Roofing Underlayment Calculator
Work out how many rolls of underlayment your roof needs before the crew gets on the ladder. Enter your roof area and pick a felt or synthetic underlayment to get a roll count and cap nail estimate.
Total area of doors, windows or other openings to subtract.
Covers cuts, breakage and layout waste. 5–10% is normal.
Everything is calculated in your browser. Nothing you type is sent anywhere or stored.
Ordering tips
—
Coverage & waste
Material profile
Underlayment coverage is based on square footage per roll, commonly measured in roofing squares (100 sq ft equals 1 square). Take your roof’s total area, add a waste percentage for overlaps and starter courses, then divide by the coverage of one roll of your chosen underlayment. Round up to the next full roll since underlayment can’t be bought by the partial roll.
Underlayment Overlap by Roof Slope
How much you overlap each course depends on how steep your roof is.
| Roof Pitch | Minimum Overlap | Notes |
|---|---|---|
| 4:12 and steeper | 2 in horizontal, 4 in vertical | Standard single-layer application |
| 2:12 to under 4:12 (low-slope) | 19 in (double coverage) | Requires ice-and-water shield or double-layer felt |
| Valleys and eaves | Extra course or membrane | Add ice-and-water shield per local code |
Underlayment roll coverage by type
| Type | Coverage / unit | Size | Best for |
|---|---|---|---|
| #15 felt (4 sq/roll) | 400 sq ft | 36 in x 144 ft | Budget underlayment |
| #30 felt (2 sq/roll) | 200 sq ft | 36 in x 72 ft | Heavier-duty felt underlayment |
| Synthetic underlayment (10 sq/roll) | 1000 sq ft | 48 in x 250 ft | Lighter, tear-resistant, most common today |
How to calculate how many underlayment rolls you need
- Measure your roof area in square feet, or use your roof’s plan dimensions.
- Add waste typically 10% for overlaps, valleys, and starter courses.
- Pick a product felt (#15 or #30) or synthetic, each with different coverage per roll.
- Divide the adjusted area by the coverage per roll, then round up to a full roll.
- Add cap nails one box generally covers a set number of rolls; the calculator sizes boxes to your roll count.
Worked example
Here’s the breakdown for a 1,200 sq ft roof using synthetic underlayment.
- Roof area1,200 sq ft
- Waste added10%
- Underlayment selectedSynthetic underlayment, 1,000 sq ft coverage per roll
- Rolls needed2 rolls
- Cap nail boxes needed1 box
- Roll cost$190.00 (2 x $95.00)
- Cap nail cost$24.00
- Total material cost$214.00
Underlayment Roll Options
| Underlayment Type | Coverage per Roll | Best Use |
|---|---|---|
| #15 felt (4 sq/roll) | 400 sq ft | Budget underlayment |
| #30 felt (2 sq/roll) | 200 sq ft | Heavier-duty felt underlayment |
| Synthetic (10 sq/roll) | 1,000 sq ft | Lighter, tear-resistant, most common today |
Ordering roofing underlayment
Underlayment goes on fast once the crew starts, so run out mid-roof and you’re stuck waiting on a supply run with the deck exposed. Order a full roll extra on any job with valleys, hips, or multiple roof planes.
- Order one roll extra on cut-up roofsvalleys, hips, and dormers eat more material than a simple gable roof, and running short mid-job means exposed decking overnight.
- Match cap nails to the underlayment typesynthetic underlayment typically needs closer nail spacing than felt, so check the roll’s spec sheet before ordering fasteners.
- Check the shingle warranty requirementssome shingle manufacturers require a specific underlayment brand or type to keep the shingle warranty valid.
- Plan ice-and-water shield separatelyeaves and valleys in cold climates usually need a self-adhering membrane in addition to the field underlayment this calculator estimates.
Common mistakes
- Skipping the waste allowanceOverlaps at every course and extra courses at valleys and eaves use up more material than the flat roof area suggests.
- Buying felt by raw square footage instead of squaresFelt rolls are labeled and sold by roofing squares (100 sq ft), not raw square footage, which trips up first-time buyers.
- Wrong nail spacingCap nails need closer spacing, about every 12 in, on synthetic underlayment than standard felt nails, so check the product spec sheet.
- Not accounting for low-slope areasRoofs under 4:12 pitch need double-coverage application, which roughly doubles the underlayment needed for that section.
What this calculator assumes
Adjust your result if your project differs from these.
- Roof area entered is the actual roof surface area, not the footprint of the house.
- Waste percentage covers standard overlaps, starter courses, and valleys.
- One cap nail box is sized to cover a set number of rolls.
- Coverage figures assume standard single-layer application on a 4:12 or steeper roof.
Common questions
How much waste should I add for roofing underlayment?
10% is typical for a standard gable roof. Add more, up to 15-20%, for roofs with lots of valleys, dormers, or hips.
What's the difference between felt and synthetic underlayment?
Felt is heavier, cheaper, and sold in smaller rolls by the square. Synthetic is lighter, more tear-resistant, and usually covers more area per roll.
Do I need ice-and-water shield too?
Most codes require ice-and-water membrane at eaves and valleys in addition to standard underlayment in cold climates. This calculator covers the field underlayment, not that membrane.
How many cap nails do I need per roll?
Coverage varies by nail spacing and pattern, but a single box of cap nails typically covers one to two rolls of underlayment.
Can I use synthetic underlayment on a low-slope roof?
Only if the product is rated for low-slope, double-coverage application. Check the manufacturer’s spec sheet before using it under 4:12 pitch.
Embed this calculator
Related calculators
Roofing Underlayment Calculator Estimate
—
materialcalculate.com/tools/roofing-underlayment-calculator/
Project
| Wall size | — |
| Openings deducted | — |
| Net area | — |
| Material | — |
| Waste allowance | — |
Shopping list
| Units needed | — |
| Mortar / mix | — |
| Estimated total cost | — |
Notes
Estimate generated by materialcalculate.com. Confirm quantities with your supplier before ordering.