Roof pitch calculator

Convert a roof pitch between degrees, X in 12 and per cent, or work it out from a rise and run, and get the rafter line length with the overhang, in millimetres or feet and inches.

Sources: Lysaght Custom Orb (minimum roof slope).

Roof pitch calculator

This calculator needs JavaScript, which is turned off or didn't load. You can still do it by hand: the formulas and worked examples below give the same answers.

Pitch in any form, or from a rise and run, and the rafter line length. The same maths as the Pitch calculator in the MeisterCalc app.

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Worked example: a 6 in 12 roof with a 4000 mm run

  • Slope: 6 ÷ 12 = 0.5, so a 50% grade.
  • Degrees: arctan(0.5) = 26.57°.
  • Slope factor: √(1 + 0.5²) = 1.1180.
  • Rafter line length with a 450 mm overhang: (4000 + 450) × 1.1180 = 4975.3 mm.

Australian roofs are usually given in degrees: 15° is 3.22 in 12, 22.5° is 4.97 in 12 and 30° is 6.93 in 12.

How it works

  • Pitch is rise ÷ run, with the run measured level. Times 12 gives X in 12; times 100 gives per cent.
  • Degrees = arctan(rise ÷ run).
  • Rafter line length = (run + level overhang) × √(1 + slope²): the line along the top of the rafter, before any plumb or seat cuts.

Pitch in MeisterCalc

Questions

What is a 4/12 pitch in degrees?

arctan(4 ÷ 12) = 18.43°, a 33.33% grade.

Is 45 degrees a 12 in 12 pitch?

Yes. At 45° the rise equals the run, so the slope is 1, the grade 100% and the pitch 12 in 12.

What is the lowest pitch for a metal roof?

It comes from the sheet's maker and the building code. Lysaght, for one, lists a 5° minimum roof slope for Custom Orb corrugated steel.

Sources

Building rules are set by the National Construction Code, the NZ Building Code or the Approved Documents and by your council, not by this page or the app.

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