Underlayment Calculator
Count underlayment rolls the way the roof actually consumes them: gross roll coverage minus the course overlap, then the roof area divided by what is left. A 10-square synthetic roll never covers 10 squares once the laps go down — at a 10% overlap it covers 9 — and that difference is exactly one missing roll on a big roof. Pick the roll type, set your overlap, and get rolls rounded up. Free, no login.
Underlayment
Rolls
Usable coverage = roll area less the overlap. Synthetic 10-sq roll = 48" × 250' (typical). Add extra for valleys and starter rows.
Underlayment — Material Order Sheet
Prepared with NORDIX — nordixhq.com/tools/underlayment-calculator
Roof area: 2000 ft² · Roll type: Synthetic (10 sq) · Overlap: 10%
| Line item | Quantity | Notes |
|---|---|---|
| Underlayment — Synthetic (10 sq) | 3 rolls | Usable 900 ft²/roll after 10% overlap |
Usable coverage = gross roll area less the entered overlap; rolls round up. Felt figures are nominal (4 sq / 2 sq) planning values — actual common rolls are 432 ft² (#15) and 216 ft² (#30); confirm coverage on the roll label before ordering. Add extra for valleys, starter rows, and steep-slope double coverage.
How this calculator works
usable per roll = gross coverage × (1 − overlap%) · rolls = ⌈roof area ÷ usable⌉
The roll figures carry two different confidence levels, and the calculator inherits both. The synthetic 1,000 ft² (10-square) roll is sourced — 48" × 250' is the common synthetic format across GAF FeltBuster, Owens Corning Titanium UDL50, and Grip-Rite product listings. The felt figures are conservative nominals: #15 at 400 ft² (“4 square”) and #30 at 200 ft² (“2 square”), while actual common rolls run 432 ft² and 216 ft² (36" × 144' ASTM D4869-labeled retail rolls) — the nominal values under-promise slightly, so confirm coverage on the roll label before ordering. Overlap is your input: ~10% covers a standard 2" head lap plus end laps; low-slope double coverage is effectively 50%.
Worked example
A 2,000 ft² roof surface under synthetic underlayment at 10% overlap:
- Usable per roll: 1,000 × 0.90 = 900 ft².
- Rolls: 2,000 ÷ 900 = 2.2 → 3 rolls (rolls always round up).
- Same roof in felt: #15 at 400 ft² nominal yields 360 usable → 6 rolls; #30 at 200 ft² nominal yields 180 usable → 12 rolls.
Enter the same roof above (2,000 ft², Synthetic, 10%) to reproduce it.
Reference: roll coverage and usable area
Gross and usable-at-10%-overlap coverage per roll, computed by the same engine. Felt figures are conservative nominal planning values (actual common rolls: 432 ft² for #15, 216 ft² for #30) — confirm against the roll label.
| Roll type | Gross coverage | Usable @ 10% overlap | Rolls per 2,000 ft² roof |
|---|---|---|---|
| Synthetic (10 sq) | 1,000 ft² | 900 ft² | 3 |
| #15 felt (4 sq) | 400 ft² | 360 ft² | 6 |
| #30 felt (2 sq) | 200 ft² | 180 ft² | 12 |
Frequently asked questions
How many rolls of underlayment do I need?
Take the true sloped roof area, divide by the usable coverage of one roll, and round up — that's what this calculator does. Usable coverage is the roll's rated area minus the overlap loss: a 1,000 ft² synthetic roll at 10% overlap nets 900 ft², so a 2,000 ft² roof takes 3 rolls, not 2. Rounding happens per roll, so a small roof can look 'over-ordered' — that partial roll is real material you can't skip.
How many square feet does a roll of underlayment cover?
It varies by product, which is why the calculator offers presets: common synthetic rolls run about 1,000 ft² (10 squares), #15 felt about 400 ft² (4 squares), and #30 felt about 200 ft² (2 squares). Those are rated gross areas — synthetics in particular come in 5-square and 10-square rolls from different makers, so check the label and pick the preset that matches, then let the overlap setting handle the loss.
What overlap should I enter?
The default 10% approximates the standard laps: a 2–4 inch horizontal (head) lap plus 6-inch end laps on a 36–48 inch wide roll eats roughly 8–12% of the gross area. On low slopes — commonly 2:12 up to 4:12 for shingle roofs — codes and manufacturers typically require much bigger laps (historically a half-lapped double layer of felt), which can push effective loss toward 50%; enter the overlap your spec actually requires rather than the default.
Should I use felt or synthetic underlayment?
Synthetic has largely taken over: a 10-square roll weighs about what a 2-square roll of #30 felt does, it doesn't tear off the cap nails in wind, it's less slick to walk when dry, and it tolerates longer UV exposure before shingling (30–180 days by product, versus days for felt). Felt still gets specced for cost, for some historic or warranty situations, and #30 under certain steep-slope materials like tile or shakes. Either way, use what the shingle manufacturer's instructions and the adopted code accept — that's what the warranty rides on.
Is underlayment required by code?
For asphalt shingles, yes — the model codes (IRC/IBC) require an underlayment layer, with the specifics (material standard, laps, fastening) set by roof slope and wind region in the adopted edition and by the shingle manufacturer's instructions. Requirements genuinely vary by jurisdiction and by slope, so treat 'one layer of felt or synthetic, lapped 2–4 inches' as the common baseline and confirm against the code your jurisdiction has adopted before bidding.
Does this calculator include ice and water shield?
No — ice barrier (self-adhered membrane) is a separate product with its own takeoff. In cold regions, codes commonly require it along the eaves extending at least 24 inches inside the exterior wall line, and good practice runs it in valleys and around penetrations too; whether it's mandatory is a jurisdiction call in the adopted code. Subtract the membrane-covered area from this calculator's input only if you're not underlaying over it — most crews run the field underlayment over the ice barrier's upper edge and don't bother deducting.
Method: usable per roll = gross roll coverage × (1 − overlap), rolls = roof area ÷ usable, rounded up. Synthetic 10-square coverage is sourced from the common 48" × 250' format (GAF FeltBuster, OC Titanium UDL50, Grip-Rite listings); #15/#30 felt use nominal 4-square/2-square planning figures that sit slightly below the 432/216 ft² of typical ASTM D4869 retail rolls — confirm the label. The count covers field courses only: valleys, starter courses, steep- or low-slope double coverage, and ice-and-water membrane at eaves and penetrations are separate line items, and the underlayment your roof requires (type and layers) is a code/manufacturer question tied to pitch.
Pair with the Roof Area & Pitch Calculator to get the surface area from a footprint, the Shingle & Bundle Calculator for the layer that goes on top, or the Roof Pitch Multiplier for the slope math behind the area.