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Tar paper is a waterproof building material that serves as an underlayment in building construction. It works as a vapor barrier by preventing the ingress of moisture as well as providing a thin layer of protection against air flow. Tar paper, also called roofing felt, is made from cellulose fibers, glass fibers, or polyester fleece compressed into thin sheets. The sheets are treated with tar or asphalt to make them waterproof. Tar paper is typically installed under roof shingles, exterior siding and hardwood floors.
Sold in rolls of various widths, weights and lengths, tar paper is inexpensive and readily available in home improvement stores and construction supply outlets. The most common size rolls are 3 feet (0.9144 meters) wide by 50 or 100 feet (about 15.25 or 30.5 meters) long. It comes in either 15-pound or 30-pound (6.8- or 13.6-kilogram) weights.
The weight designation signifies the weight of 100 square feet of tar paper. The 15-pound (6.8 kg) tar paper is less durable than the 30-pound (13.6 kg) paper and is used in less critical areas such as walls and floors. Tar paper is sometimes coated with sand to prevent it from sticking to itself while on the roll.
The process of installing tar paper is called tar paper roofing. It is typically installed on roofs over the sub-roof prior to the installation of roof shingles or gravel. In driving rains, it is possible for water to be pushed under shingles. The tar paper works as a secondary line of defense, preventing the water from reaching the interior wood structure.
Tar paper is also applied as sheathing on exterior walls. It is installed under the siding. This not only prevents water from reaching the interior wood structure, but it also helps prevent the loss of heated or cooled interior air. Tar paper is sometimes used as a vapor barrier and cushioning beneath wood floors, keeping the sub-floor dry and helping to prevent wood floors from squeaking.
Tar paper can help protect a building if exterior elements are lost or become damaged. For instance, if a shingle is blown off during a storm, the underlying tar paper can protect the sub-roof until the shingle is replaced. Although tar paper is a cost-effective way to provide an additional layer of protection, it is not weather resistant. It will deteriorate if left exposed for extended periods of time. Exposed tar paper should be covered as soon as possible to help it retain its integrity.
I personally don't advise tar paper for wood floor underlay. The reason is that some people are sensitive to the fumes that are vented, especially if they have asthma. I prefer rosin paper or a polymerized product, depending on how much vapor barrier you want. I also avoid foam because it has a shorter service life.