How to Calculate a Roof Slope
Posted By April on December 21, 2012
- 1
Place a ladder near or against the building so you can have easy access to the roof. Climb the ladder and bring along a level and ruler or tape measure.
- 2
Place the level on the roof lengthwise. The level should be at least 1 foot in length. If it is longer, then there should be a 1-foot mark from either end. The end you measure the 1 foot from should be on the high side when you place it on the roof.
- 3
Lift the low side of the level until it is horizontal. It is horizontal when the bubble in the level is centered between its indicator marks.
- 4
Measure the distance from the 1-foot mark on the level to the surface of the roof, keeping the ruler or tape measure perpendicular to the level.
- 5
Determine the roof slope in whatever form you require. If the distance from the level to the roof surface is 5 inches, you could write the pitch as “5:12” or “5 pitch.” To calculate the slope angle, take the inverse tangent–arctangent–of the ratio. For example,
5:12 = 5/12; atan(5/12) = 22.6 degrees
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