Calculated result
2rolls
Underlayment rolls to order
Formula
ROOF-UNDERLAY-001L × W × √(1 + (rise ÷ 12)²) × (1 + waste) ÷ net coverage
670.82 ft²
Pitched roof area
737.9 ft²
Area including waste
1.118 ×
Slope multiplier
Roofing calculator
Roofing underlayment calculator
Pitched roof area and whole underlayment-roll quantity using stated roll coverage.
By Feroz Sheikh · Formula ROOF-UNDERLAY-001 v1.0.0 · Tested formula
Formula
ROOF-UNDERLAY-001v1.0.0Project inputs
Enter your measurements. Switching units converts the same project, including coverage and unit rates. Package yields keep the unit printed beside each field.
Refine measurements and rates at any time. Your current values are included in the material-list PDF.
Planning estimate, not engineering advice. Confirm site measurements and product specifications. Calculation limits.
This calculator turns sloped roof area into whole underlayment rolls using the coverage printed on the selected product. Formula ROOF-UNDERLAY-001 v1.0.0. Ice-and-water, felt and synthetic do not share a roll. Keep laps as labelled coverage. Drop rolls into an estimate. Follow the specified waterproofing system.
- Measure horizontal roof dimensions and enter pitch rise per 12.
- Use net product coverage after overlap requirements, not nominal roll area.
- Confirm product instructions and special detailing at valleys, eaves and penetrations.
Add your project costs
Enter supplier prices and any labour, equipment or delivery costs. Pick a currency — badges, inputs, and totals follow.
Costs use the square, bundle or panel rate you type. Construclytic does not fetch shingle or metal prices. Tear-off, dumpsters, starter and flashing stay separate estimate lines. For GCC projects, select QAR, AED, SAR, KWD, OMR or BHD and enter local supplier rates.
How this calculator works
A transparent roofing underlayment calculator takeoff.
Feroz SheikhWritten by Feroz Sheikh · founder
This calculator converts a pitched roof area into whole underlayment rolls using the selected product's net coverage after required laps.
ROOF-UNDERLAY-001 turns sloped roof area into whole rolls using the coverage printed on the selected product. Ice-and-water, felt and synthetic do not share a roll. Formula ID stays on the sheet so a later estimator can swap the product without guessing the old coverage.
Worked example. Example: a 30 ft × 20 ft roof at 6:12 has about 671 ft² of area. With 10% allowance and 432 ft² net roll coverage, it rounds to 2 rolls.
See the example inputs and calculated result
This separate example uses the calculator's starting values in imperial units. It stays unchanged when you edit the tool above. Example rates, yields and dimensions are assumptions, not recommendations for your project.
| Input | Example value |
|---|---|
| Roof length | 30 ft |
| Roof width | 20 ft |
| Pitch rise | 6 rise per 12 |
| Roll coverage | 432 ft² |
| Waste factor | 10 % |
Example result: Underlayment rolls to order — 2 rolls.
L × W × √(1 + (rise ÷ 12)²) × (1 + waste) ÷ net coverage
Use the labelled units above when reproducing this example. To work in metric, switch the calculator to Metric; applicable measurements are converted, while product-specific units remain labelled.
- 1. Measure horizontal roof dimensions and enter pitch rise per 12.
- 2. Use net product coverage after overlap requirements, not nominal roll area.
- 3. Confirm product instructions and special detailing at valleys, eaves and penetrations.
Field notes that change the number
- Use the sloped area after pitch, not the building footprint.
- Enter the labelled net coverage after laps, not the marketing roll size.
- Valleys and eaves that get ice-and-water are a different calculator.
Common mistakes
- Ordering felt rolls from plan area on a 8:12 roof.
- Folding ice-and-water into the felt count.
- Ignoring the labelled overlap.
Need the broader workflow? Read the roofing planning guide.
How-to: How to estimate roof shingles
Questions
Questions about roofing underlayment calculator
Does one roll always cover the same area?
No. Coverage depends on the selected product and required laps; use manufacturer guidance.
Does this replace roof installation instructions?
No. Follow product instructions and local requirements for installation and detailing.
How many underlayment rolls for a roof?
A 30 × 20 ft footprint at 6:12 with 432 ft² net roll coverage and 10% waste returns 2 rolls. Use sloped area, not the building footprint.
Is ice-and-water the same as felt?
No. Ice-and-water, felt and synthetic do not share a roll. This count is for the product whose coverage you entered.
What is the formula ID?
This roofing underlayment calculator publishes formula ROOF-UNDERLAY-001, version 1.0.0. The ID stays on the result and on the estimate line.