The purpose of felt underlayment.
Standard Slope Application:
Proper application requires that a single layer of #15 felt underlayment be applied to decks with slopes of 4/12 or greater and consistent with all applicable building codes. This felt underlayment is required to achieve a Class A or C fire rating on the deck assembly.
Felt also helps to prevent leaks from wind driven rains that may, somehow, penetrate the shingles. Imperfections in the wood decking also can be a problem in the absence of felt underlayment. These imperfections will more readily telegraph through today’s thinner fiberglass shingles. Finally, many shingle manufacturers require felt to be used over a bare wood deck.
Certainteed Corp. which is a one of the best shingle manufacturers recommends that underlayment be installed but not require it except as noted below.
Low Slope Application: 2:12-4:12 (51mm/305mm)
On slopes 2" to 4" per foot apply a double layer of felt underlayment over the entire deck surface. Starting with 19" wide strip at eaves, overhanging the eaves 1/4" to 3/8", cover with full 36" strip providing a 17" exposure. Continue with 36" strips lapping each course 19" over the preceding course. End laps for low slopes shall be 12" and staggered 6 feet apart. Install drip edge over the felt underlayment at the rake and beneath the underlayment at the eaves.
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