overlap (more overlap for flatter roofs).
It can be installed in several ways, such as staples or roofing nails, but it is also sometimes applied in several layers with hot asphalt, cold asphalt (adhesive), or non-asphaltic adhesives.
Older construction sometimes used a lighter-weight tar paper, stapled up with some overlap, as a water- and wind-proofing material on walls, but modern carpenters more often use 8-or-10-foot (2.4 or 3.0 m) widths of housewrap.
In the 19th and early 20th centuries, tar paper shacks consisting of wooden frames covered with tar paper were a common form of very low-cost housing in the rural United States and Canada.