38 free calculators for trades and homeowners

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Roofing & Siding

Pitch, area, shingles, framing and cladding for roofs and exterior walls.

Why a roof is bigger than the house under it

Every roofing calculator here starts from the building footprint rather than the roof surface, because the footprint is the thing you can measure from the ground. Pitch converts one to the other, and on a steep roof the difference is substantial: a 12:12 roof has over forty percent more surface than the ground it covers.

Pitch is expressed as rise per 12 of run, not as an angle. A 6:12 roof is 26.57 degrees, and entering 6 into a tool expecting degrees produces a roof less than half as steep with every length that follows it wrong. The pitch calculator reports both figures side by side for exactly that reason.

Measuring a roof safely

  • Take the pitch from inside the loft with a level and a tape.
  • Measure the footprint on the ground, not on the roof.
  • Count hips, valleys and dormers — they drive waste.
  • Check how many bundles your shingle needs per square.

Questions

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A square is 100 square feet of finished roof surface. Shingles are packaged, priced and quoted against it, so squares are the unit to take to a supplier. Feet are what you measure in; squares are what you buy in.

Ten percent on a simple gable, fifteen or more once there are hips, valleys and dormers. Every cut through a valley leaves an offcut too small to use, so waste scales with how much cut edge the roof has rather than with its area.

Not quite. Siding is measured on the wall face, which is a flat area you can measure directly, so no pitch conversion applies. Gable ends are the exception and are entered separately, because they are triangles rather than rectangles.

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