Everywhere the ice & water shield doesn’t go, a synthetic underlayment covers the deck. It’s the second layer under your shingles, catching water that gets past them. It’s tougher than old felt paper, so it won’t wrinkle or tear if it gets wet before the shingles go on.
Tap Next on the drawing. It stays in view while you read.
Understand your roof
Ice & water shield
Ask your roofer
- How many rows go on at the eaves?
- Besides the eaves, where else does the ice & water go: valleys, walls, around the chimney and pipes?
Ask them to put the answer in writing.
Why it matters
Ice dams push water up under the shingles at the edges, in the valleys and along walls, where a regular underlayment can’t stop it.
How it should be built
Tap Next on the drawing to see it go together, layer by layer.
- Code minimum (cold climates like Michigan): from the eave to at least 24″ inside the exterior wall line. Check your local code; it varies by area.
- Best practice: at least two rows at every eave.
- Down every valley, along every wall, and around the chimney, skylights and every pipe.
- Synthetic underlayment everywhere else, under every shingle the ice & water doesn’t cover.