Roof Underlayment Types in Abilene: Felt, Synthetic, Peel-and-Stick

Underlayment is the only part of a roof you pay for and then never see again. The window in which anyone can verify what you actually got is the half day between tear-off and the first course of shingles.

Which makes it fertile ground for vagueness. "Synthetic" sounds like an upgrade and is not a specification. "30 lb felt" sounds like a weight, and ASTM stopped implying it was one by about 1982.

What underlayment for a roof is

Three jobs, in order of how often each earns its keep:

1. Backup drainage plane. Water that works under a lifted tab, a slipped tile or a failed pipe boot runs down to the eave instead of into the sheathing.
2. The temporary roof. Between tear-off and the last ridge cap the underlayment is the roof, through whatever weather turns up.
3. Separation. It keeps the covering off the deck — resin bleed, condensation, tile rocking on plywood.

Is roof underlayment waterproof?

No, with one narrow exception, and the trade uses three different words for the gradient.

NRCA's Joan Crowe, in Professional Roofing in May 2005, sets the frame: steep-slope roof systems are water-shedding. Built from separate pieces laid shingle fashion, they pass water course to course by gravity, and the sheet underneath does the same on lapped seams — a secondary weatherproofing barrier, in her words, for what gets past the covering. Pond water on it, or turn that geometry sideways, and every lap is just a joint. Synthetics she rates a notch up, as water-resistant.

Which is not nothing: D8257 puts a hydrostatic test on synthetics and D4869's shower test measures liquid water moving through felt. But both test an intact sheet, and a roof is a sheet with several thousand fasteners driven through it.

Hence the narrow exception. ASTM D1970 is the only one of these standards that asks the sheet to stay sealed around a nail: Section 7.9 drives a roofing nail through a specimen adhered to plywood, backs it out a quarter inch, and stands a 5-inch head of water on it for three days. Which is why D1970 gets specified at eaves, valleys and penetrations — where water backs up rather than runs off, an ice dam being the obvious case.

NRCA's own asterisk on that: Mark Graham argued in November 2006 that backing the nail out "gaskets" the seal against the shank, making the test less demanding than the roof.

And the word "waterproofing" appears in the title of exactly one of these standards — ASTM D226, Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing — which is the least waterproof product in the chapter.

Felt: ASTM D226, and what No. 15 and No. 30 mean

Asphalt-saturated organic felt is the original and still the cheapest. ASTM D226 covers two types: Type I, commonly called No. 15 asphalt felt, and Type II, commonly called No. 30.

The standard says "commonly called" because the felts have not weighed 15 and 30 pounds per 100 square feet for decades. Writing in IIBEC Interface in November 2019, Jeffrey Levine gives D226 Type I at roughly 11.5 to 13 lb per 100 sq ft and Type II at about 26. The names are a fossil nobody retired.

Worse, there is a second felt standard. ASTM D4869 was written for steep-slope roofing specifically, and unlike D226 it adds a liquid-water transmission pass/fail and dimensional-stability limits aimed at wrinkling. Sensible — except its lightest type runs around 8 lb per 100 sq ft, two-thirds of a D226 Type I, and both are sold at the yard as "15 lb felt." The label carries the answer. Nothing else does.

Felt's failure mode is not mysterious: it absorbs water, wrinkles as it dries, and telegraphs those wrinkles through the shingles. Over years it loses its volatiles and goes brittle, which is why a tear-off crew takes thirty-year-old felt off in confetti.

Synthetic is a category, not a standard

Synthetic underlayment is a woven or spun polypropylene or polyethylene scrim with a coating. That is the whole of what the word guarantees.

For roughly two decades there was no U.S. product standard at all. Products got into the code through an ICC-ES evaluation report written against AC188, the acceptance criteria for roof underlayments — and acceptance criteria are not consensus standards. The test requirements are settled between the manufacturer and the evaluation service, and AC188 cannot be cited in the I-Codes as a reference standard; it is an alternative compliance path for products the prescriptive code does not describe.

That changed in December 2020, when ASTM published D8257, Standard Specification for Mechanically Attached Polymeric Roof Underlayment Used in Steep Slope Roofing, after eight years of development. It tests tensile and tear strength, fastener pull-through and hydrostatic resistance after thermal cycling and accelerated weathering — deliberately, because the exposed period is when synthetics fail. NRCA's Mark Graham, in Professional Roofing in July 2021, expected it would miss the 2024 I-Codes. It did not: D8257 is in the 2024 IRC's list.

So a standard exists. A product can still be legal without meeting it, and the marketing does not distinguish.

