Materials / Synthetic Underlayment
Synthetic Underlayment
accessorySynthetic underlayment is a woven polypropylene or polyethylene sheet installed beneath asphalt shingles on steep-slope roofs as a secondary water-resistive barrier, replacing traditional asphalt-saturated felt. It is governed by ASTM D8257, Standard Specification for Mechanically Attached Polymeric Roof Underlayment Used in Steep Slope Roofing, a dedicated performance-based standard published in 2020 specifically for polymeric sheet products. The 2024 IRC added D8257 alongside D226, D1970, D4869, and D6757 in Table R905.1.1(1) as an accepted underlayment type. It is lighter and covers more roof area per roll than felt, and is commonly used under fastener-applied asphalt shingle roofing.
Category family
accessory
Slope suitability
steep
Pros
- lighter weight and covers more roof area per roll than asphalt-saturated felt
- tested against a standard written for polymeric sheet products rather than a felt standard adapted to a different material
- generally more tear- and wrinkle-resistant than felt if exposed before shingles are installed
- often more slip-resistant underfoot during installation than felt
Cons
- the dedicated standard, ASTM D8257, is recent (published 2020, added to the IRC table in the 2024 edition), so not every product or every adopted code cycle references it yet
- older felt-based standards (ASTM D226, D4869, D6757) do not test the properties relevant to polymeric sheet products, so a product's labeled compliance basis is worth checking
- code recognition depends on which IRC edition a jurisdiction has adopted
Related guides
Sources
- up.codes verified 2026-07-28