Roof Sheathing and Underlayment Selection Guide

Intermediate Level

How to choose the right combination of roof sheathing (OSB or plywood), underlayment (synthetic or felt), and ice protection for your climate zone and roofing material.

Materials Needed

Note

Ice and water shield is required by IRC at eaves in Climate Zones 5–8 (mean January temp ≤ 25°F). Check local amendments — some jurisdictions expand this requirement.

Note

Full-coverage ice and water shield creates a vapor barrier over the entire deck. Ensure adequate attic ventilation to prevent trapped moisture and sheathing decay.

Note

Synthetic underlayment is now required by most shingle manufacturers for warranty coverage. Check your specific manufacturer's installation instructions before specifying felt.

Sheathing Selection: OSB vs. Plywood

Both OSB and plywood are approved roof sheathing materials per IRC R803.1 when they meet the required span rating. CDX plywood handles moisture better than standard OSB — if a roof leak occurs, plywood dries without permanent swelling. For this reason, many builders prefer plywood for roof decks despite the 15–25% cost premium.

Underlayment by Climate Zone

Zones 1–3 (hot/mild): Synthetic underlayment over entire deck. No ice protection required. Zones 4–5 (mixed): Synthetic underlayment plus ice and water shield at eaves (24" past exterior wall). Zones 6–8 (cold): Ice and water shield at eaves and valleys; synthetic underlayment on remainder. Consider full-deck ice shield on low-slope sections.

Matching Underlayment to Roofing Material

Asphalt shingles: Standard synthetic or felt, plus ice protection where required. Metal roofing: High-temperature synthetic underlayment (not standard felt, which can stick to hot metal). Tile/slate: Two layers of underlayment for the extended installation timeline. Always follow the roofing manufacturer's requirements — using the wrong underlayment can void the warranty.