Roofing underlayment is the water-resisting layer installed directly on the roof deck, beneath the shingles or panels, and there are three main types: asphalt-saturated felt, synthetic (polypropylene or polyethylene), and self-adhered peel-and-stick membrane. Which one you need is set by roof slope, climate, and code, not preference alone. Asphalt shingle installations are governed by IRC Section R905.1.1, which requires a labeled underlayment meeting ASTM D226, D4869, D1970, or D6757. This guide breaks down each underlayment material, where each is required, and what it costs per square in 2026.
What is roofing underlayment and what does it do?
Roofing underlayment is a rolled sheet material laid over the roof deck as a secondary water barrier under the primary roof covering. The shingles or panels are the first defense; underlayment is the backup that keeps wind-driven rain, ice-dam meltwater, and construction-phase weather off the wood deck. On asphalt shingle roofs it is required by code, not optional.
Two properties separate the types: water-resistant versus waterproof. Felt and standard synthetic underlayments are water-resistant, meaning they shed water but do not seal around fasteners. Self-adhered membranes are waterproof because they bond to the deck and seal around every nail. That distinction drives where each material belongs on the roof.
What are the three types of roof underlayment?
The three types of roof underlayment are asphalt-saturated felt, synthetic underlayment, and self-adhered (peel-and-stick) membrane. Felt is the oldest and cheapest, synthetic is the lightweight modern default for full-deck coverage, and self-adhered membrane is the waterproof product reserved for leak-prone zones. Most 2026 residential roofs use synthetic across the field with a self-adhered strip at the eaves and valleys.
Asphalt-saturated felt (15-lb and 30-lb)
Felt underlayment is a paper or fiberglass mat saturated with asphalt, sold in two weights: No. 15 and No. 30. It is water-resistant, not waterproof, and complies with ASTM D226 or D4869. No. 30 felt is thicker, tears less during installation, and holds up better in wind than No. 15. Felt is the lowest-cost option but the heaviest per roll, and it cannot stay exposed more than a day or two before it dries, buckles, or leaches oils in heat.
Synthetic underlayment (polypropylene or polyethylene)
Synthetic underlayment is a woven or spun polymer sheet, usually polypropylene or polyethylene, laminated for strength. It complies with ASTM D6757 and has largely replaced felt on new residential work. Compared with felt, it is lighter, has far higher tear strength, resists fungal growth, and can stay exposed for weeks to months depending on the brand rating. It is mechanically fastened with cap nails and is the standard field underlayment on most 2026 asphalt and metal roofs.
Self-adhered (peel-and-stick) membrane
Self-adhered underlayment is a rubberized-asphalt or polymer membrane with a pressure-sensitive adhesive back, protected by a release film. It complies with ASTM D1970 and is the only fully waterproof underlayment type because it bonds to the deck and self-seals around nail penetrations. This is the category that includes ice-and-water barrier. It is the most expensive per square and is used in targeted zones rather than across the whole roof, except on low-slope or high-value assemblies.
How do the underlayment types compare on cost and performance?
The three underlayment types diverge sharply on price, weight, exposure rating, and waterproofing. Felt is cheapest but shortest-lived on the deck; synthetic costs more but installs faster and tolerates weather delays; self-adhered membrane costs the most but is the only one that seals around fasteners. Prices below are per roofing square (100 square feet), material only, at 2026 supply-house levels.
| Underlayment type | ASTM standard | Water resistance | Exposure rating | 2026 cost per square (material) |
|---|---|---|---|---|
| 15-lb felt | D226 / D4869 | Water-resistant | Up to about 1-2 days | $5 to $10 |
| 30-lb felt | D226 / D4869 | Water-resistant | Up to a few days | $10 to $18 |
| Synthetic | D6757 | Water-resistant | Weeks to 6 months (by brand) | $12 to $30 |
| Self-adhered (peel-and-stick) | D1970 | Waterproof, self-sealing | Weeks to months (by brand) | $40 to $70 |
Installed cost runs higher once labor is added, and self-adhered membrane is slower to apply, so a full-deck peel-and-stick job carries both a material and a labor premium. For a deeper cost and lifespan breakdown of the field-layer choice, see our felt vs synthetic underlayment comparison.
When is each underlayment type required?
