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Roof Underlayment: Felt vs. Synthetic vs. Self-Adhered

  • Published November 7, 2026

Felt, synthetic and self-adhered underlayment all serve as the backup water barrier under shingles, and which one you need depends on your roof’s slope and wind exposure more than on the material. Sections between 2:12 and 4:12 need two layers or another approved material, and high-wind areas can use a sealed-deck setup that limits water entry after shingles blow off.

The standard printed on the roll tells you more than the material name does. A quote that says “synthetic” or “felt” and nothing else has not told you what is going on your roof.

What Underlayment Does When Shingles Fail

Shingles shed water; they are not watertight. Wind-driven rain, ice dams and missing or damaged shingles all put water under the shingle layer, and the underlayment is what that water meets next. Its job is to carry the water down to the eave on top of the deck instead of through it.

That job matters most when the shingles are gone. FEMA’s Technical Fact Sheet No. 7.2, Roof Underlayment for Asphalt Shingle Roofs, from December 2010 and written for high-wind coastal areas, ranks three underlayment approaches by how well they hold up: “Option 1 provides the greatest reliability for long-term exposure,” while “Option 3 provides limited protection and is advocated only in areas with a modest population density and a design wind speed less than or equal to 110 mph.” The ranking makes the point: when a storm strips shingles, the underlayment becomes the roof until repairs are made.

Underlayment also protects the deck during installation, between tear-off and shingles. Where it sits in the full sequence from deck to ridge is covered in where underlayment fits the installation sequence, and where it sits among the other components is in the water barriers under the shingles.

Felt, Synthetic and Self-Adhered: What the Standards Say

Name the standard every time you name a type. The standard is what you can hold a product to.

Felt. Asphalt-saturated organic felt has two ASTM specifications. ASTM’s page for D226/D226M-25, the edition active since July 2025, “covers asphalt-saturated organic felts” used in roofing and waterproofing. ASTM’s page for D4869/D4869M, reapproved in 2025, “covers asphalt-saturated organic felt for use as an underlayment with steep slope roofing.” A felt roll should name one of the two.

Read the designation the way ASTM writes it. In D226/D226M-25, the letters and first number name the specification and the number after the dash is the edition year; in D4869/D4869M-16a(2025), the year in parentheses marks a reapproval. A roll that cites an earlier edition still cites the same specification, so the designation itself matters more than the year printed after it.

Synthetic. Synthetic underlayments are polymer sheets sold under product names, and the name tells you nothing about how the product was tested. Ask the contractor for the standard or the code evaluation report printed on the roll or the product data sheet, and confirm with your building department that it accepts that product as underlayment.

Self-adhered. Self-adhered membrane has its own specification. ASTM’s D1970/D1970M-25, active since December 2025, covers self-adhering polymer modified bituminous sheet used as steep roofing underlayment for ice dam protection; the 2021 edition’s scope describes it as “intended for use as underlayment on roof eaves.” It is sticky on the back and bonds to the deck. Beyond the eaves, some contractors also use it in valleys and on low-slope sections.

Type Standard to look for What it is Where it is specified Role in sealed-deck options
Felt ASTM D226 or ASTM D4869 Asphalt-saturated organic felt Field underlayment; two layers on 2:12 to 4:12 sections Two layers of 30# felt is one IBHS deck-sealing method
Synthetic The standard or evaluation report printed on the roll Polymer sheet sold under a product name Field underlayment where the product's evaluation and the local code allow Ask how it fits the deck-sealing method chosen
Self-adhered ASTM D1970 Self-adhering polymer modified bituminous sheet Eaves for ice dam protection; sometimes also valleys and low-slope sections A fully adhered membrane over the deck is another IBHS method

The ice barrier at the eaves is a separate code question with its own reach rule, covered in the ice barrier at the eaves.

