Roofing Underlayment Calculator
A roofing underlayment calculator divides roof area by 1,000 sq ft per synthetic roll: 3,700 sq ft needs 4 rolls, or 5 with 10% waste.
Roofing underlayment calculator formula
A roofing underlayment calculator turns roof area into whole rolls. The formula is rolls = roof area in square feet ÷ 1,000, rounded up to a whole roll.
The calculator assumes one roll of synthetic underlayment covers 1,000 sq ft, which is 10 squares. Tick the waste box and the area is multiplied by 1.10 before the rounding. The price per roll then gives the material cost.
The area to enter is the roof surface, not the footprint of the house. A sloped roof is larger than the floor below it.
Worked examples
Example 1 is the default 30 ft × 20 ft roof plane, which is 600 sq ft, at $90 per roll.
- 600 ÷ 1,000 = 0.60 rolls, rounded up to 1 roll, $90
- With 10 percent waste: 660 ÷ 1,000 = 0.66, still 1 roll
Example 2 is a 2,400 sq ft roof.
- 2,400 ÷ 1,000 = 2.40, rounded up to 3 rolls, $270
- With 10 percent waste: 2,640 ÷ 1,000 = 2.64, still 3 rolls
Example 3 is a 3,700 sq ft roof, where the waste setting changes the order.
- Without waste: 3.70, rounded up to 4 rolls, $360
- With 10 percent waste: 4.07, rounded up to 5 rolls, $450
The third job is just over the line. When the result is 4.07 rolls, you can decide whether the fifth roll is worth buying or whether a smaller allowance is enough for a simple roof.
Rolls by roof area
| Roof area | Rolls without waste | Rolls with 10% waste |
|---|---|---|
| 1,000 sq ft | 1 | 2 |
| 1,001 sq ft | 2 | 2 |
| 2,400 sq ft | 3 | 3 |
| 3,300 sq ft | 4 | 4 |
| 3,700 sq ft | 4 | 5 |
| 5,000 sq ft | 5 | 6 |
An area of exactly 1,000 sq ft needs one roll, but with the 10 percent box ticked it needs two. Rounding is applied after the waste.
Getting the roof area from the footprint
If you only know the footprint of the house, multiply by a pitch factor. The factor is the square root of 1 plus (rise ÷ 12) squared.
| Roof pitch | Multiplier |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
A 30 ft × 40 ft footprint with a 6/12 roof is 1,200 sq ft × 1.118 = 1,342 sq ft of surface, before overhangs. Add the eaves and rakes, and any dormers. The roof area calculator does this for you, and the roof pitch calculator helps if you do not know the pitch.
Types of underlayment
| Type | Notes |
|---|---|
| Synthetic | Light, tear-resistant rolls, often 10 squares each |
| Asphalt felt | Heavier and cheaper, sold as 15 lb or 30 lb rolls with less coverage |
| Self-adhering membrane | Used at eaves, valleys and penetrations, sold in smaller rolls |
Self-adhering ice and water membrane has its own coverage, and the calculator does not include it. Use the roofing felt calculator when you plan to use asphalt felt.
How to measure
- Measure each roof plane on the roof or from a drawing, using the sloped length, not the horizontal one.
- Add the planes together, including porches and dormers.
- Include the eave and rake overhang.
- Take a photo of the roof and write the lengths on it. That helps when you order.
- Ask the roofer whether the eaves and valleys will use a different membrane, and take that area out of the underlayment total.
Common mistakes
- Using the floor area of the house. The roof is larger.
- Ordering exactly one roll for 1,000 sq ft. Overlaps and cuts use more than the net area.
- Mixing up squares and square feet. One square is 100 sq ft, and a 10 square roll is 1,000.
- Choosing a product the shingle maker does not allow. Read the shingle wrapper and the warranty.
- Leaving underlayment exposed too long. Check the roll label for the exposure time, and cover it with the roofing as soon as you can.
Related calculators
Once the deck is covered, use the roofing shingle calculator to count bundles. For the deck underneath, the OSB calculator counts sheets.
Frequently asked questions
How much does one roll of synthetic underlayment cover?
This calculator uses 1,000 sq ft per roll, which is the usual 10 square roll. A square is 100 sq ft of roof. Some rolls are 4 ft × 250 ft, others are narrower or shorter, so read the roll label for the real coverage.
Is overlap already factored into the 1,000 sq ft coverage rate?
No, the calculator takes 1,000 sq ft as the net area per roll. Real rolls print overlap lines and the usable coverage after laps can be less than the full roll area. The 10 percent waste box covers part of that. Check the label, since it may list net coverage.
Why choose synthetic underlayment over traditional felt?
Synthetic rolls are lighter per square, tear less, and are often rated to stay exposed longer before the shingles go on. Felt costs less and is familiar. Pick based on your roofing manufacturer's requirements, since some shingle warranties name acceptable underlayments.
How much waste should I add for underlayment?
Ten percent is the calculator's setting. A simple gable roof may need less, and a roof with many valleys, hips and dormers needs more. Remember that whole rolls are bought, so the result is rounded up in either case.
Do I need underlayment under metal roofing too, not just shingles?
Usually yes. Most metal roofing systems call for an underlayment, and some need a high-temperature type. Follow the metal panel manufacturer's instructions and your local code, because the correct product depends on the system and the climate.
Estimates only. Verify quantities with your supplier before purchasing.