Quick Calculators · Free to use
Roofing Underlayment Calculator
Estimate roofing underlayment rolls from measured sloped area or projected area and pitch.
Your estimate
Enter your measurements to see the estimate.
Whole rolls
rolls- Sloped roof area
- sq ft
- Coverage including allowance
- sq ft
Single uniform pitch when converting projected area. Separate different roof pitches. Valleys, penetrations, ice barriers, multiple layers, fastening and product/code requirements are outside this quantity estimate.
How to Use This Calculator
Estimate roofing underlayment rolls from measured sloped area or projected area and pitch.
Decimals and fractions accepted for dimensions. Unmarked values use the labeled unit; explicit feet/inches are converted. For example, 10' 6 1/4" is an explicit length; 10.6 means 10.6 of the labeled unit. Rates, prices, counts and areas use numbers only.
Inputs and where to find them
Measured area (sq ft). Enter the roof area on the measurement basis selected below. If you measured each roof plane along its slope, use that sloped area. If you measured the horizontal footprint, use projected area and supply the pitch.
Area measurement. Choose measured sloped area when the roof’s incline is already included in your measurement. Choose projected area when starting from its horizontal footprint. Applying pitch to an already sloped measurement would count the slope increase twice.
Roof rise per 12 in run (in). For projected area, enter the inches of rise for each 12 inches of horizontal run. Obtain this from the roof plan or a level measurement. Calculate roof portions with different pitches separately.
Effective coverage per roll (sq ft). Enter the effective installed coverage of one roll after the required laps. Consult the product’s installation information rather than simply multiplying raw roll dimensions. Use the separate allowance for additional cutting or handling loss, not for the same laps again.
Allowance (%). Enter the percentage you want to add for cutting, handling loss or other conditions specific to this work. Apply it to the measured quantity rather than enlarging the measurements as well. The suggested value is editable, not a universal recommendation; enter zero when no allowance is needed.
Results, example and assumptions
Sloped roof area is the surface to cover before waste. Required coverage adds the entered allowance, and rolls round that demand up using effective installed roll coverage. These quantities do not separately estimate ice barriers, extra layers, fasteners or other roof details.
If projected, sloped area = projected area × √(12² + rise²) ÷ 12. If already measured on slope, do not apply pitch again. Rolls = ceil(sloped area × allowance factor ÷ effective roll coverage).
Worked example: 1,200 sq ft projected at 5:12 is 1,300 sq ft sloped. With 10% allowance, 1,430 sq ft requires 4 rolls of 400 effective sq ft.
Single uniform pitch when converting projected area. Separate different roof pitches. Valleys, penetrations, ice barriers, multiple layers, fastening and product/code requirements are outside this quantity estimate.