Roofing / Roll quantity
Roof Underlayment Calculator
Estimate whole roof-underlayment rolls from verified sloped roof area, project-specified layers, manufacturer-published effective coverage, and your ordering allowance. See the base area, exact rolls, whole-roll rounding, surplus coverage, and optional material cost separately.
Calculate roof underlayment rolls
Enter verified sloped roof area, project layers, and the selected product's published effective coverage. The calculator keeps allowance and whole-roll rounding separate.
Roof and layer basis
Use the true roof surface area after pitch adjustment, not the building footprint.
Use the layer count from the project requirements. The calculator does not choose it.
Roll coverage and ordering
Enter published effective coverage for the selected product and installation, not simply roll length × width.
Enter the selected product's effective coverage per roll.
Blank means 0%. The calculator does not recommend an allowance.
Enter your supplier rate or leave blank for quantities only.
The currency label does not fetch prices or convert exchange rates.
Coverage basis
- Sloped roof surface area
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- Project layers
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- Base coverage area
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- User allowance area
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- Ordering area
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Underlayment rolls
- Effective coverage per roll
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- Exact roll requirement
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- Whole rolls to order
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- Rolls added by whole-unit rounding
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- Coverage represented by whole rolls
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- Coverage beyond ordering target
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- Optional material cost
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View copyable summary
Enter valid roof area and effective roll coverage to see the summary.
Method
How to calculate roof underlayment rolls
Start with the actual sloped roof surface area. If you only have plan dimensions and pitch, use the Roof Area Calculator first. Then use the effective coverage stated for the selected underlayment and installation method.
- Base coverage area = sloped roof area × project layer count
- Allowance area = base coverage area × user allowance ÷ 100
- Ordering area = base coverage area + allowance area
- Exact rolls = ordering area ÷ effective coverage per roll
- Whole rolls = exact rolls rounded up once to the next whole roll
Roll length multiplied by roll width may describe gross material area. Effective coverage can be lower because products and installation procedures differ. Use the coverage basis published for the product and project rather than assuming gross roll area is usable coverage.
Worked examples
Imperial and Metric underlayment examples
For a 2,000 ft² sloped roof with one layer, a user-selected 10% allowance, and 1,000 ft² of effective coverage per roll, the base area is 2,000 ft² and the ordering area is 2,200 ft². The exact requirement is 2.2 rolls, so the purchase quantity is 3 whole rolls. Those rolls represent 3,000 ft² of published coverage, 800 ft² beyond the allowance-adjusted target.
For a 100 m² roof requiring two layers, the base coverage area is 200 m². A 5% allowance adds 10 m². At 50 m² effective coverage per roll, 210 ÷ 50 gives 4.2 rolls, which rounds once to 5 whole rolls.
Roofing felt and roof underlayment
Roofing felt is commonly used as a name for an underlayment installed below the finished roof covering, but “felt roof” can also describe exposed weatherproofing systems for some sheds and flat roofs. This calculator covers roll material used as roof underlayment. It does not estimate an exposed felt roof system, adhesives, caps, flashing, or finish layers.
Synthetic and felt products can have different coverage and installation instructions. GAF likewise notes that underlayment types can require different installation procedures. Use the selected product's current instructions for coverage, laps, fastening, exposure limits, and suitability. See GAF's synthetic roof underlayment overview. GAF does not certify this calculator.
Area, layers, allowance, and roll rounding
Roof surface area is geometric. Multiplying by the entered layer count creates the base material coverage target. The user-selected allowance is then shown as a separate area before the calculator divides by product coverage and rounds the final result to whole rolls.
This separation helps prevent a BOQ or purchase record from treating allowance, package rounding, or surplus as extra roof geometry. ConstructGauge does not prescribe an allowance or layer count because roof details, product requirements, weather exposure, jurisdiction, and project documents vary.
Assumptions and limitations
- The roof-area input is the exterior sloped surface, not floor area or an unadjusted building footprint.
- The layer count comes from the project specification, product instructions, and applicable requirements.
- Effective coverage per roll comes from the selected product and intended installation method.
- The user selects the ordering allowance; no universal percentage is prescribed.
- Whole-roll rounding is applied once after layers and allowance.
- Optional cost uses a user-entered roll rate and currency label; it is not a live supplier quote or installed cost.
- The calculation excludes ice barriers, caps, tapes, adhesives, fasteners, flashing, labor, and installation design.
Roof underlayment calculator FAQ
How many rolls of roof underlayment do I need?
Multiply the sloped roof area by the required layers, add your chosen allowance, divide by the selected product's effective coverage per roll, and round the final result up to a whole roll. The calculator displays each step.
Should I use roof area or house floor area?
Use the exterior sloped roof surface area. Floor area or horizontal footprint can understate material when the roof has pitch, overhangs, or multiple sections.
What coverage should I enter?
Enter the manufacturer's published effective coverage for the exact product and intended installation. Do not assume the gross area from roll length and width is the same as effective coverage.
How many underlayment layers are required?
This calculator does not decide that. Use the layer count required by the project documents, selected roof covering and underlayment instructions, and applicable local rules.
Can I use this as a roofing felt calculator?
Yes, when the felt is underlayment beneath another roof covering and you enter its effective roll coverage. The calculator is not designed for exposed shed or flat-roof felt systems.
Does the estimate include ice-and-water barrier or fasteners?
No. It returns underlayment roll quantity and optional roll material cost only. Separate roof zones, fastening schedules, accessories, labor, and installation are outside its scope.
BOQ use and calculation basis
The primary reusable result is finite integer wholeRolls, suggested as a “Roof underlayment rolls” BOQ line when the product coverage, layer count, and user allowance match the estimate. Base, allowance, ordering, purchased, and surplus areas remain separate square-metre quantities.
The pure calculation engine returns unformatted numbers and keeps geometry, layers, allowance, package rounding, supplier coverage, rate, and cost as separate fields. Metric and Imperial area conversions use the international foot relationship documented in the NIST conversion-factor reference. NIST does not certify this tool or its construction assumptions.