Roofing Underlayment Guide: Felt, Synthetic, and Self-Adhering

Underlayment is the layer between the roof deck and the visible covering. It catches water that gets past the covering and gives the roof a second chance to shed it. Choose the type after you know the slope, the covering, and the climate, because the wrong type can fail before the covering does.

Which underlayment should you use?

For most asphalt-shingle roofs on slopes of 4:12 and steeper in mild climates, one layer of synthetic underlayment or #30 felt works. Match the harder cases to a tougher type instead of upgrading everything blindly.

What are the three underlayment types?

Asphalt-saturated felt

Felt underlayment is mat saturated with asphalt. Rolls are sold as #15 and #30, which name the nominal weight per 100 square feet of material. ASTM D226 covers asphalt-saturated organic felt used in roofing and waterproofing. ASTM D4869 covers asphalt-saturated organic felt underlayment made for steep-slope roofing and adds a liquid-water test that D226 lacks. ASTM D6757 covers felt that contains inorganic (fiberglass) fibers for steep-slope roofing, which resists wrinkling better.

Felt is the cheapest option and has decades of field history. It tears more easily than synthetic, absorbs some moisture, and wrinkles when wet or overheated. Wrinkles can telegraph through shingles. Unrated felt varies in asphalt saturation, so ASTM-rated felt is the safer pick when the drawings name a grade.

Synthetic underlayment

Synthetic underlayment is made from woven or non-woven polypropylene or polyethylene. It does not absorb water, so it stays flat through wet spells and temperature swings. A roll of synthetic covers much more area than a roll of felt at a fraction of the weight, which means fewer trips up the ladder and faster installation.

Many synthetics have a textured, slip-resistant surface and printed lap lines. Every product has a printed UV exposure limit. A synthetic left in sun past its limit becomes brittle.

Self-adhering membrane

Self-adhering underlayment is a polymer-modified bitumen sheet with a sticky back, sold as peel-and-stick. ASTM D1970 covers this type for ice-dam protection on steep roofs. The adhesive bonds to the deck and seals around nails and staples, so it keeps water out where wind-driven rain or ice dams push water sideways under the covering.

It is the most expensive type and the least forgiving to install. Once it touches the deck it sticks, so alignment must be right on the first pass. The deck must be clean, dry, and warm enough for the adhesive to grab. Metal roofs need a high-temperature version.

How does roof slope change the underlayment?

On slopes from 2:12 up to 4:12, the International Residential Code requires double underlayment under asphalt shingles (IRC 2021 R905.2.2). Water moves slowly on shallow slopes, so one layer leaves too much risk.

Shingles are not permitted below 2:12. On low slopes, use a low-slope membrane system or full self-adhering underlayment per the shingle and membrane manufacturers' instructions. On 4:12 and steeper, one layer is the code baseline; the manufacturer instructions and local amendments control the final choice.

What is an ice barrier and where does the code require one?

An ice barrier is a waterproof layer at the eaves that stops meltwater from backing up under the covering when ice dams form. IRC R905.1.2 requires it in areas with a history of ice forming along the eaves causing water backup, as designated in Table R301.2(1). The barrier must be at least two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet. It must run from the lowest edges of the roof to at least 24 inches inside the exterior wall line. On slopes of 8:12 or steeper, it must also extend at least 36 inches along the slope from the eave edge.

Valleys and pitch changes get their own treatment in some jurisdictions; Oklahoma's adopted code, for example, adds 36-inch ice barriers centered in valleys and at pitch changes where the lower roof is flatter. Ask the local building department which edition and amendments apply to the project.

How is underlayment installed?

Lay courses shingle fashion, starting at the eave and working up, so each upper course overlaps the lower one. For the code-level two-layer installation used in high-wind regions, the National Roofing Contractors Association describes this method: a 19-inch-wide starter course parallel to and starting at the eave, then full-width courses lapped 19 inches, with end laps of at least 4 inches offset at least 6 feet between courses. Fasten with corrosion-resistant cap nails of at least 0.083-inch diameter annular-ring or deformed shank, in a 12-inch grid between side laps and 6 inches on center at side and end laps. Caps must be at least 1 inch in diameter, and nails must penetrate at least 3/4 inch into the sheathing.

With self-adhering membrane, unroll from the eave, peel the release liner progressively, and press out air. Overlap per the manufacturer instructions. Never install it on a wet, dusty, or frozen deck, and check the minimum application temperature on the label.

For the roll math after the type is chosen, use the roofing underlayment measuring guide: convert the sloped roof area into roll courses after laps, count rolls, and apply the low-slope double-layer rule.

Which underlayment belongs under a metal roof?

Metal runs hot, and standard self-adhering membrane can soften and bleed under sustained heat. Use a high-temperature rated self-adhering membrane or a synthetic underlayment approved by both the panel manufacturer and the membrane manufacturer. Felt can wrinkle under metal, and those wrinkles can show through thin panels. Verify the combination before ordering, because a voided panel warranty costs more than the underlayment saved.

How long can underlayment stay exposed?

Every product has a printed exposure limit. Synthetic underlayment tolerates much longer exposure than felt, but no underlayment is a temporary roof for an indefinite period. Sun degrades asphalt and embrittles polymers. Schedule the covering crew before stripping the old roof, and if the schedule slips, check the roll label before trusting what is already down.

What mistakes cause underlayment failures?

• Installing one layer on a 3:12 shingle roof where the code requires two.
• Fastening with staples where cap nails are required; staples pull through under wind.
• Short or reversed laps that channel water into the deck instead of off it.
• Sticking self-adhering membrane to a wet, dusty, or cold deck, so it never bonds.
• Leaving underlayment exposed past its printed UV limit.
• Putting asphalt-based underlayment against a material it stains or reacts with without checking compatibility.
• Treating ice-barrier zones as optional because the last winter was mild.

Safety and professional scope

Underlayment work happens on sloped, sometimes slippery surfaces. Wet felt is slick, and synthetic surfaces vary by product. Fall protection, ladder setup, and deck inspection follow the job safety plan and applicable law, not this guide.

Manufacturer instructions and the locally adopted code edition control the accepted type, laps, and fasteners. This guide explains the options; it does not replace the product data sheet or the building department.