Low-Slope Sections Need Two Layers

Slope changes the requirement. The City of Bloomington, Minnesota reroofing sheet, dated June 2023 and based on the 2020 Minnesota Residential Code, states: “For roof slopes from 2:12 to 4:12, two layers of underlayment or other approved material is required.” The rule traces to the International Residential Code (IRC). That model code reaches your house only through the edition your state or city adopted, so ask your building department whether its version says the same.

The reason is water speed. On a shallow slope, water moves slowly and can be pushed uphill under shingles by wind, so a second layer gives it another barrier before it reaches the deck.

Check your roof for any slope in that range: porch roofs, shed dormers and additions are the places to look. If the quote treats the whole roof as one slope, ask whether those sections get two layers or an approved substitute, and which.

High-Wind Areas: Sealed-Deck Underlayment Options

In hurricane-prone and other high-wind areas, the goal shifts from shedding water to keeping the deck sealed if the shingles are torn off. DOE’s guide to roof deck sheathing and sealing for sloped roofs, last updated in September 2020, states that “IBHS Fortified allows three methods for sealing the roof deck”: a fully adhered membrane, taped seams, or two layers of felt. The felt method is specific: “Cover the roof decking with two layers of 30# building felt.”

Notice that two layers of felt are on that list. “Synthetic always beats felt” is not how the program sees it; each method is accepted when it is installed to its own details.

Outside high-wind areas, these methods are optional upgrades. If a bidder offers one, ask for it as a separate line item so you can see what it adds to the price.

A sealed deck is part of the IBHS FORTIFIED standard, which requires it. IBHS publishes that standard in dated versions, so ask the roofer which version applies to your job before relying on the list above; what the rest of the standard asks for, and which checklist version it reflects, is in the FORTIFIED roof standard. Why sealed decks matter in hurricane construction, and how they fit with deck nailing and edge details, is covered in why sealed roof decks matter.

Reading the Underlayment Line in Your Quote

With the types, the slope rule and the wind options in hand, you can read the underlayment line on a quote. A line that says only “synthetic underlayment” leaves out everything you need to compare bids.

Hypothetical example: a complete line reads more like “Field underlayment: [product name], meeting [standard or evaluation report]; two layers on the 3:12 porch roof; ASTM D1970 ice barrier at all eaves; deck seams taped where FORTIFIED is specified.”

Use this checklist to fill the gaps:

  • Type and product name for the field underlayment
  • The standard it meets: ASTM D226 or D4869 for felt, ASTM D1970 for self-adhered, and the printed standard or evaluation report for synthetic
  • Layer count on every section between 2:12 and 4:12
  • The ice barrier at the eaves, if your area requires one, listed as its own line
  • Any deck-sealing method, if you are in a high-wind area or pursuing FORTIFIED
  • Installation per the maker’s instructions, including laps and fastening
  • The maximum time the product may be left exposed before shingles cover it, from the data sheet

Ask the contractor to fill in anything missing in writing before you sign. Then ask for photos during the job: each slope after the underlayment goes down and before shingles cover it, including the low-slope sections and the eaves. Underlayment disappears as soon as shingles go on, so those photos are the only record of the layer count. If two bids name different products, compare them on the standard and the layer count, not on the brand.

Some roofs need a different answer. Under metal roofing, ask the panel maker which underlayment its installation instructions require. Under tile, the underlayment carries more of the long-term job, which is covered in underlayment under tile.

Which Underlayment Should You Request?

Match the underlayment to your roof’s slope and wind exposure, then require the standard in writing. On slopes of 4:12 and steeper, the two-layer rule above does not apply; ask for a field underlayment that both the shingle maker’s instructions and your building department accept, such as felt meeting D226 or D4869 or a synthetic with a printed standard. On any section between 2:12 and 4:12, require two layers or an approved material, named in the quote. In a high-wind area, ask for one of the IBHS deck-sealing methods and have the quote say which.

If a contractor pushes one material as always better, ask which standard it meets and how it fits your slope and wind exposure. The answer to those two questions decides the request, and the next step is to send your contractor the checklist above and get each line filled in.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

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