Roofing & Exterior
Estimate underlayment purchase quantity from a verified roof surface and the effective coverage of the selected roll after using an editable overlap/detail allowance.
Change the defaults to match your project and the product you plan to buy.
Project result
See the area target after overlap/detail allowance and the whole-roll purchase quantity.
Planning estimate only. Verify product coverage, package size, supplier specifications and project requirements before purchasing.
Choose the right tool
You know roof surface area and effective roll coverage and need underlayment rolls.
Use Roofing Shingle for shingle bundles; this tool does not select membrane type or lap rules.
Methodology
measured roof surface
Use sloped area.
area × (1 + allowance)
Represents laps, cuts and details.
target ÷ roll coverage
Rounds to whole rolls.
rolls × price
Optional underlayment-only cost.
Begin with the actual roof surface area rather than the building footprint. If different roof sections use different membranes or application methods, calculate them separately. Use the product documentation to determine whether the stated roll coverage is gross roll area or effective installed coverage.
Underlayment selection and lap requirements depend on the roof system, slope, climate, manufacturer and applicable requirements. The calculator does not choose a membrane or lap width. Enter the effective coverage and allowance that match the approved installation plan.
Reference
| Planning item | Starting point | Verify / adjust |
|---|---|---|
| Roof area | sloped surface | Use same measurement basis as roofing material. |
| Allowance | laps/details | Project specific. |
| Roll coverage | effective product value | Gross roll area can overstate coverage. |
| Special membranes | separate scope | Eaves/valleys may differ. |
Questions
Only if it matches the effective installed coverage. Required laps can make usable coverage smaller.
Not separately. Different membrane zones should be calculated as their own scope.
Use sloped roof area unless your product planning method explicitly accounts for pitch elsewhere.
Planning guides
Roofing & Exterior
Plan underlayment rolls using effective installed coverage after laps and details rather than gross roll dimensions alone.
Read guide →Estimate simple gable roof surface area from building footprint and roof pitch, with separate eave/overhang inputs and no material waste.
Open calculator → Roofing & ExteriorEstimate roofing squares and shingle bundles from measured roof area, waste allowance and actual bundle coverage.
Open calculator → Roofing & ExteriorEstimate siding purchase coverage from exterior wall dimensions, major openings, waste allowance and product package coverage.
Open calculator →