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Choosing Roofing Underlayment: Felt vs. Synthetic, and What the Code Recognizes
Underlayment is the cheapest layer on a roof and the one most often decided by whatever is on the truck. It is also the layer where the paperwork has changed recently: synthetic underlayment got a standard written for it in 2020, and it entered the IRC in the 2024 edition, which most of the jurisdictions in this directory have not adopted yet. Here is what each product is, which standard governs it, and what that means when you are comparing two bids.
Three products, two different jobs
The word "underlayment" gets used for three products that are not interchangeable.
- Asphalt-saturated felt and synthetic underlayment do the same job: a secondary water-resistive barrier across the whole roof deck, under the shingles, mechanically fastened. These two compete with each other.
- Ice and water barrier does a different job. It is a self-adhering membrane that bonds to the deck and seals around fastener penetrations, used at eaves, valleys and other vulnerable areas to stop water driven backwards by an ice dam. It is not a whole-roof substitute for field underlayment, and it is not priced like one.
A bid that says "underlayment included" has told you almost nothing. The useful question is which of the three, where on the roof, and to which standard.
Felt: ASTM D226, and what No. 15 and No. 30 mean
Asphalt-saturated felt is organic felt impregnated with asphalt. It is manufactured to ASTM D226, "Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing," in two weights: Type I, commonly called No. 15 felt, and the heavier Type II, No. 30 felt. A related standard, ASTM D4869, covers steep-slope felt specifically.
IRC Section R905.1.1 (Underlayment) requires labeled underlayment conforming to ASTM D226 Type I or II, or to D4869, beneath asphalt shingles. Felt's case is its track record and its price: it is code-recognized, cheap, available everywhere, and installs with ordinary cap nails. Its weaknesses are all about exposure. It absorbs moisture and can wrinkle or buckle if it gets wet before the shingles cover it, it tears more easily during installation, and it degrades faster under prolonged UV if the roof sits open.
Catalogued example: TAMKO No. 30 UL publishes ASTM D226 Type II.
One wrinkle worth knowing before you read any label. GAF Shingle-Mate is a fiberglass-reinforced asphalt felt rather than a conventional organic one, and GAF's data sheet says it "meets or exceeds the physical requirements of ASTM D226, Type II and ASTM D4869." That is a statement about physical requirements, not a claim of certified compliance to the standard, and the two are not the same thing. We record hedged wording like that as the manufacturer's own prose rather than as a standard the product carries.
Synthetic: ASTM D8257 is newer than most adopted codes
Synthetic underlayment is a woven polypropylene or polyethylene sheet. It is lighter than felt, covers more roof area per roll, resists tearing and wrinkling better if the deck is left exposed, and is usually less slippery underfoot during installation.
The part worth understanding is the paperwork. For years synthetics were sold against felt standards, because those were the standards the code named. ASTM D8257, "Standard Specification for Mechanically Attached Polymeric Roof Underlayment Used in Steep Slope Roofing," was published in 2020 as a dedicated performance standard written for polymeric sheet products. The 2024 IRC added D8257 alongside D226, D1970, D4869 and D6757 in Table R905.1.1(1) as an accepted underlayment type.
Two practical consequences follow, and they are the reason this guide exists:
- Code recognition depends on which IRC edition your jurisdiction has adopted. D8257 is in the 2024 table. A jurisdiction on the 2018 or 2021 IRC is working from a table that does not name it. Check the jurisdiction's own adopted code before assuming.
- Read whether the label claims certification or merely resemblance. Older felt standards do not test the properties that matter for a polymeric sheet, so which standard a synthetic cites tells you something — but how it cites it tells you more. GAF FeltBuster, a non-asphaltic polypropylene synthetic, is described on GAF's sell sheet as meeting "or exceeding the physical requirements of ASTM D226 Type II, ASTM D8257, and ASTM D4869 Types III and IV" — a broad list, but phrased as meeting physical requirements rather than as certified compliance. Atlas Summit 60 publishes ASTM D8257 without that hedge. Others answer with an evaluation report instead: CertainTeed RoofRunner carries ICC-ES ESR-4103, and Owens Corning ProArmor lists ICC-ES AC188 among its compliance documents. All three are legitimate answers; they are not the same answer.
