Underlayment Calculator
Calculate rolls of roof underlayment (synthetic or felt) plus ice & water shield for eaves and valleys, using the IRC 24-inch-inside-wall ice-barrier rule.
Used for the ice-barrier slope-depth conversion.
~10%. Synthetic laps more efficiently than felt.
Past the exterior wall.
One roll width centered in each valley.
Underlayment takeoff
Synthetic (~10 sq/roll)
Eave + valley coverage.
2 course(s); horizontal 3.5 ft → 24" inside wall.
- Eave I&W area
- 480sq ft
- Valley I&W area
- 90sq ft
- Roof area
- 2,000sq ft
- Squares
- 20
- Waste applied
- 10%
Two products: synthetic/felt underlayment covers the whole field; ice & water shield (self-adhering) goes at eaves and valleys.
Cold-climate code (IRC R905.1.2) requires the ice barrier to extend from the eave edge to at least 24 inches INSIDE the exterior wall line — that’s why coverage depth depends on your overhang and pitch.
Valleys get ice & water shield too — typically one roll width centered in the valley.
Synthetic underlayment covers more per roll and laps more efficiently than felt.
Verify ice-barrier requirements against your local code — not all climates require it; some require more.
How this calculator works
A roof needs two different membranes, and this calculator sizes both. Field underlayment — synthetic or felt — covers the entire roof under the shingles. Ice & water shield is a separate self-adhering membrane that goes only where water concentrates: along the eaves and in the valleys. Because they cover different areas and come in different roll sizes, they are calculated separately.
Field underlayment
This is straightforward: roof area plus a waste factor for overlaps, divided by the coverage per roll, rounded up. The product matters — synthetic covers about 10 squares per roll and laps efficiently, while #30 felt covers only about 2 squares, so the roll count swings widely with your choice.
The 24-inch ice-barrier rule
The interesting part is the eave ice barrier. IRC R905.1.2 requires it to reach from the eave edge to at least 24 inches inside the exterior wall line in cold climates. That horizontal distance is your overhang plus 24 inches — but the membrane lies on the slope, so the calculator converts that horizontal depth to slope distance using the pitch multiplier. A 12:12 roof needs a noticeably deeper eave course than a 4:12 roof for the same wall line, which a simple “two feet at the eave” estimate misses. The depth is then billed in whole courses of the roll width.
Valleys and verification
Valleys get their own course of ice & water shield, typically one roll width centered in the valley, which the calculator adds to the eave total. Ice-barrier requirements vary: not every climate requires them, and some jurisdictions require more than the IRC minimum, so verify against your local code before ordering.
Frequently asked questions
- How much roof underlayment do I need?
- Divide your roof area by the coverage per roll and round up, after adding about 10% for overlaps. A synthetic roll covers roughly 10 squares (1,000 sq ft), so a 2,000 sq ft roof needs about 3 rolls with waste. Felt covers far less per roll, so it takes more. The calculator does this for whichever product you pick.
- How far up does ice and water shield go?
- Per IRC R905.1.2, in cold climates the ice barrier must run from the eave edge to at least 24 inches inside the exterior wall line. Because the membrane lies on the slope, that horizontal distance — your overhang plus 24 inches — has to be converted to slope distance using the pitch. A steeper roof needs a deeper course measured along the slope, which is why a naive "2 feet at the eave" undershoots.
- Synthetic vs. felt underlayment — how much per roll?
- Coverage differs a lot. Synthetic underlayment typically covers about 10 squares (1,000 sq ft) per roll, #15 felt about 4 squares (400 sq ft), and #30 felt about 2 squares (200 sq ft). Synthetic is lighter, stronger, and laps more efficiently, so it both covers more and wastes less. Always confirm the printed coverage on your specific product.
- Do valleys need ice and water shield?
- Yes. Valleys concentrate water and are a classic leak point, so they get a course of self-adhering ice & water shield too — typically one roll width (about 3 feet) centered in the valley. The calculator adds valley coverage to the eave coverage when you enter a valley length.
- Do I need ice and water shield in a warm climate?
- Often not for the ice-dam reason — the IRC ice-barrier requirement applies where there is a history of ice forming at the eaves. But many builders still run self-adhering membrane at vulnerable spots like valleys, low-slope sections, and around penetrations regardless of climate. Check what your local code requires; some jurisdictions require more than the IRC minimum.
- How much overlap or waste should I add for underlayment?
- About 10% is a good default. Underlayment laps a few inches horizontally and more at the ends, and you lose some at cuts and details. Synthetic laps more efficiently than felt, so you can lean toward the lower end with synthetic and the higher end with felt.