Roof Area Calculator

The formula

When every roof plane has the same pitch, the sloping roof area equals the flat (plan) area covered by the roof multiplied by the slope factor. Plan area = (length + 2 × overhang) × (width + 2 × overhang). Slope factor = √(rise² + run²) ÷ run, or 1 ÷ cos(angle) when you enter degrees. Roof area = plan area × slope factor. Wastage is then added as a percentage.

For sheets, number of sheets = area with wastage ÷ (sheet length × sheet width), rounded up. For shingles, a roofing square is 100 sq ft (9.29 m²); bundles = squares × bundles per square, rounded up. Three bundles per square is the default and is common for standard shingles, but the pack states its own coverage.

Worked example

A building 10 m long and 8 m wide with a 0.5 m overhang all round has a plan area of 11 × 9 = 99 m². At a pitch of 6 in 12 (rise 6, run 12) the slope factor is √(6² + 12²) ÷ 12 = 1.1180 and the angle is 26.6°. The roof area is 99 × 1.1180 = 110.69 m². With 10% wastage it becomes 121.75 m². Covered with sheets whose effective cover is 3 m by 0.8 m (2.4 m² each), that is 121.75 ÷ 2.4 = 50.7, so 51 sheets.

Rise and run, or degrees

• Rise and run: a pitch of 6 in 12 means the roof rises 6 units for every 12 units of horizontal run. Use the same unit for both. The default run of 12 suits the usual X-in-12 way of stating pitch.
• Degrees: enter the roof angle directly. A 30° roof has a slope factor of 1 ÷ cos 30° = 1.1547. Angles from 0 to 80 degrees are accepted.
• A flat roof (rise 0 or 0 degrees) has a factor of 1, so its area equals the plan area.

Limits

• It assumes every plane of the roof has the same pitch, as on a simple gable, hip or single-slope roof. Roofs with different pitches, dormers, valleys, chimneys and skylights need to be measured plane by plane.
• The overhang is added on all four sides. For a gable roof with overhang only at the eaves, enter 0 for the overhang and add twice the eave overhang to the building width yourself.
• The area is the roof surface, not the amount of material you can lay. Overlaps, ridge and hip caps, starter courses, flashing and underlay are not counted.
• Wastage is entered by you. Cut roofs with many angles need more than a simple rectangle.
• Enter the sheet's effective covering size after the overlaps are deducted, not the size on the packing label.

More worked examples

Imperial with shingles: a 40 ft × 30 ft building, 1.5 ft overhang, pitch 4 in 12 and 15% wastage. The plan area is 1,419.0 sq ft and the slope factor is 1.0541 (18.4°). The roof surface is 1,495.8 sq ft, which is 1,720.1 sq ft with wastage, or 17.20 roofing squares. At 3 bundles per square you need 52 bundles.

Metric with an angle: a 10 m × 8 m building at 30° has a slope factor of 1.1547, a roof area of 92.38 m² and 101.61 m² with 10% wastage. At 45° a 24 ft × 20 ft roof (480 sq ft in plan) is 678.8 sq ft, or 6.79 squares and 21 bundles.

Sheets: a 6 m × 4 m building with a 0.3 m overhang and a 45° (1 in 1) pitch has a plan area of 30.36 m², a roof of 42.94 m² and 47.23 m² with 10% wastage. Sheets covering 2.4 m × 0.9 m (2.16 m² each) need 22.

Limits on the pitch inputs

• A rise of 0 (or an angle of 0) gives a slope factor of exactly 1, so the roof area equals the plan area. Leave the overhang at 0 and the answer is the building footprint.
• Angles from 0 to 80 degrees are accepted. At 80° the factor is 5.7588 and the 10 m × 8 m roof is 460.70 m². 85° shows "The pitch angle should be between 0 and 80 degrees."
• A blank rise shows "Please fill in the rise and the run." For sheets, missing sheet dimensions show "Please fill in the sheet length and width."