Roofing Underlayment Calculator
Enter the building size and pitch, or a known roof area. You get rolls of synthetic underlayment for the whole deck and rolls of ice-and-water barrier for the eaves.
Underlayment rolls
2 rolls
10-square synthetic rolls · 1,550 sq ft with 10% extra
2 rolls of 10-square underlayment and 2 rolls of ice-and-water barrier.
- Roof area1,409 sq ft (130.9 m²)
- Area with laps1,550 sq ft (144.0 m²)
- Eave length used84.0 ft (estimated)
- Ice barrier area252 sq ft (23.4 m²)
- Ice-and-water rolls (2 squares)2
- At your price per rollAdd a price
Synthetic rolls are counted at 10 squares each and ice-and-water rolls at 2 squares. How far up the slope the barrier must go is set by your local code and the shingle maker.
Planning estimate, not a quote. Disclaimer
How the underlayment calculator works
The whole roof deck gets underlayment, so the roll count starts from the roof area plus an allowance for laps. A 10-square synthetic roll such as GAF's FeltBuster or Deck-Armor (48 in × 250 ft) is listed at 1,000 sq ft and GAF notes that figure excludes laps; the calculator adds your overlap allowance, 10% by default, which is our assumption and not a manufacturer figure. Ice-and-water barrier goes along the eaves, so its area is the eave length times the width of the barrier course. A GAF StormGuard roll is 3 ft × 66.8 ft, listed as 200 sq ft.
Ice-and-water area = eave length × barrier width up the slope
Ice-and-water rolls = area ÷ 200, rounded up
Example: the default roof is 1,409 sq ft. With 10% for laps that is 1,550 sq ft, or 2 rolls of 1,000 sq ft. The eaves are about 84 ft; a 3-ft course along them is 252 sq ft, or 2 rolls of ice-and-water barrier.
Check these before you buy
- How far up the slope the barrier must go is set by local code and the shingle maker. Many codes measure from the interior wall line, so a wide overhang needs a wider course than a narrow one. Change the barrier width to match.
- Low-slope roofs (below 4:12) need a double layer under many shingles. GAF, for example, requires two layers of its underlayment for Royal Sovereign shingles at 2:12 to 4:12, plus a leak barrier at the eaves. Double the roll count if that is your case.
- Valleys, chimneys and skylights often get barrier too. Add a roll if you have several.
Use the shingle calculator for the shingles that go on top.
Frequently asked questions
How many rolls of underlayment do I need?
Divide the roof area plus laps by the coverage per roll. A 1,409 sq ft roof with 10% for laps needs 2 rolls of 1,000 sq ft underlayment.
How much does a roll of synthetic underlayment cover?
GAF lists its 48 in × 250 ft synthetic rolls at 1,000 sq ft (10 squares) and notes that excludes laps. Felt rolls are smaller and vary by weight, so check yours.
How many rolls of ice-and-water shield do I need?
Multiply the eave length by the course width and divide by the roll coverage. GAF StormGuard is 200 sq ft per roll; a 3-ft course on 84 ft of eaves needs 2 rolls.
How far up the roof does ice-and-water barrier go?
It depends on local code and the shingle manufacturer. Many jurisdictions require it to extend past the interior wall line. Ask your building department.
Do I need underlayment under metal roofing?
Often yes, but the requirement depends on the panel and the code. Check the panel manufacturer’s installation instructions.
Related
Other roofing calculators
Sources
- GAF: FeltBuster data sheet (10 squares per roll, excludes laps) (Oct 2022; checked Nov 12, 2026)
- GAF: StormGuard technical data sheet (3 ft x 66.8 ft, 200 sq ft roll) (checked Nov 12, 2026)
- GAF: Minimum slope for shingles (checked Nov 12, 2026)