Roof Underlayment Explained: Felt, Synthetic and Ice-and-Water Shield

Roof underlayment is the sheet material laid over the wood deck before the shingles go on. ARMA, the asphalt shingle makers' trade group, says asphalt shingles should not be installed directly on a wood deck and that the deck should first be covered by underlayment ¹. It is a second line of defense: it shields the deck from wind-driven rain that gets past the shingles, keeps the deck dry until the shingles are on, and, ARMA notes, is a required component of a fire-classified roof assembly ¹.

There are three common kinds: asphalt-saturated felt, synthetic sheets, and self-adhering membranes often called ice-and-water shield. They are often combined: ARMA notes that self-adhering membranes are most frequently used as ice dam protection at the eaves ¹, with felt or synthetic covering the rest of the deck.

Felt underlayment

Asphalt-saturated organic felt is the traditional choice. ARMA notes it is water-resistant because of its asphalt, but its felt base allows some vapor permeability and moisture absorption ¹. Two standards cover it ¹:

• ASTM D226, with Type I, commonly called No. 15 felt, and the heavier Type II, often called No. 30 felt. Perforated felts under this standard are made for low-slope built-up roofs and are not suitable under shingles.
• ASTM D4869, the first standard written specifically for steep-slope underlayment, with four types differentiated mainly by weight and a water-shedding "shower test."

NRCA, the roofing contractors' association, recommends a single layer of No. 15 felt for asphalt shingles on slopes of 18 degrees or more, and at least two layers of No. 15 on slopes between 14 and 18 degrees. It says No. 30 is appropriate under a heavier shingle with a long projected service life ². A double layer is not the same as one heavier layer: NRCA explains that a double layer overlaps by 19 inches, while a single layer overlaps by 2 inches ².

Synthetic underlayment

Synthetic underlayments are mechanically fastened polymer sheets, covered by ASTM D8257. ARMA notes they are lighter than saturated felts yet typically cover a larger roof area per roll ¹. A related product, ASTM D6757 felt, uses inorganic fibers in place of organic felt, which allows less asphalt and may improve dimensional stability ¹. Codes do not accept every product everywhere; ARMA cautions that some underlayments may not be approved by the codes in certain areas ¹.

Self-adhering membranes (ice-and-water shield)

Self-adhering polymer-modified bitumen membranes, covered by ASTM D1970, stick to the deck and seal around the shanks of the nails driven through them. ARMA says that is why water backing up from an ice dam is unlikely to get through, and that these membranes are also used on ridges, hips and valleys and around vents, skylights, pipes and chimneys ¹. ARMA adds a caution: self-adhering membranes act as vapor retarders, so if one covers the whole deck, attic ventilation matters even more ¹. See roof ventilation explained.

Where the code calls for each

Most building departments enforce a version of the International Residential Code, and ARMA notes that section R905.1.1 of the 2024 IRC sets underlayment requirements that vary by roof covering ¹. Local editions and amendments differ, which a few building departments' handouts show:

• Ice barrier at the eaves. In areas with a history of ice forming along the eaves, the 2015 IRC text calls for an ice barrier of two layers of underlayment cemented together or a self-adhering membrane, from the lowest roof edge to a point at least 24 inches inside the exterior wall line, and on slopes of 8 in 12 or steeper, at least 36 inches up the slope from the eave, as quoted by Hilltown Township, Pennsylvania ⁵. Firestone, Colorado, under the 2018 IRC, lists ice barriers as required, to at least 24 inches inside the exterior wall line ³.
• Low slopes. Firestone requires double underlayment on pitches from 2 in 12 up to 4 in 12 ³. ARMA describes the general rule as a double layer of fastened underlayment or a single layer of self-adhering underlayment on those slopes ¹. Meriden, Connecticut, under the 2022 Connecticut State Building Code, calls for continuous ice-and-water barrier from eave to ridge on slopes from 2 in 12 up to 4 in 12 ⁴.
• Valleys and deck joints. Meriden calls for a 36-inch-wide strip of ice-and-water barrier in valleys and a 4-inch strip over all plywood joints in the deck ⁴. The joint-strip rule does not appear in the other two handouts quoted here.

Your roofer should know the local version, and the permit office can confirm it. Our article on whether you need a permit to replace a roof covers how to check.

How it meets the drip edge

The order of layers at the roof edges matters for water. ARMA's general guideline is to put the drip edge under the underlayment along the eaves and over the underlayment on the rakes ⁸, and Meriden's code handout says the same ⁴. ARMA notes that where a self-adhering membrane is used at the eave, some makers and codes permit the drip edge on top of it, with an extra strip of membrane beneath ⁸. The roof flashing guide covers drip edge and the other metal pieces.

What it costs

Fixr reports that standard roofing felt is typically included in base roof pricing, and that ice-and-water shield adds about $0.50 per square foot, roughly $50 per square (updated February 4, 2026) ⁶. For metal roofs, Fixr lists underlayment options from $0.05 to about $0.50 per square foot including materials and labor (updated January 23, 2026) ⁷. On a quote, look for the underlayment brand or type, the ASTM standard it meets, where the self-adhering membrane goes, and how far up from the eave.

Questions to ask the roofer

• Which underlayment is quoted for the field of the roof, and which standard does it meet?
• Where will self-adhering membrane go: eaves, valleys, around chimneys and skylights?
• How far up from the eave does the ice barrier run, and does that meet the local code?
• Does the shingle maker require a particular underlayment for its warranty?
• If the old roof is coming off, is new underlayment over the whole deck included? See tear-off vs. roof-over.

Underlayment is one layer of the roof system; types of roofing materials covers the covering on top. When you are ready, you can compare quotes from roofing companies and ask each to list its underlayment line by line.

About these figures

• ¹ Reported: ARMA technical bulletin (May 2025) on underlayment types, ASTM D226, D4869, D6757, D8257 and D1970, the 2024 IRC reference and the 2 in 12 to 4 in 12 rule, checked November 11, 2026.
• ² Reported: NRCA consumer answers on No. 15 and No. 30 felt, slope thresholds of 14 and 18 degrees, and the 19-inch and 2-inch overlaps, checked November 11, 2026.
• ³ Official record: Town of Firestone reroofing requirements under the 2018 IRC, ice barrier to 24 inches inside the wall line and double underlayment from 2 in 12 to 4 in 12, checked November 11, 2026.
• ⁴ Official record: City of Meriden handout under the 2022 Connecticut State Building Code on underlayment, 4-inch joint strips, 36-inch valley strips, low-slope ice barrier and drip edge order, checked November 11, 2026.
• ⁵ Official record: Hilltown Township's quotation of 2015 IRC section R905.1.2 on ice barriers (24 inches inside the wall line, 36 inches on slopes of 8 in 12 or more), checked November 11, 2026.
• ⁶ Reported: Fixr roof replacement cost guide, ice-and-water shield at about $0.50 per square foot or $50 per square, updated February 4, 2026, checked November 11, 2026.
• ⁷ Reported: Fixr metal roof cost guide, underlayment at $0.05 to $0.50 per square foot, updated January 23, 2026, checked November 11, 2026.
• ⁸ Reported: ARMA technical bulletin (May 2025) on drip edge placement at eaves and rakes, checked November 11, 2026.