How this is worked out
Pitch is rise over run, said as inches of rise for every 12 inches of run, so 4 inches over 8 is a 6/12. The angle is the arctangent of rise over run. The slope multiplier is how much longer the roof surface is than the level ground under it: the square root of one plus the rise over run squared, 1.118 for a 6/12.
The roof's area is the footprint times that multiplier, plus 10% waste for cutting at ridges, hips and valleys, or what you type. A roofing square is 100 sq ft, and the calculator uses three bundles a square, which covers most shingles; check the wrapper on yours. Bundles round up. This is the same math as Handyman Calc's roofing screen.
Every pitch from 1/12 to 12/12 is worked out in the roof pitch chart, with rafter lengths and a PDF to print.
On your phone, on the job site
Handyman Calc, our free app, has this roofing screen and a rafter calculator and fourteen more calculators. It works with no signal, and it's free.
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Charts worked out with the same math:
- how many bags of concrete a slab takes
- how many sheets of drywall a room takes
- a fraction to decimal chart for inches
- how much paint a room takes
- how much mulch a bed takes
- a roof pitch chart
- how many steps a rise takes
- the diagonal that squares a frame
- board feet for common lumber
More calculators:
Frequently asked questions
How do you calculate roof pitch?
Measure how far the roof rises over a level run, then scale it to 12. A roof that rises 4 inches over 8 inches of run is 4 ÷ 8 × 12 = 6, a 6/12 pitch. A level and a tape on the rafter or the roof surface is enough.
How do you convert roof pitch to degrees?
Take the arctangent of rise over run. A 6/12 roof is arctan(6 ÷ 12) = 26.6°, a 4/12 is 18.4° and a 12/12 is 45°.
How many bundles of shingles do I need?
Three bundles cover a roofing square of 100 sq ft for most three-tab and architectural shingles. A 40 × 30 ft house with a 6/12 roof is about 1,476 sq ft of roof with 10% waste, which is 45 bundles.
How do you find a roof's square footage?
Multiply the footprint, length times width including the overhangs, by the slope multiplier for the pitch: 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12 and 1.414 for 12/12.
What is the lowest pitch for shingles?
2/12 under the IRC (R905.2.2), and from 2/12 up to 4/12 it asks for two layers of underlayment. Flatter roofs need a low-slope roofing system instead.