Underlayment quantity here comes from roof footprint, pitch, layer count and material type, with 10% waste built in. A medium (2,000 sq ft) home at standard pitch, single-layer synthetic, runs about $860–$1,960 installed — but the same home at a low pitch, running the double layer the building code requires there in asphalt felt, can actually cost less, because material choice moves the price more than layer count does.
How this calculator sizes an underlayment job
Underlayment is the water-resistant sheet material installed directly on the deck, underneath the shingles — a backup layer if water gets past them. This calculator converts your home's footprint to roof area with the same pitch multipliers used across this site's roofing tools (1.03 low, 1.12 standard, 1.25 steep), multiplies by however many layers you select, adds 10% for waste and overlap, and divides by the coverage per roll for your chosen material.
When the code requires a second layer
This calculator's "layers" input isn't just an optional upgrade — on a real roof it's sometimes mandatory. The IRC sets minimum slopes for asphalt shingles and tile, and requires double underlayment across a specific pitch band for each. Per IRC R905.2.2, asphalt shingles have a minimum slope of 2:12, and from 2:12 up to 4:12 the code requires double underlayment. R905.3.2 sets the same double-layer requirement for clay and concrete tile between 2.5:12 and 4:12. That's exactly the pitch range this calculator's "Low (2/12–4/12)" bucket describes — so if your roof pitch lands there and you're shingling or tiling it, the two-layer option in this calculator isn't a preference, it's the minimum the code allows.
That's the surprise worth sitting with: the code-mandated double-layer felt job (Example B) comes in cheaper than the single-layer synthetic job (Example A), because felt's per-square-foot material cost is roughly a third of synthetic's even at twice the coverage. Layer count is not the lever that moves this number the most — material selection is.
What's happening to felt material prices
Asphalt felt itself has a federal price index behind it. The Bureau of Labor Statistics' Producer Price Index for Asphalt Felts and Coatings (WPU136) — tracked via FRED — stood at 365.850 in July 2026 on a 1982=100 base, not seasonally adjusted. That index covers the felt and coating materials this calculator's 15lb and 30lb felt options represent; it is not a shingle price index, so don't read it as a signal for the material that goes on top of this layer.
Where the labor dollars go
Underlayment installation is usually rolled into the same crew doing the tear-off and reroof, so its labor cost follows the same wage data as the rest of a roofing job. The Bureau of Labor Statistics puts the May 2025 mean hourly wage for roofers at $27.95, with a 10th percentile of $18.01 and a 90th of $39.29 — a spread wide enough on its own to explain most of the labor-cost difference between two otherwise identical underlayment quotes in different metros.
Underlayment is also a small fraction of a full reroof's cost. The 2025 Cost vs. Value report from the Journal of Light Construction puts a national average asphalt-shingle roof replacement job at $31,871 — underlayment sits inside that figure as one line among several (tear-off, decking repair, underlayment, shingles, flashing), which is worth remembering if a quote's underlayment line looks small relative to the total; it should.
What this estimate excludes
This page prices underlayment material and its installation labor only. It does not include:
- ·The decking underneath. Underlayment goes over the deck, not in place of repairing it — see the roof decking calculator if the deck itself needs work.
- ·Shingles or tile on top. This is a substrate cost, not a finished-roof cost.
- ·Flashing at penetrations. Chimneys, vents and skylights need separate flashing work beyond a sheet of underlayment run past them.
- ·Permit fees. Underlayment bundled into a reroof shares that project's permit; standalone underlayment work rarely triggers one on its own, but check locally.
There's no federal data series tracking underlayment material choice by home, either. The Census Bureau's Characteristics of New Housing (CHARS) survey — which does track square footage, bedroom and bathroom counts and exterior wall material for new homes — does not publish roofing material data of any kind, underlayment included, confirmed on its landing and 2025 Highlights pages.
How to read a contractor's underlayment line
- Does the quote name a specific product (felt weight, or synthetic vs. premium synthetic) rather than just "underlayment"? Material type moved this page's own worked examples more than layer count did — it's the single biggest lever in the price.
- If your pitch is between 2:12 and 4:12 for asphalt shingles (or 2.5:12–4:12 for tile), does the quote show a double layer? IRC R905.2.2 and R905.3.2 require it in that range — a single-layer quote there is under code, not a discount.
- Is ice-and-water shield priced as a separate line from field underlayment? The two materials serve different purposes (eaves/valleys vs. the general field) and shouldn't be quoted as interchangeable.
Methodology
Roll counts and cost bands are read directly from this calculator's own module: pitch multipliers (1.03 low / 1.12 standard / 1.25 steep), a 10% waste factor, per-material coverage and cost-per-sq-ft bands for 15lb felt, 30lb felt, synthetic standard, synthetic premium, and ice-and-water shield. The double-underlayment code requirement is IRC R905.2.2 (asphalt shingles, 2:12–4:12) and R905.3.2 (tile, 2.5:12–4:12). Material price-trend context is BLS PPI WPU136 (asphalt felts and coatings); wage context is BLS OEWS. No figure on this page beyond the module's own constants is estimated or interpolated.
Sources
- International Code Council — 2021 IRC Chapter 9, Roof Assemblies (R905.2.2, R905.3.2) — accessed 2026-09-05
- FRED (Federal Reserve Bank of St. Louis) — BLS PPI, Asphalt Felts and Coatings (WPU136) — accessed 2026-09-05
- U.S. Bureau of Labor Statistics — OEWS Profile, Roofers (47-2181), May 2025 — accessed 2026-09-05
- Journal of Light Construction — 2025 Cost vs. Value Report — accessed 2026-09-05
- U.S. Census Bureau — Characteristics of New Housing (CHARS) — accessed 2026-09-05