Roof Underlayment: Your Roof's Critical Second Layer (2026)
Roof underlayment is a water-resistant barrier installed directly on the roof deck beneath shingles, metal, or other roof coverings. It serves as a secondary weather barrier if the primary roofing is compromised. Modern synthetic underlayments have largely replaced traditional felt due to superior tear resistance and UV stability.
Key Features
- Secondary weather barrier; Synthetic and felt options; Tear-resistant synthetics; UV stable for temporary exposure; Required by building code
Specifications
Applications
- Under asphalt shingles; Under metal roofing; Under cedar shakes; Under tile roofing; Temporary weather protection
Installation Overview
Roll horizontally starting at the eave. Overlap courses minimum 2 inches horizontally and 6 inches at end laps. Fasten with cap nails or staples. Install ice barrier at eaves in cold climates before underlayment.
Pros & Cons
Pros
- ✓ Code-required weather protection; Prevents deck rot; Tear-resistant synthetics; Easy to install; Multiple product options
Cons
- ✗ Felt can tear in wind; Felt wrinkles when wet; Synthetics cost more; Limited UV exposure time; Adds to labor time
Top Brands
Code Requirements
Required per IRC R905.2.7. Minimum one layer of approved underlayment under asphalt shingles. Two layers for slopes 2:12 to 4:12. Self-adhered per R905.2.7.1 in ice dam regions.
Buying Tips
Synthetic underlayment is worth the premium for most projects. Choose a product with a high UV exposure rating if the project may be delayed. Non-skid surface is essential for worker safety.
Frequently Asked Questions
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