How to Use This Roof Underlayment Calculator
Choose "From building size" to enter the length, width and overhang of a gable or hip roof, or switch to "From roof area" if you already know the sloped area. Enter the pitch, pick your underlayment and set a waste allowance. Tick the ice barrier box if your area needs it. The result shows how many rolls to buy, how many cap nails to order, and a side-by-side comparison of every underlayment type.
Why This Matters
Underlayment is the backup layer under your shingles, tiles or metal. It sheds wind-driven rain, protects the deck while the roof is being installed, and is the layer that stops water at the eaves when ice dams form. Ordering the right amount matters because the roof is only as dry as its weakest bare patch. A typical 2,000 sq ft ranch on a 6:12 roof has about 2,400 sq ft of surface, or 24 "squares". That is far more than the footprint suggests, because pitch adds area.
Running short mid-job means a supply run with the deck exposed to weather. Over-ordering ties up money in rolls that may not be returnable. Roll sizes also differ a lot: a #15 felt roll nets about 408 sq ft, while a synthetic roll nets over 900 sq ft. Homeowners comparing quotes, contractors writing material lists and DIYers planning a weekend tear-off all need the same number: rolls, rounded up, with the right overlap and waste built in.
How It's Calculated
The calculator turns your building size into sloped roof area, then divides by the usable coverage of one roll:
Headlap is 2 in. for felt, 4 in. for synthetic and 3 in. for self-adhered rolls. Layers are 2 for pitches under 4:12 (double coverage per IRC R905.2.7) and 1 otherwise. Rolls are always rounded up.
For ice barrier, a 36-in. roll covers one course. A strip wider than 36 in. needs extra courses, each adding 33 in. after a 3-in. lap. Rolls = ⌈(courses × eave length + valley length) × (1 + waste) ÷ 67 ft⌉. Under IRC R905.1.2 the barrier must reach at least 24 in. inside the exterior wall line, so the tool also shows that minimum width, (overhang + 24 in.) ÷ 12 × slope factor, and warns if your strip is shorter. Cap nails are estimated at 2 per sq ft of underlayment, including waste, in boxes of 3,000. Match your manufacturer's nailing pattern if it is tighter. The ice barrier area is not subtracted from the main underlayment, which errs on the safe side.
Worked Example
A 40 ft × 28 ft gable house with 12-in. overhangs, a 6:12 pitch, synthetic underlayment and 10% waste (the tool's defaults):
- Plan area with overhang: (40 + 2) × (28 + 2) = 42 × 30 = 1,260 sq ft.
- Slope factor: √(1 + 0.5²) = 1.118. Roof area = 1,260 × 1.118 = 1,408.7 sq ft (14.09 squares).
- Pitch is 6:12, so 1 layer. With 10% waste: 1,408.7 × 1.10 = 1,549.6 sq ft.
- Synthetic net coverage: (48 − 4) ÷ 12 × 250 = 916.7 sq ft per roll. 1,549.6 ÷ 916.7 = 1.69, so 2 rolls.
- Cap nails: 1,549.6 × 2 = 3,100 nails, which is 2 boxes of 3,000.
With the ice barrier ticked and a 4 ft strip: the gable eaves are 2 × 42 = 84 ft, and a 48-in. strip needs 1 + ⌈12 ÷ 33⌉ = 2 courses. That is 168 lineal ft × 1.10 = 184.8 ft ÷ 67 = 2.76, so 3 rolls. The minimum width to reach 24 in. inside the wall is (12 + 24) ÷ 12 × 1.118 = 3.35 ft, so 4 ft clears it. In the comparison table, #15 felt needs 4 rolls, #30 felt 8 rolls and self-adhered 9 rolls.
Tips & Common Mistakes
- Don't use the footprint. Floor area ignores pitch and overhangs. A 6:12 roof has about 12% more surface than its plan, and a 12:12 roof about 41% more.
- Raise the waste allowance for cut-up roofs. Hips, valleys, dormers and chimneys use more material. Use 10–15% for these and 15–20% for very complex roofs.
- Don't subtract skylights or chimneys. The offcuts are rarely usable, and the leftover covers mistakes and partial rolls.
- Check low slopes. From 2:12 up to under 4:12 you need double coverage, and asphalt shingles aren't permitted below 2:12. Those roofs need a low-slope system.
- Mind exposure limits. Felt and synthetics can only stay uncovered for a limited time, from days to months depending on the product. Order roofing so the underlayment isn't left exposed.
- Read the roll label. Brand roll sizes and laps differ. Use the custom coverage field if your roll doesn't match the defaults.
Frequently Asked Questions
How many rolls of underlayment do I need for 1,000 square feet?
With a 10% waste allowance you need to cover 1,100 sq ft. That is 2 rolls of synthetic (about 917 sq ft net each), 3 rolls of #15 felt, or 6 rolls of #30 felt. Add double coverage if the pitch is under 4:12.
What is a roofing "square"?
One square is 100 sq ft of roof surface. A 2,400 sq ft roof is 24 squares. Roofing materials and labor are commonly priced per square.
Is synthetic underlayment better than felt?
Synthetics are lighter, tear-resistant, and cover more per roll, and many tolerate longer exposure. Felt is cheaper and familiar. Your shingle manufacturer and local code decide what is acceptable, so check the shingle wrapper and local requirements.
Do I need two layers of underlayment on a low-slope roof?
Under the IRC, slopes from 2:12 up to under 4:12 need double underlayment coverage with asphalt shingles. This calculator applies two layers automatically for those pitches. Below 2:12, asphalt shingles aren't permitted at all.
How far up the roof should ice and water barrier go?
Where code or local history calls for it, the barrier should extend from the eave edge to at least 24 in. inside the exterior wall line, measured horizontally. Some installers go further, so confirm with your local building department.
Should I subtract for skylights and chimneys?
No. Small openings save very little material and the offcuts are hard to reuse. Keep the full roof area and use the waste allowance to cover cuts and mistakes.