Now an essential part of the roofing process, felt paper was once used as only a temporary protection against the elements and to keep sap from deteriorating shingles. Felt paper keeps water out and provides an extra layer of protection when shingles go missing. Different felt weights are necessary depending upon the roofing material used, state and local codes and roof pitch.
The Roofing System
Shingles are not the only thing that makes a roof. Decking, usually plywood sheets, is installed over the trusses. Underlayment is then applied over the decking. The roof covering, such as shingles or clay tiles, is then installed. Each layer protects the layer beneath it to make a complete roofing system.
Felt paper has been used in the construction industry for five decades. Its first use was to prevent sap from rising from the wood decking and deteriorating the shingles above. It now has a different purpose and is still just as vital to the roofing system. It does double duty in protecting the decking against moisture before shingles are laid and also as a water barrier when damage occurs to the roof.
The most common weights are No. 15 and No. 30. Joan Crowe, technical manager of the National Roofing Contractors Association of Illinois, explains that, "The name changed because today's 15 lb. felt is actually lighter than it used to be, because of modern technology and production.” Felt paper is saturated with asphalt to make it more weather-resistant. Some papers are only coated with asphalt on the exterior layers while other papers are saturated all the way through.
Installation is fairly simple. The paper is rolled out on the decking and nailed down with fasteners. Bobvila.com recommends using, “low-profile capped-head nails or thin metal tins and roofing nails.” Tins should have a diameter of at least 1 inch. Nails should be spaced a maximum of 6 inches apart on all seams and 12 inches everywhere else. Galvanized fasteners should be used if the roof is within 3,000 feet of salt water. It's also recommended to overlap each row of underlayment by at least 19 inches to ensure proper coverage and protection.
There are some downsides to working with asphalt underlayment. Hot weather and direct sunlight will cause the asphalt to become slick and is a potential safety hazard. Moisture can damage the underlayment. Rain will not always ruin it, but if the paper is buckled enough, the roof covering will not lay flat and the paper will have to be replaced. Underlayment does not breathe, so it also traps any moisture from the roofing system.