Roof Area Calculator

The short answer

Roof area is the footprint divided by the cosine of the pitch angle. A 6-in-12 pitch is about 26.6 degrees with a cosine of 0.894, so a 24 by 40 foot house with a 960 square foot footprint has about 1,073 square feet of roof. Add 10 to 15% for starter courses, hips, valleys and cutting waste, which puts the order at roughly 1,200 square feet, or 12 squares.

What it depends on

Pitch is the dominant variable, and the relationship is not intuitive: a 12-in-12 pitch increases the area by about 41% over the footprint, not 100%. Complex roof shapes with dormers, valleys and hips waste more than a simple gable of the same footprint, and a steeper roof needs extra material for handling and exposure. Measuring from the ground or from a satellite image introduces error in both the footprint and the pitch.

Where this stops being useful

This is an area estimate for material planning. Roof structure, load capacity, underlayment, ventilation, flashing and ice dam detailing are all specified by code and by manufacturer, and roofing usually requires a permit. A qualified roofing contractor can measure and specify the system. General information and planning only, not engineering, structural or code advice.

Roof area is not the same as the footprint of the building, and using the plan figure is the classic error. On a simple gable roof, each slope is the length of the building times the sloped width, and the sloped width is longer than the horizontal width by the rise of the roof. That difference is significant on a shallow pitch and minor on a steep one. On a hip roof there are two trapezoids and two triangles, and the triangles are the part people forget. Measure each slope separately if you can, and account for overhangs if the roof projects beyond the walls.

The roof area figure also decides the order of work on the job, because underlayment has to go down before the shingles and the ventilation has to be right before either. Getting the number wrong does not just mean a wrong order; it can mean the wrong sequencing.

Once you have the slope area, everything else in roofing is calculated from it: shingles and underlayment by the square, starter strips by the eave run, and ridge or hip capping by the length of the peak. Ventilation calculation is different again — it depends on the attic floor area rather than the roof area, and it is what determines whether you need ridge vents, soffit vents or gable vents. If you are comparing a metal roof against shingles, the slope area is the common number, but metal panels come in fixed widths and the coverage per sheet is not the same as the panel's total area.

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.