How this calculator works
Roof pitch is stated as rise over run. In Canada and the United States the usual form is the rise for every 12 in of run, so a 6/12 roof climbs 6 in over each foot of horizontal travel. The angle is the inverse tangent of the same ratio, and the multiplier is the length of slope per unit of horizontal length, which is what turns a footprint area into a sloped roof area.
Formula
rise over run as a ratio r = rise ÷ run; pitch = 12r per 12; angle = atan(r) in degrees; multiplier = √(1 + r²).
Worked example
A 6/12 roof has r = 0.5, so it is written 6:12, its angle is 26.57 degrees and its multiplier is 1.1180: every 100 ft² of footprint is 111.8 ft² of roof. An 8/12 roof gives 33.69 degrees and a multiplier of 1.2019.
Advertisement
Frequently asked questions
Why does Canada quote degrees when the US quotes rise per 12?
Both describe the same slope. Canadian drawings and code discussions often use degrees, while shingle tables and roofing suppliers in both countries use the rise per 12 figure. Showing both saves a conversion.
How do I get the sloped roof area?
Multiply the footprint area by the multiplier. The roofing calculator does this for you and adds the shingle waste allowance.
What is a shallow or steep roof?
Below about 3/12 is usually called low slope and needs different underlayment and products; 4/12 to 9/12 is the common shingle range; above 9/12 is steep and slower to work, so labour allowances rise.