What Roof Underlayment Does
Underlayment is the layer installed directly on the roof deck before shingles go on. Its functions:
- Secondary water barrier — if shingles fail, blow off, or are damaged, underlayment prevents water from reaching the deck and structure
- Deck protection during installation — underlayment protects the deck from rain while shingles are being installed
- Code compliance — building codes in most jurisdictions specify minimum underlayment types and coverage
- Dimensional stability — helps shingles lay flat and uniform
Types of Underlayment
- 15 lb asphalt felt: The traditional minimum. Economical, but tears easily and wrinkles when wet. Suitable for re-roofing in mild climates where it will be quickly covered by shingles.
- 30 lb asphalt felt: Heavier and more durable than 15 lb. Better tear resistance. Still absorbs moisture and can wrinkle.
- Synthetic underlayment: Polypropylene or polyester construction. Lighter and stronger than felt, slip-resistant for workers, UV-stable if exposed temporarily. The current standard for most professional installations.
- Ice and water shield: Self-adhering waterproof membrane. Required at eaves in cold climates (typically 24 inches past the interior warm wall), in valleys, and around all penetrations. The only truly waterproof underlayment option.
Where Ice and Water Shield Is Required
Even in warm climates, ice and water shield is required or recommended at:
- Eaves — minimum 24 inches past the interior wall in cold climates; often required at eaves everywhere
- Valleys — where two roof slopes meet and concentrate water flow
- Around chimneys, skylights, and roof vents
- At rake edges and transition areas
Related: Ice Dams and Roof Damage
What to Ask Your Roofer
When reviewing a roofing contract or estimate, ask specifically:
- What underlayment type will be used on the main field of the roof?
- Where will ice and water shield be installed?
- Is this included in the quoted price, or are these line items quoted separately?
- Does the underlayment selection meet or exceed local code requirements?
Underlayment is invisible once shingles are installed. A contractor cutting costs on underlayment saves little but creates significant long-term risk — and the homeowner cannot verify it after the fact.
Frequently Asked Questions
What is roof underlayment and why does it matter?
A water-resistant layer installed on the deck beneath shingles. It serves as a secondary barrier if shingles are damaged, and protects the deck during installation. Without adequate underlayment, a single damaged shingle can allow significant water entry.
What is the difference between felt and synthetic underlayment?
Felt is traditional asphalt-saturated mat — economical but tears easily and wrinkles when wet. Synthetic is lighter, stronger, and slip-resistant. Most professional roofers now default to synthetic for better performance.
What is ice and water shield and do I need it?
A self-adhering fully waterproof membrane required at eaves in cold climates and around all penetrations. Unlike felt or synthetic, it is fully waterproof. Your contractor should include it at eaves and all penetrations at minimum.
Find a Roofer Who Uses Quality Materials
This page is for general informational purposes only. Building code requirements for underlayment vary by jurisdiction and climate zone. Consult a licensed roofing contractor and your local building department for requirements in your area.