Know Your Roof · Underlayment & Roof Deck
Roof Underlayment: The Layer Under Your Shingles
Quick answer
Underlayment is the water-resistant layer between the deck and the shingles. It's the backup if water gets past the shingles. Most roofs use synthetic or felt underlayment across the deck, with self-adhered membrane at the eaves, valleys and around penetrations, where leaks are most likely to start.
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What it does
Shingles shed most of the water, but wind-driven rain, ice dams and damaged shingles can let some through. Underlayment catches that water and sends it down the roof. It also protects the deck while the roof is being installed, a step roofers call the dry-in.
Types
| Item | What it is | Where it's used |
|---|---|---|
| Synthetic | A polymer sheet that resists tearing and doesn't soak up water | Across the deck |
| Felt | Asphalt-saturated paper or fiberglass, also called tar paper | Across the deck, the traditional choice |
| Self-adhered | A peel-and-stick waterproof membrane, such as ice and water barrier | Eaves, valleys and around penetrations |
Where each type goes
Self-adhered membrane goes where water is most likely to get in: along the eaves, in the valleys and around chimneys, skylights and vent pipes. Synthetic or felt covers the rest of the deck, lapped so water always runs over the seams, never into them. Local code and the shingle manufacturer's instructions set the details.
Signs of an underlayment problem
You can't see underlayment from outside, so problems usually show up as leaks after shingles are lost, stains on the underside of the deck in the attic, or wrinkled and torn felt found during a tear-off. Underlayment left exposed after storm damage breaks down, so it needs a tarp until the repair.
Is underlayment replaced with a new roof?
Yes. On a roof replacement, the old roofing comes off down to the deck, the deck is checked and repaired, and new underlayment goes on before the shingles.
Looking for another word? See the roofing terms glossary, A to Z.