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What is roof underlayment?

The sheet you never see: rolled over the bare deck before the shingles go on, and the only thing between the deck and the rain if the shingles ever leave.

Updated September 12, 2026 · Every figure links to its source · Sources

DefinitionComponentAlso called: roofing felt, tar paper, synthetic underlayment

Underlayment — one or more layers of felt, sheathing paper, non-bituminous saturated felt or other approved material laid over the roof deck beneath the roof covering, as the 2021 IRC, Section R905.1.1 describes it, serving as a secondary barrier against water that gets past the covering; the IBC's definition of underlayment is the same, and the NRCA glossary records the term.

What it is

Three families cover nearly every roof. Asphalt-saturated felt — the traditional "tar paper", in Type I (once "15-pound") and heavier Type II ("30-pound") grades to ASTM D226 or D4869 — is cheap, breathable and easily torn. Synthetic underlayment, a woven polypropylene or polyethylene sheet to ASTM D8257, is lighter, stronger, lies flatter, stays walkable in the wet and does not wrinkle, and has become the default on new shingle roofs. Self-adhering polymer-modified bitumen sheet to ASTM D1970 — ice and water shield — bonds to the deck and seals around nails, and is used where water stands or nails cluster; the Asphalt Roofing Manufacturers Association publishes technical bulletins on all three.

The IRC, Section R905.1.1 lists the acceptable standards for each covering and sets how underlayment is laid: for asphalt shingles, one layer on slopes of four in twelve and steeper, two layers lapped half-width between two and four in twelve, with the sheets lapped, offset and fastened as the manufacturer directs. Metal panels, tile and wood shingles have their own lines in the same table, and metal roofs generally call for a high-temperature product; the directory's entry on underlayment for metal roofs explains that case.

Why it matters

Shingles are not a waterproof membrane; they are a shedding surface that relies on gravity, and wind-driven rain, ice-dam water and the days between tear-off and shingling all put water on the deck. The underlayment is what keeps that water out of the house — and after a windstorm that has stripped a slope, it is the roof. The IBHS FORTIFIED standard builds its sealed-roof-deck requirement on exactly this: a deck whose seams are taped or covered with a qualifying underlayment keeps most of the water out of a house that has lost its shingles.

It also matters for the warranty. Shingle manufacturers specify the underlayment their warranties assume, and the longer system warranties usually require the manufacturer's own; a quote that says "felt" when the warranty document says otherwise is worth a question.

What it costs and who does it

Underlayment is a line on the re-roof estimate priced by the square — synthetic a little more than felt, self-adhering several times either — and installed by the roofing crew immediately after the tear-off and any decking repair. It is not worth economising on: it is a small share of the job and the whole of the roof's second line of defence. Ask the estimate to name the product and where the self-adhering membrane runs. The directory's guide to a new roof installation shows where it sits in the sequence, and its nails-per-shingle entry covers the fastening that goes through it.

Frequently asked questions

Short answers; the sections above carry the detail and the sources.

Is synthetic underlayment better than felt?
For most shingle roofs, yes: it is stronger, lighter, flatter, safer to walk on and does not wrinkle or tear in wind before the shingles go on. Felt is cheaper and breathes more; some slate, tile and cold-climate assemblies still specify it. Either must meet the standard the IRC lists for the covering.
How many layers of underlayment does a roof need?
Under the 2021 IRC (Section R905.1.1), one layer under asphalt shingles on slopes of 4:12 and steeper, and two layers lapped half-width on slopes between 2:12 and 4:12. The ice barrier at the eaves is additional where ice damming has a history.
Can new shingles go on without underlayment?
Not under the IRC, which requires underlayment beneath every listed covering, and not under any shingle manufacturer’s warranty. A roofer who proposes skipping it is proposing a roof with no second line of defence.

See also

Next step

Find a roofer who does this

The underlayment is specified in one line of the estimate, and that line tells you how the contractor thinks. The directory lists roofing contractors by state and city.

Find a roofer for a shingle replacement

Sources

  1. 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)
  2. 2021 International Building Code, Chapter 15: Roof Assemblies and Rooftop Structures — International Code Council (ICC)
  3. Roofing glossary — National Roofing Contractors Association (NRCA)
  4. Technical bulletins on asphalt roofing — Asphalt Roofing Manufacturers Association (ARMA)
  5. FORTIFIED Roof: requirements and rated roof covers — Insurance Institute for Business & Home Safety (IBHS)