How to use it
- Choose what you know: rise and run, an x/12 pitch, an angle or a slope percent.
- Read the pitch in every form and check the diagram.
- Enter the building span, length and overhangs for rafter length and roof area.
- In feet, read the number of roofing squares to order materials.
How it works
Pitch is the ratio of rise to run. In North America it is written as the rise per 12 units of run (6/12 means 6 inches up for every 12 inches across). The angle is arctan(rise ÷ run) and the slope percent is rise ÷ run × 100. The pitch multiplier, √(1 + (rise ÷ run)²), converts a horizontal distance into the length along the slope.
For a gable roof the run of each rafter is half the span; for a shed roof it is the whole span. Rafter length is (run + eaves overhang) × multiplier, measured along the top of the rafter from the ridge line to the end of the overhang; it does not subtract half the ridge board or add the plumb cut. Roof area is the plan area including overhangs multiplied by the pitch multiplier. A roofing square is 100 ft² of roof surface.
The area formula is exact for simple gable and shed roofs. Hips, valleys, dormers and different pitches on different faces need each plane calculated separately, and you should add waste for cutting and starter courses when ordering.
Examples
6/12 gable roof
A 24 × 40 ft house with 1 ft overhangs and a 6/12 pitch: angle 26.57°, multiplier 1.118, rafters 14.53 ft, roof area about 1,221 ft², or 12.21 squares.
Converting 30 degrees
A 30° roof is a 6.93/12 pitch, a 57.7 % slope, with a multiplier of 1.1547.
Frequently asked questions
What angle is a 4/12 pitch?
18.43°. The table of common pitches under the calculator lists angles, percentages and multipliers from 1/12 to 18/12.
Is a 45° roof a 12/12 pitch?
Yes. At 45° the rise equals the run, so the pitch is 12/12 and the slope is 100 %.
How many squares do I need?
Divide the sloped roof area in square feet by 100. Add waste for cuts, hips and valleys and starter rows before ordering shingles.
Does the rafter length include the overhang?
Yes, the horizontal eaves overhang is included and measured along the slope. Cuts at the ridge and the tail are not deducted.
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