How to Calculate Roof Area From Roof Dimensions

Roof area formula

Roof surface area = roof plan area × slope factor, where the slope factor for a rise-in-12 pitch is √(rise² + 12²) ÷ 12. A 30 ft × 40 ft roof plan at 6-in-12 is 1,200 × 1.118 ≈ 1,342 ft². If you can measure the roof itself, the same area is the eave length × the rake length (eave to ridge) of each plane, added up.

Footprint area vs. roof area

A building's footprint is the flat, bird's-eye-view area — length × width, as if you were looking straight down. A pitched roof's actual surface area is larger, because the roof slopes up from the eaves to the ridge, and the steeper the pitch, the bigger the difference. Three measurements are easy to mix up:

• Building footprint: the outline of the exterior walls.
• Roof plan: the roof's outline seen from above, out to the eaves and rakes — the footprint plus the overhangs.
• Roof surface: the sloped area you cover, equal to the roof plan × the slope factor.

Measuring eaves, rakes and overhangs

Use the roof plan, because the overhangs are covered too. Measure the roof outline from edge to edge, or add the overhang on both sides to the building's length and width: a 30 ft × 40 ft building with 1 ft overhangs all round has a 32 ft × 42 ft roof plan, about 1,503 ft² of roof at 6-in-12 instead of 1,342. Measure eave overhangs horizontally, as seen from above; a measurement along a sloped rafter tail reads slightly long. Roof Area vs Floor Area explains why the numbers differ.

The slope factor

The slope factor converts plan area into true sloped area: √(1 + (rise/run)²), the same as √(rise² + 12²) ÷ 12 for a rise-in-12 pitch. A flat roof (0-in-12) has a factor of 1.0; a 6-in-12 roof has about 1.118, so its surface is about 11.8% larger than its plan.

The roof pitch chart lists the factor for every pitch from 2/12 to 12/12, and the guide to calculating roof pitch shows how to measure an unknown pitch.

Putting it together

Roof area = roof plan area × slope factor. When every plane has the same pitch, this works for gable, hip and combination roofs alike: hips and valleys change the shape of each plane, not the total, though they add hip, ridge and valley length and cutting waste for materials. Keep full precision until the end and round the final area; the slope factor has no units, so the method works the same in metres.

Roofs with more than one pitch

Where pitches differ — a shallow porch roof against a steeper main roof, or a dormer — split the roof plan into areas of equal pitch, apply each factor and add the results, without counting any plan area twice. Dormer side walls are walls, not roof area.

Measuring the roof surface directly

Dimensions taken on the roof already include the slope, so no slope factor is needed — never multiply a measured surface by it again. The Asphalt Roofing Manufacturers Association's residential manual finds a gable roof's area as the eave length × the sum of its rake lengths, and splits complex roofs into rectangles and triangles (base × height ÷ 2, with the height measured up the slope) that are added together. For the 30 ft × 40 ft roof above, each rake is 15 × 1.118 = 16.77 ft, and 40 × (16.77 + 16.77) = 1,341.6 ft². ARMA recommends leaving on-roof measuring to a roofing professional; the plan-and-pitch method works from the ground or from drawings.

What roof area is used for

Roof area is the geometric starting point for every roofing material calculation — shingles, underlayment, metal panels. Each material calculator then adds its own waste allowance on top of this geometric figure, since material coverage and cutting waste vary by product.

Roofing is ordered in squares of 100 ft², so divide the roof area by 100: 1,342 ft² is 13.42 squares before waste. The shingles guide and the Roof Shingle Calculator turn squares into bundles.