Underlayment Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Underlayment rolls are roof area divided by the roll coverage. A 2,000 sq ft roof needs 2 rolls of synthetic underlayment at 1,000 sq ft per roll, or 5 rolls of #15 felt at 432 sq ft per roll.
Your measurements
Enter the values you know — results update live.
Use the true sloped area, not the footprint.
Most felt and synthetic rolls are 36 or 48 inches wide.
Typical synthetic rolls are 250 ft; #15 felt is 144 ft.
Common overlaps are 4 in for synthetic, 2 in for felt, 6 in for ice-and-water.
Analysis
Live results from your measurements.
Show the working
- Net coverage:
250 × (4 − 0.333)= 916.7 sq ft - Area with waste:
2,000 × 1.1= 2,200 sq ft - Rolls:
ceil(2,200 ÷ 916.7)= 3 - Linear feet:
3 × 250= 750 ft
What this calculator does
Roof underlayment is the water-shedding layer installed between the deck and the finished roofing material. It comes in rolls, and the number of rolls depends on the roof area, the roll dimensions and the required overlap. #15 asphalt felt is the traditional choice and covers about 432 sq ft per roll with a 2 inch overlap. Synthetic underlayments are lighter, stronger and cover closer to 1,000 sq ft per roll. Ice-and-water shield is a self-adhering membrane used at eaves, valleys and penetrations; it is wider and has a larger overlap, so coverage per roll is lower.
How to use it
- Calculate the true sloped roof area, not the footprint.
- Enter the roll width and length from the product label.
- Enter the required horizontal overlap for the product and code jurisdiction.
- Subtract the overlap from the roll width to get the net coverage per roll.
- Divide the roof area plus waste by the net coverage and round up.
The formula
Net coverage per roll
net = roll length × (roll width − overlap)
The overlap strip is not usable coverage, so it is subtracted from the width.
Rolls required
rolls = ceil((roof area × (1 + waste)) / net coverage)
Rounds up because suppliers do not sell partial rolls.
Worked example
A 2,400 sq ft roof covered with 48 in × 250 ft synthetic underlayment at 4 in overlap, 10% waste.
- Net width: 4 − 0.333 = 3.667 ft
- Net coverage: 250 × 3.667 = 916.7 sq ft per roll
- Area with waste: 2,400 × 1.10 = 2,640 sq ft
- Rolls: 2,640 ÷ 916.7 = 2.88 → 3 rolls
Order 3 rolls of synthetic underlayment.
Reference chart
| Product | Roll size | Net coverage | Rolls at 10% waste |
|---|---|---|---|
| #15 felt | 36 in × 144 ft | 396 sq ft | 6 |
| #30 felt | 36 in × 72 ft | 210 sq ft | 11 |
| Synthetic | 48 in × 250 ft | 917 sq ft | 3 |
| Ice & water | 36 in × 75 ft | 206 sq ft | 11 |
Underlayment type characteristics
| Type | Typical overlap | Best for | Notes |
|---|---|---|---|
| #15 asphalt felt | 2 in | Budget jobs, short-term exposure | Tears easily when wet. |
| #30 asphalt felt | 2 in | Heavier duty, longer exposure | Thicker and more tear-resistant. |
| Synthetic | 4 in | Most residential metal and shingle roofs | Light, strong, UV-stable for months. |
| Ice & water shield | 6 in | Eaves, valleys, penetrations | Self-adhered; not for full roof. |
Common mistakes
- Using the gross roll coverage instead of subtracting the overlap.
- Forgetting waste for valleys, ridges and dormers.
- Using footprint area instead of sloped roof area.
- Mixing underlayment types without checking code requirements.
- Ordering exactly the calculated amount with no spare for repairs or rework.
Assumptions and limits
- Rolls are installed with uniform horizontal overlap.
- Vertical end laps are handled by ordering full rolls.
- The roof area entered is the true sloped surface area.
- Ice-and-water shield is calculated separately if used at eaves and valleys.
Frequently asked questions
How many square feet does a roll of underlayment cover?
A standard roll of #15 felt covers about 432 sq ft gross, but only about 396 sq ft after a 2 inch overlap. Synthetic rolls often cover 1,000 sq ft gross and about 917 sq ft net.
How many rolls of underlayment for a 2000 sq ft roof?
Two to three rolls of synthetic underlayment, or five to six rolls of #15 felt, depending on the overlap and waste allowance.
What overlap is required for roofing underlayment?
#15 and #30 felt typically need 2 inches. Synthetic underlayment usually needs 4 inches. Ice-and-water shield requires 6 inches or more per manufacturer.
Do I need underlayment under metal roofing?
Yes. A high-temperature synthetic underlayment is standard under metal. It prevents condensation damage and provides a slip surface.
Can I use #15 felt under shingles?
Yes, #15 felt meets the IRC underlayment requirement for standard asphalt shingles. Many roofers prefer synthetic for durability during construction.
Should underlayment be fastened with staples or cap nails?
Cap nails are preferred in high-wind areas and under synthetic underlayment because they hold better than staples. Staples are acceptable for felt in many jurisdictions but are increasingly restricted.
Related guides
- Minimum Roof Pitch by Material: IRC Limits for Every CoveringEvery covering has a slope below which water gets under it. Here are the code minimums, the underlayment upgrades that unlock them, and what to use when your roof is too flat.
- Roofing Squares and Shingle Bundles: How Many You NeedThe estimating unit every roofer quotes in — plus the starter strip, hip and ridge cap, underlayment and nail quantities that never appear in the headline square count.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.