How long does roof felt last?

What felt is

Roofing felt is an organic or fiberglass-reinforced mat saturated with asphalt, sold in No. 15 and No. 30 grades. It was the standard underlayment for decades and is still accepted by the residential code, though synthetic underlayments have replaced it on many new roofs.

How it ages under shingles

Heat from the shingles above and the attic below slowly draws oils out of the asphalt. On old roofs, felt often becomes brittle and crumbles during tear-off, which is normal. Felt that stayed dry did its job as a backup layer; the shingles and flashings keep most water out.

Exposed felt

• UV breaks it down and dries it out.
• Georgia humidity and dew make it wrinkle and buckle, and wrinkles telegraph through shingles installed over them.
• Wind tears it loose around fasteners.

Manufacturers set short exposure limits, so felt is meant to be shingled over within days, not left for weeks.

When felt fails early

Leaks at flashings, condensation from a poorly vented attic, and long exposure before shingling all shorten its life. Felt also tears easily when walked on in summer heat.

Felt vs synthetic today

Synthetic underlayment is lighter, stronger, safer to walk on and tolerates sun longer, which is why many roofers now use it. Felt still works when installed properly and covered promptly. See synthetic underlayment vs felt and roof anatomy explained.