Underlayment type is dictated by roof slope, climate zone, and roof covering, with the baseline set by IRC R905.1.1 for asphalt shingles. Synthetic or felt covers the field; self-adhered membrane is required or strongly recommended at eaves in cold climates, in valleys, and on low-slope sections. The table below maps common roof conditions to the type that belongs there.
| Roof condition | Recommended underlayment | Why |
|---|---|---|
| Standard slope 4/12 or steeper, mild climate | Single layer synthetic (or 15-lb felt) | Field-layer water resistance is enough; shingles shed the rest |
| Low slope 2/12 to 4/12 | Double-layer underlayment per R905.1.1 | Code requires two-ply application on reduced slopes |
| Eaves in a cold or ice-dam climate | Self-adhered membrane at eaves | Seals against meltwater backing up under shingles |
| Valleys, penetrations, low-slope tie-ins | Self-adhered membrane | High-flow, leak-prone areas need a waterproof seal |
| High-wind coastal zones | Self-adhered or high-wind-rated synthetic | Resists uplift and wind-driven rain intrusion |
The exact ice-barrier requirement, including how far up the slope the membrane must extend, is set at the local level and detailed in the ice dam protection membrane code requirements under IRC R905.1.2.
What does IRC R905.1.1 require?
IRC Section R905.1.1 requires asphalt shingle roofs to use an underlayment labeled to ASTM D226, D4869, D1970, or D6757, applied and attached per the code tables. For slopes from 2/12 up to 4/12, a double layer of underlayment is required. For slopes 4/12 and steeper, a single layer is permitted. Local amendments in high-wind and cold regions add self-adhered membrane requirements at eaves and rakes on top of this baseline.
Which underlayment is best for your roof?
The best roof underlayment for most 2026 residential jobs is synthetic across the field, paired with a self-adhered membrane at the eaves and valleys. This combination gives high tear strength and long exposure tolerance where the whole deck is covered, plus a waterproof seal exactly where leaks start. Felt still makes sense on tight budgets and short-timeline jobs where the roof covering goes on the same day.
- Choose the field layer. Synthetic for durability and weather delays; 15-lb or 30-lb felt to cut material cost on a same-day install.
- Add the waterproof zones. Self-adhered membrane at eaves in cold climates, in every valley, and around penetrations.
- Check the slope. If any section falls between 2/12 and 4/12, plan for the double-layer application R905.1.1 requires.
- Match the covering. Metal and tile often call for high-temperature-rated underlayment; confirm the manufacturer spec.
Underlayment is one line in a much larger material picture; see our full roofing materials list for how it fits with decking, shingles, and flashing. For the waterproof zones specifically, our guides on peel-and-stick underlayment and ice and water shield cover product picks and install detail.
Frequently asked questions
What are the three types of roofing underlayment?
The three types are asphalt-saturated felt, synthetic underlayment, and self-adhered peel-and-stick membrane. Felt is the cheapest and heaviest, synthetic is lightweight with high tear strength and long exposure tolerance, and self-adhered membrane is the only waterproof type that seals around nails. Most modern roofs combine synthetic in the field with self-adhered membrane at eaves and valleys.
Is felt or synthetic underlayment better?
Synthetic is better for most jobs because it is lighter, resists tearing, and can stay exposed for weeks, while felt can only sit a day or two before it buckles or dries out. Felt still wins on upfront material cost, running about $5 to $18 per square versus $12 to $30 for synthetic, so it fits tight budgets and same-day installs.
Do I need ice and water shield everywhere?
No. Ice and water shield, a self-adhered membrane, is a targeted product used at eaves in cold climates, in valleys, and around penetrations, not across the whole deck on standard-slope roofs. Full-deck coverage is generally reserved for low-slope sections or high-value assemblies. Local code sets the exact eave coverage, often extending 24 inches inside the exterior wall line.
How much does roof underlayment cost per square?
In 2026, material-only pricing runs roughly $5 to $10 per square for 15-lb felt, $10 to $18 for 30-lb felt, $12 to $30 for synthetic, and $40 to $70 for self-adhered peel-and-stick membrane. A square covers 100 square feet. Installed cost is higher once labor is added, and self-adhered membrane carries the steepest labor premium because it is slower to apply.
What underlayment does code require for asphalt shingles?
IRC R905.1.1 requires asphalt shingle roofs to use an underlayment labeled to ASTM D226, D4869, D1970, or D6757. Slopes from 2/12 to 4/12 require a double layer; slopes 4/12 and steeper allow a single layer. Cold and high-wind jurisdictions add self-adhered membrane requirements at eaves and rakes, so always confirm local amendments.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.