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Tar paper is a heavy-duty paper used in construction. Tar paper is made by impregnating paper 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 theses two products are used the same way and sometimes are informally be used as synonyms.
Tar paper has been in use for over a hundred years. Originally felt was made from recycled rag but today felts are made of recycled paper products (typically cardboard) and sawdust. The most common felt product is the so-called #15 felt. Before the oil crisis felt weighed about 15 pounds per square (one square = 100 square feet) and hence the asphalt-impregnated felt was called "15#" or "15 pound felt". Modern felts no longer weigh 0.73 kg/m2, and to reflect this fact the new felts are called "#15" asphalt felt. In fact, #15 felts can weigh from 7.5 to 12.5 pounds/sq depending on the manufacturer and the standard to which felt is made (i.e., CGSB, ASTM, or none). Thirty pound felt, of 30# felt, is now #30 felt, and actually usually weighs between 16 and 27 pounds per square. Hence, to get a product similar to a 15# felt of old, one could specify a modern #30 felt.
Tar paper is more accurately a Grade D building paper (the Grade D designation derives from a US federal specification) which is widely used in the west. Building paper is manufactured from virgin kraft paper, unlike felts, and then impregnated with asphalt. The longer fibres 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 colour when a small boat-like sample is floated on water. Common grades include 10, 20, 30, and 60 minute. The higher the rating the more moisture resistant and the heavier. A typical 20 minute paper will weigh about 3.3 pds per square, a 30 minute paper 3.75, and a 60 minute paper about six. The smaller volume of material however does tend to make these papers less resistant to moisture than heavier felts.
Tar paper is used, among other things, for waterproofing roofs to prevent ingress of moisture. It 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).
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 uses 8-or-10-foot (2.4 or 3.0 m) widths of "housewrap".
Many roofers now use a synthetic underlayment TPO (thermoplastic polyolefin). These membranes, usually made of advanced fabrics, have advantages over traditional roofing felt and tar paper. They are more durable and less prone to puncture and tear, lighter, stronger, and wider rolls though quite recent (2003) in the market. There are also breathable variations, which allow water vapour to pass through the felt; when used in conjunction with proper ventilation, they help minimize condensation in loft spaces.