There are three basic types of underlayment used beneath roofing materials:
- asphalt-saturated felt;
- rubberized asphalt; and
- non-bitumen synthetic.
One of the most common types of underlayment used in residential, steep-slope applications is black, ashphalt-saturated felt paper. Felt underlayment may be made from either organic or fiberglass substrate, although the organic is much more common. It's called "organic" underlayment because it has a cellulose base. Felt underlayment is water-resistant, but not waterproof. It's available in two thicknesses: 15-pound and 30-pound. Fifteen-pound felt has a perm rating of about 5, although this number can rise in high-humidity conditions. Thirty-pound felt is more resistant to damage during installation of the roof-covering material, and will protect the roof longer if it should somehow become exposed to weather. The difference is obvious, once you see them together. Thirty-pound felt is much thicker and stiffer.
INSTALLATION OF FELT UNDERLAYMENT
Underlayment: overlap comparison
Slope Limitations
In low-slope roofs, which include 2:12 up to 4:12, felt courses should overlap a minimum of 19 inches. This will provide a double layer of underlayment across the entire roof. In steep-slope roofs (4:12 and steeper), the upper courses of felt underlayment should overlap lower courses by at least 2 inches. You can see the difference between the underlayment overlapped 19 inches on the roof to the right and overlapped 2 inches on the roof to the left. In Figure 1 the lower roof is low slope with a 19-inch overlap and the upper roof is steep slope with a 2-inch overlap.
windstrip installed Fastening Felt is usually fastened with staples, but in high-wind areas, plastic windstrips may be used along the edges to prevent tearing.
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