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Roofing term
Roof Underlayment
Definition: Roof underlayment is a water-resistant sheet, such as felt or synthetic material, installed over the roof deck and under the shingles as a secondary barrier against water.
Why it matters
Shingles shed most water, but wind-driven rain and ice dams can push water underneath them. Underlayment is the layer that keeps that water off the roof deck, and building codes set where it is required and how it overlaps.
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