South Bay Roof Report
Materials & Lifespan

Roof Underlayment Types: Felt, Synthetic and Self-Adhered

South Bay Roof Report · Updated 2026-10-11 · 8 min read
Roof Underlayment Types: Felt, Synthetic and Self-Adhered
In briefThe three main roof underlayment types are asphalt-saturated felt, mechanically fastened synthetic sheets, and self-adhered polymer-modified bitumen membranes. Underlayment sits on the roof deck beneath shingles, tile or metal and backs up the covering against water that gets past it. The right type depends on the covering, roof slope, local code and the manufacturer's instructions. Roof work is a fall hazard, so leave deck and underlayment inspection to a licensed roofing professional.
Contents

What are the main types of roof underlayment?

Roof underlayment is the sheet layer fastened or bonded to the roof deck (the plywood or board sheathing) beneath the shingles, tile or metal you see from the street. Homes mostly use three types: asphalt-saturated felt, mechanically fastened synthetic sheets, and self-adhered polymer-modified bitumen membranes, often sold as "ice and water" barriers. The right one is set by the covering, the slope, the deck, the local code and the covering manufacturer's instructions, not by preference alone.

What does roof underlayment actually do?

Picture a roof as a water-management system, not a single surface. The covering sheds most rain. Flashings, the formed metal or membrane pieces at valleys, walls, chimneys and vents, steer water around the awkward places. The underlayment sits beneath both and catches what slips past.

A Fine Homebuilding guide to synthetic underlayments by Martin Holladay lists three jobs: underlayment "keeps the sheathing dry until the roofing is installed," provides "some protection against leaks in case wind-driven rain gets past the roofing," and adds "a slight improvement in a roof's fire resistance." Note how modest that middle promise is: on a shingle roof, underlayment is a backup, not a second roof.

Tile changes the balance. Concrete and clay tiles are laid with overlaps and open joints rather than sealed seams, so the layer beneath them carries more responsibility than its name suggests. The U.S. Department of Energy's Building America Solution Center, in its clay and concrete tile guide, notes that when tiles are lost, the result is "exposing the underlayment, flashing, and sheathing as the only protection against rainwater."

How do felt, synthetic and self-adhered underlayment compare?

Type What it is Where it tends to make sense What to ask about
Asphalt-saturated felt Paper-based felt soaked in asphalt, sold in lighter and heavier weights (No. 15 and No. 30) Long-established option that appears throughout the code tables Which grade; how long it will sit exposed before the covering goes on
Synthetic (mechanically fastened) Woven or laminated plastic sheet, nailed or capped to the deck Common modern field layer under shingles, tile and metal Which product and listing; exposure limit; ventilation of the attic or roof space
Self-adhered membrane Peel-and-stick sheet of rubberized (polymer-modified) asphalt that bonds to the deck Valleys, eaves, around penetrations, low-slope transitions, or the whole deck Where exactly it goes; temperature rating if it sits under metal; compatibility with the covering

Felt

Holladay describes felt as "recycled corrugated paper mixed with sawdust" in which "the paper is impregnated with asphalt." It has the longest track record of the three. Its weaknesses are physical: the same guide's comparison says felt "wrinkles when wet; cracks and splits in cold weather" and "tears easily in high winds and under foot traffic." Weight matters here. The heavier grade is commonly called No. 30 felt, and the Building America Solution Center's roof deck sealing guide identifies that grade by its ASTM designations, D226 Type II or D4869 Type III or IV.

Synthetic

Synthetics are, in Holladay's words, "sheet products made of laminated polypropylene or polyethylene plastic." They resist tearing and tolerate weather longer before the covering goes on: the guide reports that manufacturers' data sheets list maximum exposure times "ranging from two months to 30 months." The caution worth carrying forward is ventilation. Holladay wrote that "most synthetic roofing underlayments are vapor barriers, so they shouldn't be used on unventilated roofs." That article dates from 2011, so the specific product's data sheet should settle whether it suits a given attic or vaulted ceiling.

Self-adhered membranes

Because this sheet bonds to the deck instead of merely lying on it, water has a much harder time travelling underneath. It costs more than felt or synthetics, so it usually goes where water concentrates: valleys, eaves, around skylights and pipes, and at steep-to-low-slope joints. Storm-resilience programs sometimes cover the entire deck. The Solution Center's sealed-deck guide lists that as one option, "Cover the entire roof deck with a layer of self-adhering polymer-modified bitumen membrane," and adds that "IBHS recommends covering the membrane with #15 felt if asphalt shingles will be installed."

Which underlayment does the California code allow?

The California Residential Code sets the floor. As reproduced on a code-reference page for the 2025 California Residential Code, Section R905, Table R905.1.1(1) lists the acceptable underlayment standards by covering. For asphalt shingles in areas where wind design is not required, it lists "ASTM D226 Type I or II; ASTM D1970; ASTM D4869 Type I, II, III or IV; ASTM D6757; ASTM D8257." For clay and concrete tile in the same areas it lists "ASTM D226 Type II; ASTM D1970; ASTM D2626; ASTM D6380 Class M; ASTM D8257." Where wind design is required, both lists narrow.

You do not need to memorize these numbers. A written proposal can name the product, and its label or data sheet states which standard it meets; ask about any mismatch before work starts. Your local building department decides which code edition and local amendments apply to your permit; our guide to roofing permits in San Diego covers where to verify that.

