Felt paper, also called builder's felt or roofing felt, is used as a moisture barrier between the roof's sheathing and the roofing material. It is sold in rolls and sized according to the area it covers. Installing felt paper is a preventive measure that saves money that might be lost due to water damage during and after roof installation.
Felt paper protects the house from the elements, especially rain, while a new roof is being installed. Felt paper also provides a layer of protection in the event that roofing material is blown off in a storm. With a layer of felt paper between the shingles and the sheathing, it is also easier to remove old shingles. The paper, which is easy to attach, prevents the shingles from sticking and enables roofers to remove an old roof quickly. New technology has enabled manufacturers to provide many other features apart from moisture protection. Some types have a breathable feature, which allows moisture to pass with ventilation to reduce condensation under the roofing material. Organic roofing felt, which is typically treated with asphalt, provides extra protection against heavy rains.
Felt paper is typically made of fibreglass or polyester fleece material that is waterproofed with asphalt, tar or bitumen. Sand may be added as well to prevent the material from sticking when it is rolled. Roofing felt does tend to tear, particularly in warmer conditions.
Roofing felt paper can be installed using fasteners, hot or cold asphalt, non-asphaltic adhesives and heat. On the surface are guide lines that are marked horizontally on the rolls and provide a way to ensure the shingles run straight across the roof during installation. Roofing felt is installed under the roof's drip edge. This allows water that runs under the shingles to flow away from the roof, into the eaves and out again.
Felt paper is sold in rolls and is classified by weight as #15 or #30. #15 felt paper will cover about 400 square feet of roof and #30 rolls cover about 200 square feet. The standard #15 is thin and is usually used as a temporary covering or under inexpensive shingles that will be replaced after a short time. #30 felt paper is thicker and is used for heavier shingles and wood shingles. It may be used under slate and tile roofing as well.