Synthetic Underlayment vs. Felt: What the Data Says for Residential Roofs

Underlayment is the roof component with the worst public relations. Homeowners never see it, adjusters barely scope it, and half the industry still calls it "tar paper" like it's 1975. Yet it's the layer that keeps a roof dry between tear-off and shingles, backs up every shingle for the life of the roof, and quietly decides whether a wind-driven rain ends up in the attic.

The synthetic underlayment vs. felt question is settled for most residential roofers — synthetics have won the market — but "everyone switched" isn't the same as understanding why, where felt still holds its ground, and how to write either one into a scope that gets paid. Here's the data-first version.

What Each One Actually Is

Asphalt-saturated felt — "felt paper," "tar paper," #15 and #30 — is an organic mat (paper and cellulose fibers) saturated with asphalt. It's sold by weight class: #15 (lighter, ASTM D226 Type I or D4869 Type I/II) and #30 (heavier, D226 Type II or D4869 Type III/IV). Rolls are typically 3 feet wide, covering roughly 2 to 4 squares each.

Synthetic underlayment is a woven or spun polypropylene/polyethylene sheet, often with a scrim for tear strength and a coated or textured walking surface. Rolls are typically 4 feet wide and 250 feet long — around 10 squares per roll — at a fraction of felt's weight per square.

Same job, radically different materials. Which is why the comparison needed its own test standard — more on that in a moment.

The Head-to-Head

The pattern: synthetic wins nearly every performance row. Felt wins on material price and vapor permeability — and vapor permeability matters more than most roofers realize.

The Standards: Why "ASTM-Rated" Used to Mean Nothing

Here's the part of this comparison that separates roofers who know from roofers who repeat: for twenty years, synthetic underlayments were tested against felt standards — ASTM D226 and D4869 — which essentially check unrolling, breaking strength, and pliability. A synthetic labeled "meets ASTM D226" told you it was at least as good as tar paper on three properties. That's a low bar for a plastic sheet.

In December 2020, ASTM published D8257, the first standard written specifically for synthetic (polymeric) underlayments. It tests what actually matters for a plastic sheet under a roof: fastener pull-through resistance, water vapor transmission, accelerated weathering, dimensional change, hydrostatic resistance, and thermal cycling.

The practical takeaway: when specifying synthetic, look for D8257 compliance on the label, not just a felt-era standard. And when a carrier or inspector asks what the product meets, "D8257" is the answer that ends the conversation.

Where the Code Comes In (IRC R905.1.1)

The IRC governs underlayment through tables keyed to your roof covering and whether your jurisdiction requires wind design (high-wind regions per the code's maps). The short version:

• Underlayment must meet the applicable ASTM standard for the roof covering, applied per the tables and manufacturer instructions
• Wind design regions carry stricter application and attachment requirements — specified overlaps, fastening patterns (typically with cap nails/fasteners), and in some jurisdictions self-adhering underlayment or self-adhering strips over all deck joints as an alternative path
• Local amendments are real: some jurisdictions disallow #15 felt outright on re-roofs, requiring #30 or synthetic minimums

As with every code-required component, the locally adopted text is the last word — pull your jurisdiction's re-roof requirements once and keep them handy. And note the recurring theme: because the code requires application per manufacturer instructions, the manufacturer's spec (including its cap-fastener requirement on most synthetics) is code-enforceable.

The Two Things Synthetic Can't Do

Honest data cuts both ways. Synthetic has two genuine limitations:

1. It is not ice & water shield. Synthetic underlayment is a water-shedding layer, not a self-sealing waterproof membrane. It does not replace self-adhering polymer-modified bitumen at eaves in ice-dam regions or in valleys — those remain their own code- and manufacturer-required components. Roofers who "upgrade" a whole roof to synthetic and skip the ice barrier have downgraded the eave.

2. It usually doesn't breathe. Most synthetics are vapor barriers. On a properly ventilated attic, that's fine — the attic dries downward and outward. On an unvented or marginally ventilated assembly, a vapor-barrier underlayment can trap moisture in the deck from below. Felt's permeability is an actual advantage there, as are the breathable synthetics built for exactly that case. Match the underlayment to the assembly, not just the price sheet.

The Verdict — and the Cost Math

For the typical vented residential re-roof, synthetic is the right default: it survives the tear-off-to-dry-in window, holds through wind events, protects crews with better traction, installs faster, and outlasts felt under the shingles. The material premium is real but shrinking, and the labor savings offset a meaningful chunk of it. Felt remains a legitimate choice on a tight budget, a same-day dry-in, or an assembly that needs to breathe.

Whatever you install, the estimate has to say so — and this is where a lot of money quietly disappears:

• Mind the felt suffix. Xactimate's main shingle lines come in with-felt and without-felt ("S") variants. Installing synthetic on a with-felt shingle line and adding a synthetic underlayment item double-scopes the layer — a carrier flag. Installing it on the with-felt line without a synthetic item gives the upgrade away free. The correct scope is the without-felt shingle line plus the explicit synthetic underlayment line.
• Build it into the macro. Your underlayment decision should be encoded once in your base macro — shingle-line variant and underlayment item paired correctly — so every estimate carries the right combination by default. It's one of the most common estimate mistakes precisely because it's decided fresh, and wrong, on every claim. A complete macro library makes it a decision you make once.
• Photograph the deck. Wet, wrinkled, or torn existing felt, and any deck condition under it, is the documentation that supports both the underlayment scope and any sheathing replacement it reveals.

The layer nobody sees deserves a scope everybody can defend. Choose by the data, install to the standard, and write it like it matters — because under the shingles, it's what's actually keeping the house dry.