How does roof slope change the underlayment?

Slope, expressed as inches of rise per 12 inches of run, is a major driver: water moves slowly on a gentle roof and has more time to find a way in. The same code page states that asphalt shingles "shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater," and that between 2:12 and 4:12 "double underlayment application is required." At 4:12 or steeper, the table calls for a single layer "applied shingle fashion, parallel to and starting from the eave and lapped 2 inches," with end laps of 4 inches "offset by 6 feet."

Tile follows the same logic at a slightly steeper threshold. The code page states that for clay and concrete tile on slopes "from 2 1/2 units vertical in 12 units horizontal (25-percent slope) to 4 units vertical," double underlayment is required.

Many San Diego-area homes mix slopes: a pitched main roof, a gentle porch roof, a nearly flat addition. Each plane may need a different approach, and the transitions are where a careful specification earns its keep. Flat sections use a different system entirely, in which the membrane is the roof.

Do San Diego roofs need an ice barrier?

The code's ice-barrier rule is tied to climate history, not geography in the abstract. Section R905.1.2 applies "in areas where there has been a history of ice forming along the eaves causing a backup of water," and requires either "not fewer than two layers of underlayment cemented together" or a self-adhering polymer-modified bitumen sheet, extending "not less than 24 inches (610 mm) inside the exterior wall line of the building."

Coastal and inland valley neighborhoods lack the freezing winters that build ice dams, but San Diego County also includes mountain communities. Whether the rule applies to a specific address is for the local building department to say. Self-adhered membranes still turn up in valleys and around penetrations in any climate, because water concentrates there.

What about wildfire areas and tile?

Wildfire rules can reach the underlayment, particularly under profiled coverings. The 2025 California Wildland-Urban Interface Code, Section 504.2.1, as reproduced on the same code-reference site, addresses a roof covering installed "over a combustible deck" where "the roofing profile creates an airspace." Barrel and S-shaped tile are the familiar examples of such a profile. The options listed include "a 72 lb. (32.7 kg) cap sheet complying with ASTM D3909" or "a listed and labeled roof underlayment that has been tested as part of the roof assembly" for a Class A fire classification, and in both cases "hip and ridge caps shall be mudded in to prevent intrusion of fire or embers."

The Solution Center's tile guide puts the general principle more simply: "Fire-resistance-rated underlayment products are recommended in all wildfire areas." Whether these wildfire provisions apply to a particular parcel depends on its designation, so check with your local building department. For the broader material decision, see our comparison of the best roof for San Diego.

Does metal roofing need underlayment?

Usually, yes. The exception in R905.1.1 on the code page is for "structural metal panels that do not require a substrate or underlayment." For metal roof shingles, the application table says to "apply in accordance with the manufacturer's installation instructions." The Solution Center's metal roof guide adds a reason that has nothing to do with rain: under some conditions "condensation can occur on the underside of the metal roofing," so "a continuous layer of roof underlayment (felt, synthetic underlayment, or membrane) is needed" to protect the sheathing. Metal heats up in sun, so ask whether any self-adhered membrane specified under it is rated for that use.

How long does underlayment last, and why does it matter?

Underlayment ages on its own schedule, and under long-lived coverings such as tile it can wear out while the covering above still looks sound. Our guide to how long a roof lasts covers those ranges and the tile "lift-and-relay," in which a contractor removes the tiles, replaces the underlayment and flashings, and reinstalls sound tiles.

What should a homeowner check in a roofing bid?

Underlayment work happens on the roof, which is a fall hazard. Leave inspection of the deck and underlayment to a licensed roofing professional; a homeowner's part is reading the paperwork. A useful proposal names:

Underlayment pricing varies widely by product, region and roof, so compare bidders on the same named products, not a single total. Our bid comparison checklist shows how to line up two proposals side by side.

Sources

FAQ

What does roof underlayment do?

Underlayment is the sheet layer between the roof deck and the visible covering. It keeps the sheathing dry during installation, gives backup protection if wind-driven rain gets past shingles, tile or metal, and adds a modest improvement in fire resistance. On tile roofs, which have unsealed overlaps, the underlayment and flashings carry more of the water-management load than they do beneath shingles.

Is synthetic underlayment better than felt?

Not automatically. Synthetic sheets resist tearing and generally tolerate longer weather exposure before the covering goes on, while felt has the longer track record and can wrinkle or tear. Many synthetics are vapor barriers, which matters on unventilated roofs. The covering manufacturer's approved list, the product's data sheet and local code should decide, so ask each bidder to name the exact product.

Where does ice and water shield go on a roof?

Self-adhered membranes, often sold as ice and water barriers, are usually placed where water concentrates: valleys, eaves, around skylights, pipes and chimneys, and at low-slope transitions. Where an ice barrier is required, the California code calls for it to extend at least 24 inches inside the exterior wall line. Some storm-resilience approaches cover the entire deck. Whether your address requires one is a local building department question.

Do low-slope roofs need two layers of underlayment?

Under the California Residential Code, asphalt shingles on slopes from 2:12 up to 4:12 require double underlayment, and clay or concrete tile on slopes from 2.5:12 to 4:12 also requires double underlayment. Shingle roofs at 4:12 or steeper use a single layer lapped shingle fashion. Check which code edition and local amendments your building department applies before work begins.