Roof felt vs synthetic underlayment

	Felt (D226 / D4869)	Synthetic (D8257 or an ESR)
Standard	Two, both product-specific	One, since 2020 — and optional
Weight per roll	Heavy; a square is a lift	A fraction of felt, for more coverage
Tear strength	Tears at the fastener in wind	Several times felt, on decent product
Water absorption	Absorbs, wrinkles, telegraphs	None
Walkability	Grippy, predictable	Grippy to genuinely dangerous
Vapor	Breathes as it ages	Usually under 1 perm, so a retarder

Self-adhered modified bitumen: the one with real numbers on it

ASTM D1970 is the peel-and-stick membrane, and it is a real specification with its application written into its own title: …Used as Steep Roofing Underlayment for Ice Dam Protection. It bonds to the deck rather than sitting nailed over it.

The number that matters is thermal stability, and D1970's own baseline is exposure to about 176°F. Fine under asphalt shingles on a ventilated deck. Not fine under tile or metal, where the cavity and the panel underside run far hotter — Polyguard puts the environment a metal roof's underlayment must survive at roughly 240–265°F, and high-temperature grades are formulated for that band rather than the standard's floor. Specifying general-purpose D1970 under standing seam is the commonest expensive mistake in the subject.

The trade-off is permeability: fully adhered over the whole deck puts a vapor barrier on the outside of the assembly, the wrong side in most climates, so full-coverage peel-and-stick belongs on the roofs that need it and not everywhere by default. How long it lasts under tile, and why that clock schedules your spending, is in how long does a tile roof last.

Problems with synthetic roof underlayment

Slip. Many are slick wet or frosted. Coarse-surfaced versions exist precisely because the smooth ones put crews on the ground.

Ultraviolet exposure. The 90 to 180 day rating is a warranty term rather than a performance promise, and the sun does not read warranties.

Vapor. A sub-1 perm sheet slows drying on the underside of a cold deck, exactly where nobody can see it. Graham's point too: some are vapor-open, some are not, and the packaging rarely leads with it.

Fastening. A staple through a synthetic is a hole with nothing holding the sheet. Cap nails are what the product is designed around.

Laps, caps and the order it goes down

Underlayment fails at its edges, so the geometry is code, not preference. Table R905.1.1(2), asphalt shingles:

• 4:12 and steeper: one layer, shingle fashion from the eave, lapped 2 inches, end laps 4 inches, end laps offset 6 feet course to course.
• 2:12 to under 4:12: two layers — a starter half a sheet wide at the eave, then full 36-inch sheets each overlapping the sheet below by half a sheet plus 2 inches.
• In wind design areas, mechanically fastened underlayment takes corrosion-resistant annular ring or deformed shank nails with 1-inch-diameter metal or plastic caps in a 12-inch grid between side laps, 6 inches at side and end laps, penetrating not less than 3/4 inch into the sheathing.

Two field notes. The six-foot offset exists so end laps never stack into a channel, and skipping it is invisible from the ground forever. And underlayment goes over the drip edge at the eave and under it at the rakes — a reversal worked through in what is a drip edge.

What is code and what is the manufacturer's

Both are enforceable, by different routes.

Code. IRC R905.2.2 is the slope rule behind those two cases: asphalt shingles at 2:12 minimum, double underlayment below 4:12. R905.1.1 is the general underlayment section: it lists the acceptable standards — ASTM D226, D1970, D2626, D4869, D6380 Class M, D6757 and D8257 — and requires materials to bear a label indicating compliance. That label is your only practical verification tool. Eave ice barriers sit in a separate provision, R905.1.2, and are not a national requirement. Section numbers read on UpCodes against the 2024 residential code, since ICC's own site refuses automated access; the numbering is stable back through the 2018 edition.

Manufacturer's. IRC R904.1 requires roof coverings to be applied per the chapter and the manufacturer's installation instructions — which is how a shingle maker's demand for a particular underlayment, or a stated exposure limit, becomes something an inspector can enforce. Firestone, Colorado's re-roof handout says exactly that, and requires a mid-roof inspection with every elevation dried in and no more than a quarter of the covering on. Whether your jurisdiction runs one is worth asking before the job starts.

Who is permitted to do the work in Texas

Relevant here because the underlayment is the one part no homeowner can audit afterwards, so the permit and the credential are most of what you have — whatever roofing credential your state issues, which in several states is none at all.

Texas issues no roofing license of any kind, so a state number is not something any roofer here can show you. A contractor also may not act as a public adjuster on a property it may work on, even holding an adjuster license.

Bottom line

Ask for the product name and the standard it meets, not the category, and under tile or metal ask about the temperature rating specifically. Then photograph the roll label and the dried-in deck, because that is the last day the evidence exists.

Abilene Premium Roofing does written roof inspections, roof leak repair and full roof replacement across Abilene and west central Texas, and we put the underlayment product on paper before we order it. Call (877) 881-2921.