Roof Area Calculator
Calculate sloped roof surface from its rectangular plan.
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What this tool does
Calculate sloped roof surface from its rectangular plan. All roof planes have the same pitch; irregular extensions require separate sections. Bundles assume three per 100 square feet.
How to use the Roof Area Calculator
- Enter or select length in ft.
- Enter or select width in ft.
- Enter or select roof pitch (rise per 12).
- Enter or select overhang on each edge in ft.
- Enter or select waste allowance (%).
- Read the calculated result; change any measurement to compare alternatives.
Formula
roof area=(length+2×overhang)×(width+2×overhang)×sqrt(1+(pitch/12)²); squares=area/100; bundles=ceil(area×(1+waste/100)/bundleCoverage)
- length
- Length (ft, before metric conversion)
- width
- Width (ft, before metric conversion)
- pitch
- Roof pitch (rise per 12)
- overhang
- Overhang on each edge (ft, before metric conversion)
- waste
- Waste allowance (%)
All roof planes have the same pitch; irregular extensions require separate sections. Bundles assume three per 100 square feet.
Worked example
For roof area calculator, the following measurements illustrate the exact method: Length: 20; Width: 10; Roof pitch (rise per 12): 6; Overhang on each edge: 1; Waste allowance (%): 10.
Inputs
- Length20 ft
- Width10 ft
- Roof pitch (rise per 12)6
- Overhang on each edge1 ft
- Waste allowance (%)10
Result
- Roof surface area295.16 sq ft
- Roofing squares3.25
- Area with waste324.68 sq ft
Results explained
- Roof surface area
- Roof surface area from the formula above, expressed in sqft before metric conversion. All roof planes have the same pitch; irregular extensions require separate sections. Bundles assume three per 100 square feet.
- Roofing squares
- Roofing squares from the formula above. All roof planes have the same pitch; irregular extensions require separate sections. Bundles assume three per 100 square feet.
- Area with waste
- Area with waste from the formula above, expressed in sqft before metric conversion. All roof planes have the same pitch; irregular extensions require separate sections. Bundles assume three per 100 square feet.
Frequently asked questions
Multiply horizontal footprint including overhangs by the slope factor; equal-pitch hip and gable roofs share this total area.
Use plan length/width, pitch and overhang.
Use 5–10% for simple layouts and 10–15% for diagonal cuts or complicated outlines; the allowance is editable.
Split the outline into rectangles or triangles and calculate each section separately. Do not count overlapping sections twice.
This is a purchasing estimate, not a structural design. Confirm measurements and local requirements before buying or building.