What Is Underlayment?

Three families are common. Felt is paper or fiberglass saturated with asphalt and sold by weight, with the heavier grade thicker and stiffer. Synthetic underlayment is woven or sheet polymer: lighter per roll, harder to tear, and steadier when wet. Self-adhered membrane, covered under ice and water shield, is the third option for special zones. Whichever is used, sheets are lapped so each upper course sheds onto the one below, then fastened with cap nails or cap staples. It is water-resistant, not waterproof. Its job is to protect the deck during installation and catch what the shingles miss.

Almost no homeowner ever sees it, which makes it the place where bids quietly differ. Two proposals can match line for line on shingles and still specify different underlayment, and the lower number may be the one using the lowest grade. Saying what is going on the deck, and why, makes the rest of the estimate easier to trust. Adjuster scopes usually carry an underlayment line by the square, and moving up to a synthetic is normally the homeowner’s choice rather than the carrier’s obligation.

The name gets mangled constantly. People say tar paper, house wrap, or ice and water shield as if the terms were interchangeable, and they are not. Exposure time is another trap. Felt left uncovered for a long stretch can wrinkle and soak up moisture, and each synthetic product carries its own limit on how long it can sit in the sun before shingles go over it.