How this calc works

Roofing is sold and bid by the square — 100 square feet of roof surface. This calc takes your roof area and divides by 100 to give you squares, the unit you actually order shingles, felt, and labor in.

The catch every estimator knows: the square footage that matters is the sloped surface area, not the footprint you measure off the ground or off a plan. A 24x40 footprint is 960 square feet flat, but on an 8/12 pitch the real surface is closer to 1,150 — about 12 squares, not 9.6. If you are starting from the footprint, run the roof-area or pitch-multiplier calc first, then bring that number here.

Common mistakes

The margin-killer is bidding off the flat footprint and forgetting the pitch. On a steep roof that error is double-digit squares — enough to lose money on a job you thought you priced right.

The second is ordering exactly the squares calculated with no waste allowance. Hips, valleys, and starter courses eat material; add 10% for a simple gable and 15% or more for a cut-up roof with multiple valleys.

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