Roof Area Calculator

A roof is a sloped surface, and a slope always covers more area than the building underneath it. Measuring the footprint — the flat outline of the house — and treating that as the roof area is the most common estimating mistake, because it understates the material needed by a margin that grows with the steepness of the pitch. The correction is the pitch multiplier. A roof pitch is described as rise over run, such as six-in-twelve, meaning six inches of rise for every twelve inches of horizontal run. The slope length is the hypotenuse of that triangle, and dividing it by the run of twelve gives the multiplier: about 1.12 for a six-twelve roof, 1.25 for a nine-twelve, and 1.41 for a twelve-twelve. Multiplying the footprint by this figure gives the true sloped area. Overhangs add to the footprint before any of this, because the roof extends past the walls on every side and that projecting area has to be covered too. Add the overhang to both dimensions, then apply the multiplier. For a roof with more than one plane, such as a hip or a gable with a garage wing, work out each plane separately and add them, since different sections often have different pitches. This calculator takes the building dimensions, overhang and pitch, and returns the true roof area in square feet and in roofing squares, plus an estimate of shingle bundles. It assumes a simple gable-shaped roof; complex roofs with valleys, dormers and multiple ridge lines need each plane measured individually. Use the total to compare contractor quotes, which are usually priced per square.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.