Quick definition
A water-resistant layer installed over the roof deck and beneath the roof covering. It gives secondary protection against water that gets past the primary roofing material.
Shingles, tile and metal shed most of the water that hits a roof, but not all of it. Underlayment is the layer underneath that catches what gets past them, and it keeps the deck dry while the roof is being installed.
What is underlayment?
Underlayment is a sheet material rolled out over the roof deck before the roof covering goes on. It is lapped shingle-fashion, with each upper course overlapping the one below, so water runs down and off rather than under the laps.
What does underlayment do?
- Acts as a secondary water barrier when wind-driven rain, ice or damage gets past the roof covering.
- Protects the deck from rain during installation, before the roofing is finished.
- Separates the roof covering from the deck.
- Forms part of the tested assembly behind many fire and wind ratings, which is why manufacturers specify which underlayment to use with their products.
What are the types of underlayment?
| Type | What it is | Strengths | Watch for |
|---|---|---|---|
| Asphalt-saturated felt | An organic or fiberglass mat soaked in asphalt, sold in weights such as #15 and #30 | Low cost, long track record | Tears more easily and can wrinkle when wet |
| Synthetic | Woven or spun polypropylene or polyethylene | Light, strong, tear-resistant, safer to walk on | Each product has a limit on how long it can be left exposed to sun |
| Self-adhered membrane | A rubberized-asphalt sheet with a sticky back, often called ice and water shield | Seals around nails; used where leaks are most likely | Costs more; hard to remove at the next replacement |
Where is underlayment installed?
Felt or synthetic underlayment covers the whole deck. Self-adhered membrane goes where water concentrates or backs up: along the eaves in cold climates, in valleys, and around chimneys, skylights and other penetrations. Some roofs, especially in high-wind or low-slope areas, are covered entirely with self-adhered membrane.
What problems happen with underlayment?
- Wrong lap direction: a lower sheet lapped over an upper one channels water under the lap.
- Wrinkles and buckles: felt that gets wet before it is covered can telegraph ripples through the shingles.
- Too much sun: underlayment left exposed longer than the manufacturer allows can degrade.
- Tears and loose fasteners: from foot traffic or wind before the roofing goes on.
- Gaps at valleys and penetrations: the places it matters most.
Do building codes require underlayment?
What it means for you
What Underlayment means for homeowners
You won't see underlayment once the roof is on, so check the quote. It should name the type (felt, synthetic or self-adhered) and where self-adhered membrane will go, such as valleys and eaves.
Frequently asked questions
Sources
- Underlayment and shingle manufacturer installation instructions. Technical Reference.
Tier 1 authority 路 manufacturer - The NRCA Roofing Manual: Steep-slope Roof Systems. NRCA.
Tier 2 authority 路 industry - Residential Asphalt Roofing Manual. ARMA.
Tier 2 authority 路 industry - International Residential Code, Chapter 9: Roof Assemblies. International Code Council.
Tier 1 authority 路 code