Calculate roof area from footprint and pitch

A sloped roof has more surface area than its horizontal footprint. For a simple rectangular footprint with one uniform pitch, a slope factor connects the two.

Include the overhang on both sides

For a 30 × 20 ft building with a 1 ft overhang on every edge, the adjusted footprint is (30 + 2) × (20 + 2) = 704 square feet. An overhang measured on each edge is added twice to each building dimension.

Convert pitch to a surface factor

A 6:12 pitch rises 6 units for every 12 horizontal units. Its factor is √1.25, approximately 1.118034. The example footprint therefore gives about 787.096 square feet of roof surface before an allowance.

Do not enter an angle in degrees into the rise-per-12 field. For an angle θ, rise per 12 = 12 × tan(θ). A 30° angle corresponds to about 6.9282:12, with a surface factor of approximately 1.1547. The roofing calculator labels its pitch field as rise per 12.

From surface area to whole bundles

A chosen 10% allowance brings the 6:12 example to about 865.806 square feet. At an illustrative 33.33 square feet of coverage per bundle, that requires 26 whole bundles. Use the actual product coverage; do not assume every product comes three bundles to a square.

One US roofing square is 100 square feet, or exactly 9.290304 m². It is a trade unit of area, not a count of individual pieces. If the product coverage already accounts for its required overlap, do not subtract the overlap again.

Where this model stops

The model assumes one uniform pitch across a rectangular footprint. Roofs with multiple pitches, dormers, valleys, openings or complex cut patterns need separate measured planes and a layout-specific allowance. This quantity estimate is not a roof design, a load check or approval of a product for a particular slope.

Keep your footprint, overhang, pitch and product coverage with the quote. Changing one assumption in a saved estimate makes comparisons easier. Fasteners, flashing, underlayment, delivery and labor are separate items.

Calculate roof area and bundles →

See our formulas and rounding rules. All sizes, allowances and coverage values above are arithmetic examples, not installation specifications.