Roof underlayment: synthetic, felt or ice and water shield.

Shingles, tile and metal get the attention. Under every one of them sits the underlayment, a sheet fastened to the wood deck before the roofing goes on. When wind lifts a shingle, when a storm drives rain sideways, or when a flashing lets go, the underlayment is what keeps water off the wood until the roof is put right.

This guide covers the kinds of underlayment by the standard printed on the roll, what the building code asks for on each slope, how long a sheet can sit in the sun, what goes under tile and metal, and how to read the underlayment line on a quote. For where it sits in the whole roof, see the parts of a roof; for the rest of a bid, see how to compare roofing estimates.

The Asphalt Roofing Manufacturers Association, the trade body of the shingle makers, starts its bulletin on underlayment with a plain rule: asphalt shingles should not be installed directly on a wood deck. Its reasons, in short:

• It is a second barrier under the shingles, there for the wind-driven rain the shingles let through.
• It keeps the deck dry between tear-off and the last course of shingles.
• Building codes require it, and the shingle makers set requirements for it.
• It is part of what gives a shingle roof its fire rating.
• It hides small differences in the thickness of the deck sheets, which otherwise show through as a grid on the finished roof, and it keeps resin in the wood away from the shingles.

Each kind has its own standard, and the code requires the felt and membrane standards to be labeled on the product, so the roll tells you what it is. These are the ones you will see on a Texas roof.

• Asphalt-saturated felt (ASTM D226). The traditional sheet, made of organic felt soaked in asphalt. Type I is the lighter weight roofers call No. 15 felt; Type II is the heavier No. 30. It sheds water, but it also absorbs some and lets some vapor through.
• Steep-slope felt (ASTM D4869). The first standard written for steep-slope underlayment, with four weights and a water-shedding test the industry calls the shower test.
• Felt with inorganic fibers (ASTM D6757). Looks and works like felt, uses less asphalt, and may hold its shape better.
• Synthetic underlayment (ASTM D8257). Polymer sheets fastened with nails. Lighter than felt, and a roll covers more roof. The 2021 code's table for shingles names only the felt standards, so a synthetic should come with its approval for use under shingles.
• Self-adhered membrane (ASTM D1970). Rubberized asphalt with a sticky back, sold as ice and water shield. It bonds to the deck and seals around the shank of every nail driven through it.

Texas cities adopt the International Residential Code as their residential building code, and its underlayment rules turn on the slope of the roof.

• 4:12 and steeper. One layer, laid like shingles from the eave up, each course lapped 2 inches over the one below, end laps of 4 inches, and end laps staggered 6 feet apart.
• From 2:12 up to 4:12. Two layers: a 19-inch strip along the eave, then 36-inch sheets, each overlapping the one below by 19 inches, so every point on the deck is covered twice. The manufacturers' association names one layer of self-adhered membrane as the other way to do it.
• Below 2:12. No asphalt shingles at all. A roof that flat needs a low-slope system.
• High-wind areas. Where the code requires wind design, the lighter felts drop out: the sheet must be ASTM D226 Type II or ASTM D4869 Type III or IV.

The code's ice barrier rule, a cemented or self-adhered strip reaching 24 inches inside the wall line, applies only where the local code table records a history of ice forming at the eaves. We use self-adhered membrane on Texas roofs for a different reason: it seals the places where water gathers and wind drives it sideways.

Felt is named in the code's own table. Synthetics are newer, and the manufacturers' association warns that some of the products it lists may not be approved by the codes in force everywhere, so ask for the approval. On the roof, the association notes that felt absorbs some moisture and lets some vapor through, while synthetic sheets are lighter and cover more of the roof from each roll.

We cover the deck with synthetic underlayment on every roof. If a quote says only "underlayment", ask which product: the label on the roll decides which standard it meets.

Membrane costs more than felt or synthetic, so it goes where it earns its keep. The manufacturers' association lists the places: the eaves, the valleys, the hips and ridges, and around every penetration, from plumbing vents and skylights to chimneys. On a Mammoth roof, Igloo Shield™ ice and water shield goes at every eave and valley and around every penetration, and Pelt Felt™ synthetic underlayment covers the rest of the deck.

Membrane over the whole deck is a different case. The association warns that a fully adhered deck acts as a vapor retarder, so attic ventilation becomes more important, not less: without it, water vapor can condense under the deck. That matters on the roofs where we do cover the whole deck, such as under metal, and it is one reason we size the attic vents on every job. Our ventilation guide covers the rule.

Every underlayment has a limit on how long it can be left uncovered, and the limit is set by the product, not by a general rule. The manufacturers' association says shingles should go on as soon as possible after the underlayment. The makers put numbers on it in their warranties.

CertainTeed's warranty for its RoofRunner synthetic underlayment, for example, excludes damage from exposure to the weather, except sun damage on a sheet that was covered within 90 days; its RoofRunner AIR version is made to resist ultraviolet light for up to 180 days. Most single-story homes we reroof are finished in one day, so on our jobs the question rarely comes up. On a job that stalls for weeks with the underlayment showing, it should.

On a tile roof, the underlayment is the real waterproofing. Clay and concrete tile lasts 50 years or more, but the underlayment under it gives out at 20 to 25, and when it does the roof leaks while the tile looks fine. The fix is a re-lay: the tile comes off, the deck is checked, a high-temperature membrane and new flashings go down, and the same tile goes back on. Our tile roofing page covers how we do it.

Under metal, the whole deck gets a high-temperature underlayment instead of standard felt, a stronger second barrier beneath the panels. The details for each metal system are on our metal roofing page.

The underlayment usually carries a warranty of its own, and it often follows the shingles. CertainTeed's RoofRunner warranty runs for the same length as the CertainTeed shingles installed over it, and for ten years under another maker's shingles. It ends if either the underlayment or the roofing over it is removed, and it excludes failures caused by poor attic ventilation.

The shingle warranty cares about it too. The manufacturers' association tells installers to follow the shingle maker's requirements for the underlayment, and CertainTeed's shingle warranty excludes damage from any installation that does not follow its instructions. A crew that lays one layer on a 3:12 slope has built outside the code and put the maker's coverage at risk. Our guide to roof warranty types explains what each document covers.

Picture a ranch house with a main roof at 6:12 and a covered patio off the back at 3:12. Same shingles, same crew, same day, two different underlayment jobs.

• The main roof (6:12). Membrane along every eave and in both valleys and around the vent pipes, then one layer of synthetic underlayment over the rest, lapped 2 inches course over course with 4-inch end laps staggered 6 feet apart.
• The patio roof (3:12). Below 4:12, so two layers: a 19-inch starter strip at the eave, then 36-inch sheets lapped 19 inches over each other, or one layer of self-adhered membrane over the whole patio deck. Where the patio roof meets the house wall, membrane runs up behind the step flashing.

A quote for this house should show both. If it lists one line, "synthetic underlayment, whole roof", ask how the patio is being done, because the slope there changes the rule.

• The product by name, so you can look up its standard and its warranty.
• Where the self-adhered membrane goes: eaves, valleys, walls, penetrations.
• Whether the main underlayment covers the whole deck.
• How any slope under 4:12 is handled: two layers or membrane.
• For tile or metal, the high-temperature membrane and whether it covers the whole deck.

"Underlayment as needed" is not an answer. Ask until the line names a product and a place.