Peel and Stick Underlayment vs Felt for Steep Slope

Why this matters

Underlayment is the second line of defense once shingles fail or wind drives water under them. Choice between #15 organic felt, #30 organic felt, synthetic underlayment, and full coverage peel and stick (also called self adhered underlayment, or SAU) drives both cost and life expectancy. On a steep slope (4:12 or greater) the IRC permits all four, but the code minimum, the warranty requirement, and the right answer for the climate are often three different things.

What the code actually requires

IRC 2021 R905.1.1 sets the underlayment requirement at one layer of ASTM D226 Type I (#15 felt), ASTM D4869 Type I, or ASTM D8257 (synthetic) on slopes 4:12 and greater. On slopes between 2:12 and less than 4:12, two layers shingle lapped are required. Below 2:12, asphalt shingles are not permitted at all (a low slope membrane is required).

R905.1.2 adds an ice barrier requirement in areas with a history of ice forming along the eaves and backing water up, as designated by the jurisdiction in Table R301.2. It is a local designation, not a temperature formula in the code text, so check the adopting jurisdiction rather than guessing off a climate map. The ice barrier must extend from the eave edge to a point at least 24 inches inside the exterior wall line of the building. This is typically peel and stick (ice and water shield) compliant with ASTM D1970.

So at code minimum: one layer of felt or synthetic on the field, ice and water shield at the eave in cold climates, around penetrations, and in valleys.

Asphalt felt: the legacy default

#15 felt is approximately 11 to 15 lb per 100 sq ft, made from cellulose or fiberglass felt saturated with asphalt. #30 felt is roughly twice the weight. They are NOT the same ASTM class: #15 is ASTM D226 Type I, #30 is ASTM D226 Type II. When a spec or a code table calls for Type II, #15 does not satisfy it. Properties:

• Cheap (lowest material cost of any option).
• Tears under foot traffic, especially when wet or hot.
• Wrinkles when exposed to humidity, telegraphing through finished shingles.
• 30 day maximum exposure per most manufacturer instructions before water absorption degrades it.
• Wind resistance limited; pulls free from cap nails in moderate winds during install.

#30 felt is the right call for a quick reroof in a stable climate with a same day shingle install. It is rarely the right call for any job where the deck might sit dried in for more than a couple of days.

Synthetic underlayment: the modern standard

Polypropylene or polyester scrim with a polymer coating. Major brands: GAF Tiger Paw / Deck Armor, CertainTeed Roofers Select / DiamondDeck, Owens Corning Deck Defense / RhinoRoof U20, IKO Stormtite, Atlas Summit, Tamko TW Underlayment.

Properties:

• Light: typically 4 to 8 lb per square, roughly half the weight of #15 felt and a quarter of #30 for the same coverage.
• High tear strength; survives heavy foot traffic on steep pitches.
• Walkable surface; non slip pattern molded in.
• UV exposure rating typically 30 to 180 days depending on product. Tiger Paw is 180; some are 90.
• Does not wrinkle, does not absorb water, does not telegraph through shingles.
• Higher material cost than felt but lower than peel and stick.
• Cap nailed (1 inch plastic cap nails) at the manufacturer specified spacing, typically 12 inch on field and 6 inch on overlaps.

Synthetic is now the field default on nearly every reroof in the US. It is what major shingle manufacturers spec for full system warranty coverage.

Peel and stick (self adhered underlayment): the premium / problem solver

ASTM D1970 compliant SAU is a polymer modified asphalt sheet with a release liner backing. Originally developed for ice damming protection (Grace Ice and Water Shield, the original brand), it now ships in lighter "high temp" versions for full deck coverage:

• GAF WeatherWatch (eave / valley) and FeltBuster / StormGuard.
• CertainTeed WinterGuard HT and DiamondDeck SBS / WaterTight.
• Owens Corning WeatherLock G / WeatherLock Mat.
• Henry Blueskin / Polyglass Polyflex SA / Soprema Soprastick.

Properties:

• Self sealing around fasteners (the rubberized asphalt closes around the nail shank).
• Watertight even before shingles are installed.
• Long exposure rating: high temp variants typically 90 to 180 days.
• Heaviest weight and highest cost.
• One way street: hard to remove or reposition once stuck.
• Must be installed on clean, dry, primed deck (some products require primer on green lumber or dusty OSB).

Decision matrix

Compatibility gotchas

Standard ice and water shield is asphalt based and does not belong in direct contact with single ply membrane. With PVC it is plasticizer migration, the plasticizer leaves the sheet into the asphalt and the PVC goes brittle. With EPDM it is the reverse direction, the rubber swells and softens in contact with asphalt. TPO is the least reactive of the three but most manufacturers still prohibit direct contact. Use the manufacturer's compatible peel and stick, or a separator sheet, when transitioning a steep slope into a low slope membrane.

Under metal panels, use the high temperature version of the SAU. A metal panel runs hotter at the deck than a shingle roof does, and standard SAU that exceeds its service temperature softens, re bonds to itself or to the underside of the panel, and telegraphs ghost lines through the finish. Do not eyeball this: read the service temperature printed on the product data sheet, confirm it exceeds the panel manufacturer's stated deck temperature, and keep the pair of data sheets in the job file.

Synthetic underlayment under metal roofing: confirm the synthetic is rated for metal contact. Some products specifically prohibit metal roof use; the slip surface is too aggressive and the panel walks during thermal cycling.

Wind and warranty leverage

Most major shingle manufacturers offer extended wind warranties (130 mph, sometimes 150 mph) only when the full system includes:

• Their starter strip product (not field shingles cut and reversed).
• Their underlayment, often synthetic or peel and stick.
• Their hip and ridge accessory.
• Their ventilation product.

Using a generic underlayment with a system shingle drops you back to the shingle's base limited wind warranty, which is materially lower than the enhanced number in the brochure. The base and enhanced figures vary by manufacturer and product line, so read the actual warranty document for the shingle you are installing rather than quoting a number from memory. On a coastal job, the enhanced warranty is the value proposition.

References

• IRC 2021 R905.1 (Underlayment) and R905.1.2 (Ice Barriers).
• ASTM D226 / D226M Standard Specification for Asphalt Saturated Organic Felt Used in Roofing and Waterproofing.
• ASTM D4869 / D4869M Standard Specification for Asphalt Saturated Organic Felt Underlayment Used in Steep Slope Roofing.
• ASTM D1970 / D1970M Standard Specification for Self Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Slope Roofing Underlayment for Ice Dam Protection.
• ASTM D8257 Standard Specification for Mechanically Attached Polymeric Roof Underlayment Used in Steep Slope Roofing.
• GAF, CertainTeed, Owens Corning, IKO underlayment installation instructions.
• NRCA Roofing Manual: Steep Slope Roof Systems (current edition).