Roofing, Insulation & Exterior calculator
Roof Underlayment Calculator
Estimate roof underlayment roll coverage from horizontal footprint area, pitch, waste and effective product coverage.
Estimate roof underlayment roll coverage from horizontal footprint area, pitch, waste and effective product coverage. Enter the project measurements first, then replace any default material or performance value with data for the product you intend to use. Check roll laps and effective coverage carefully. Nominal roll area overstates installed coverage when overlaps are required. Use the product's effective coverage or include lap allowance.
What you need to enter
The Roof Underlayment Calculator uses the following project information to calculate the result.
- Building footprint area
- Roof pitch
- Waste (%)
- Coverage per pack/unit
Calculator result
Adjusted roof area and optional packs/units
How the Roof Underlayment Calculator works
This page is built around footprint adjusted for pitch, waste and unit coverage. Applied to roof underlayment rolls from sloping area and roll coverage, the engine first converts compatible measurements to a common basis and then calculates adjusted roof area and optional packs/units. Verify roll laps and effective coverage before treating the output as a buying, sizing or set-out figure.
Roof Underlayment Calculator worked example
Roof Underlayment Calculator example: For a 100 m² horizontal roof footprint at 30°, dividing by cos(30°) gives about 115.5 m² of simple sloping area. A 10% allowance brings that to roughly 127 m² before product-specific laps and detailing for roof underlayment rolls from sloping area and roll coverage. This example shows the arithmetic only; on a live job, review roll laps and effective coverage before turning the result into an order or final size.
Important calculation assumptions
For roof underlayment rolls from sloping area and roll coverage, nominal roll area overstates installed coverage when overlaps are required. Use the product's effective coverage or include lap allowance.
Frequently asked questions
What should I write down before calculating with the Roof Underlayment Calculator?
Use horizontal footprint area and true roof pitch, then enter waste and the effective coverage of the chosen roof product. Those entries form the basis for estimating roof underlayment rolls from sloping area and roll coverage.
How sensitive is the Roof Underlayment Calculator result to roll laps and effective coverage?
Nominal roll area overstates installed coverage when overlaps are required. Use the product's effective coverage or include lap allowance. Treat the assumptions behind roll laps and effective coverage as project-specific in the Roof Underlayment Calculator.
Does the Roof Underlayment Calculator include laps, hips and valleys?
For the Roof Underlayment Calculator, use the result in context: use adjusted roof area with the manufacturer's effective coverage to estimate packs/panels. Add separate quantities for ridge, hip, valley, starter, flashing and trim products.