Roofing Underlayment

Definition

Roofing underlayment is a water-resistant or waterproof layer installed between the roof deck and the shingles. It serves as a secondary barrier against water infiltration if the shingles fail or are damaged. The two main types are felt underlayment (traditional asphalt-saturated paper) and synthetic underlayment (polymer-based sheets). Synthetic has largely replaced felt in modern roofing because it is lighter, stronger, tear-resistant, and lasts longer when exposed. Underlayment is required by building code on almost all sloped roofs.