Roof Underlayment Guide
Compare felt, synthetic and self-adhered roof underlayments, where each is used and which details matter in a replacement proposal.
Roof underlayment is the layer installed over the roof deck and beneath the visible covering. It provides secondary protection during construction and when wind-driven rain, ice or damaged roofing allows water past the outer surface. Underlayment is important, but it is not intended to rescue poor flashing or remain exposed indefinitely.
Main underlayment types
| Type | Typical characteristics | Questions to ask |
|---|---|---|
| Asphalt-saturated felt | Traditional roll material; weight and standards vary | Which designation, exposure limit and fastening method? |
| Synthetic underlayment | Light, tear-resistant woven or spun polymer sheets | Is it approved for the roof covering and exposure period? |
| Self-adhered membrane | Peel-and-stick layer that seals around many fasteners | Where is it required, and is the deck/temperature suitable? |
Felt underlayment
Traditional felt can provide dependable service when kept dry and installed correctly. It may wrinkle when wet and can tear in wind before the roof covering is installed. Product designation does not always equal the finished roll’s exact weight, so compare published specifications rather than names alone.
Synthetic underlayment
Synthetic products are commonly lighter and more resistant to tearing. Printed grid lines can aid layout, and some products allow longer temporary exposure. They vary in vapor permeability, slip resistance and temperature rating. “Synthetic” does not automatically mean suitable beneath every metal, tile, slate or shingle system.
Self-adhered ice and water membranes
Self-adhered membranes are commonly used at eaves, valleys, penetrations and other vulnerable areas, and some projects use them across the entire deck. Local ice-barrier requirements differ. Full coverage changes drying potential and must be coordinated with ventilation, insulation and the covering manufacturer.
Where details matter
- Layers should lap so water drains outward rather than behind lower courses.
- Valleys and roof-to-wall transitions need a planned sequence with flashing.
- Fastening must resist wind before the covering is installed.
- High-temperature roofs may require membranes and adhesives rated for the surface temperature.
- Wrinkles, fishmouths and trapped debris can telegraph through or interfere with roofing.
Can underlayment be reused?
During full replacement it is normally removed with the old covering so the deck can be inspected and a compatible new layer installed. A roof-over proposal should explain how deck condition will be assessed and whether local code and product warranties permit leaving existing layers.
Questions for a proposal
- Which manufacturer and product are included?
- Where will self-adhered membrane be installed?
- What happens if rain occurs before the covering is complete?
- Is the underlayment compatible with the covering and warranty?
- How will valleys, eaves, walls and penetrations be layered?
Underlayment works with flashing, not instead of it. Continue with the roof flashing guide.