Synthetic Roof Underlayment
Synthetic roof underlayment is a woven or non-woven polypropylene or polyethylene sheet installed over roof sheathing before the final roofing material. It has largely replaced traditional #15 and #30 felt in new construction due to superior tear resistance, UV stability, and lighter weight.
Fast Facts
- What Is It?
- Lightweight, tear-resistant polymer sheet installed on roof decking as a secondary water barrier beneath shingles, metal, or tile.
- Common Uses
- Under asphalt shingles, metal roofing, tile, and slate. Required by code on all sloped roofs (IRC R905).
- Cost Range
- $50–$120 per roll (10 squares coverage); $0.05–$0.12/sq ft material only.
- Durability
- UV exposure: 4–6 months (can serve as temporary roof during construction). Under roofing: full service life of the building.
Specifications
| Property | Value |
|---|---|
| Standard | ASTM D226 Type I (15#) / Type II (30#) for felt; ASTM D4869 for synthetic |
| Weight | Synthetic: 3–8 lbs/square vs. Felt: 15–30 lbs/square |
| Tear Strength | Synthetic: 20–50 lbs (MD); Felt: 5–10 lbs |
| Permeance | Varies: 5–20 perms (vapor-permeable); <1 perm (vapor barrier types) |
| UV Exposure | 4–6 months (synthetic); 30 days (felt) |
| Coverage | 48" wide × 250' roll = 10 squares (1,000 sq ft) |
Synthetic vs. Felt Underlayment
Synthetic underlayment has largely replaced felt paper (#15 and #30) in new construction for good reason: it is 4–5× stronger in tear resistance, 3–4× lighter for easier handling, and remains stable on the roof for months during construction. Felt absorbs water and wrinkles, creating bumps under shingles. Synthetic lays flat and sheds water.
Vapor Permeability Matters
Not all synthetic underlayments have the same vapor permeability. In cold climates, a vapor-permeable underlayment (>5 perms) allows moisture to dry through the roof deck, reducing the risk of sheathing rot. In hot-humid climates, a low-perm underlayment may be preferred to prevent inward vapor drive. Match the underlayment permeance to your climate zone.
Frequently Asked Questions
Is synthetic underlayment required by code?
IRC R905.2.7 requires underlayment beneath asphalt shingles. The code allows either ASTM D226 felt (#15 or #30) or ASTM D4869 synthetic. Most manufacturers now require synthetic underlayment for their warranty — check your shingle manufacturer's installation instructions.
Can synthetic underlayment be left exposed?
Synthetic underlayment has limited UV exposure ratings — typically 4–6 months. It can serve as a temporary weather barrier during construction, but it is not designed as a final roofing surface. Extended UV exposure degrades the polymer and voids the warranty.
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