None of this makes synthetic better or worse than felt. It makes the two harder to compare on a line item, which is exactly when it pays to ask.
Ice and water barrier is a different decision
Ice and water barrier is a self-adhering, polymer-modified bituminous sheet. Its rubberized asphalt underside bonds directly to the deck and seals around fasteners, which is what ordinary felt cannot do. IRC Section R905.1.2 permits it as an alternative to two cemented layers of underlayment in areas with a history of ice damming, and requires the sheet to comply with ASTM D1970. CertainTeed Vycor Ice & Water Shield is a catalogued example carrying D1970.
Whether your jurisdiction requires one, and how far up the roof, is a local question with genuinely contradictory answers between neighbouring cities. That map is its own guide: Where Code Requires an Ice and Water Barrier, and the per-jurisdiction cards are on the ice & water shield facet.
What to actually check on a bid
Four questions, in the order they matter:
- Which product, and over what area? Field underlayment across the deck is one line; ice and water barrier at eaves and valleys is another. If they are bundled into one word, ask for the split.
- What standard is the product labeled to? D226 Type I or II and D4869 are the felt standards; D8257 is the polymeric one; D1970 is the ice-barrier one. A product carrying an ICC-ES evaluation report is answering the same question a different way.
- What has your jurisdiction adopted? The standard named on the label only helps if the adopted code edition recognizes it. Most of the jurisdictions we cover carry their adopted code, quoted and sourced, on their permitting record; where we have not established it, the record says nothing rather than guessing. Start at the permitting hubs, or the 2024 I-Codes facet for the jurisdictions already on the newer editions.
- How long will the deck be open? This is where felt and synthetic genuinely differ in the field rather than on paper. A roof that will sit exposed through weather is the case synthetics are built for.
Related decisions that ride along with this one: roofing nails and wind zones for the fasteners that hold both the underlayment and the shingles, and wind and fire ratings explained for what the letter classes on a shingle wrapper do and do not mean.
Frequently asked questions
Is synthetic underlayment approved by code?
It depends on which code edition your jurisdiction has adopted. ASTM D8257 is the dedicated standard for mechanically attached polymeric steep-slope underlayment, published in 2020, and the 2024 IRC added it to Table R905.1.1(1) alongside the felt standards. A jurisdiction still on an earlier edition is working from a table that does not name D8257, so synthetics there are typically accepted on another basis such as an ICC-ES evaluation report. Check the jurisdiction’s own adopted code rather than assuming either way.
Is felt underlayment still allowed?
Yes. IRC Section R905.1.1 requires labeled underlayment conforming to ASTM D226 Type I or Type II, or to ASTM D4869, beneath asphalt shingles, and felt is what those standards describe. Felt has not been superseded; synthetic was simply added as another recognized option.
What is the difference between No. 15 and No. 30 felt?
They are the two weights defined by ASTM D226. Type I is commonly called No. 15 felt and Type II is the heavier No. 30. Both are code-recognized under IRC R905.1.1; the heavier sheet is more robust during installation and exposure.
Does ice and water barrier replace regular underlayment?
No. It does a different job. Ice and water barrier is a self-adhering membrane used at eaves, valleys and other vulnerable areas to resist water pushed backwards by an ice dam, and IRC Section R905.1.2 permits it in place of two cemented layers of underlayment where there is a history of ice damming, with the sheet required to comply with ASTM D1970. Field underlayment across the rest of the deck is still its own line item.
How do I check what an underlayment product actually complies with?
Read the manufacturer’s data sheet rather than the marketing page, and look for the standard designations: D226 Type I or II and D4869 for felt, D8257 for polymeric sheet, D1970 for ice barrier. Some products answer with an ICC-ES evaluation report number instead, which you can look up directly. Our product pages list the standards each catalogued product publishes, with the source document linked.