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Roof and Rafter Plain answers about roofs and the crews who fix them

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Roof Area Estimator: Squares from Footprint and Pitch

Roofers quote in squares, and a square is 100 square feet of roof surface. If you know the length and width of the house, the overhang and the pitch, you can check a quote's quantities yourself before you talk to anyone.

Footprint with overhang
1,344 sq ft
Roof surface area
1,502.6 sq ft
Roofing squares
15 squares
Squares with waste allowance
16.5 squares

The overhang is added to every side of the house length and width, which gives the footprint. The footprint is then multiplied by the pitch factor, the square root of 1 plus (rise divided by 12) squared, because a sloped surface is larger than the ground beneath it. One roofing square is 100 square feet. The waste allowance adds a percentage on top for cuts and offcuts. What the boxes hold when the page opens is a worked example: 40 by 30 feet, a 12 inch overhang, a 6 in 12 pitch and 10 percent waste.

How this works

The ground footprint of the roof is the house length and width plus the overhang on every side. A sloped roof covers more surface than that footprint, and the pitch tells you how much more. Pitch is written as rise over run: a 6 in 12 roof climbs 6 inches for every 12 inches it runs horizontally. The pitch factor is the square root of 1 plus (rise divided by 12) squared, so a 6 in 12 roof is about 1.118 times its footprint and a 12 in 12 roof is about 1.414 times.

Roof area is footprint times pitch factor. Divide by 100 for roofing squares. The waste allowance adds a percentage for cuts, starter courses and ridge pieces; a plain gable usually needs less than a roof with many valleys and hips, and your roofer or the manufacturer's guidance will name the figure for your job.

What it leaves out

This is an estimate for a roof whose planes all share one pitch, such as a gable or a hip roof. Dormers, porches, attached garages with their own pitch, chimneys and skylights are not counted, and a quote should measure them separately. Treat the result as a way to sanity-check quantities, not as an order sheet. Local code and the product manufacturer's installation instructions govern what the finished roof needs. For the vocabulary on a quote, see the roofing terms page, and for what to ask a crew, the hiring guide.