Materials / Asphalt-Saturated Felt Underlayment
Asphalt-Saturated Felt Underlayment
accessoryAsphalt-saturated felt underlayment is an organic felt sheet impregnated with asphalt and installed between roof decking and asphalt shingles as a secondary water-resistive barrier. It is manufactured to ASTM D226, Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing, in two weights: Type I (commonly called No. 15 felt) and the heavier Type II (No. 30 felt). IRC Section R905.1.1 (Underlayment) requires labeled underlayment conforming to ASTM D226 Type I or II, or to the related ASTM D4869 steep-slope felt standard, beneath asphalt shingles. It is used almost exclusively on steep-slope roofs.
Category family
accessory
Slope suitability
steep
Pros
- code-recognized under IRC R905.1.1 for asphalt shingle roofs
- inexpensive and widely available at lumber and roofing suppliers
- long field track record across decades of steep-slope installations
- simple to install with standard cap nails or staples
Cons
- absorbs moisture and can wrinkle or buckle if wetted before shingles cover it
- tears more easily during installation than synthetic underlayments
- degrades faster than synthetics under prolonged UV/weather exposure before covering
- heavier and bulkier per roll than comparable synthetic underlayment
Related guides
Sources
- up.codes verified 2026-07-28