How to Calculate Roof Pitch Angles

The pitch, or slope, of a roof is defined as the amount of vertical rise per unit of horizontal run. In the US, roof pitch is expressed as inches of rise per foot, or 12 inches, of run. A "4 pitch" or "4:12" roof will have 4 inches of rise per foot of run. In most other countries, however, roof pitch is expressed as an angle. By using basic trigonometric identities, the measurements for rise and run lengths can easily be converted to a slope angle in degrees.

Things You'll Need

• Ladder
• Level
• Tape measure or ruler
• Calculator

Instructions

1. Find a safe and secure location to place the ladder that will allow access to the roof. Climb the ladder with the level and the tape measure or ruler. The ruler should be at least one foot long. If it is longer than one foot and does not have a length scale on it, make a mark to indicate a length of one foot from either end.

2. Place the level vertically on the roof slope so the 1 foot mark is toward the low side and the end it was measured from is on the high side. Lift the low end of the level until it is indicates level with horizontal. Usually this means centering the middle bubble.

3. Measure the distance from the roof surface to the one foot mark on the level. Keep the ruler or tape measure perpendicular to the level. Record or commit to memory the pitch measurement. For example, if your rise measurement was 3 inches, the pitch would be "3:12."

4. Calculate the pitch ration by dividing the rise by the run and take the inverse tangent of the result to determine the slope angle. For example:
   3:12 = atan(3/12) = 14 degrees.

Tips & Warnings

• One foot, or 12 inches, is the standard used in the US, but you may use and distance for the run. A rise of 1 inch with a run of 4 inches would yield: atan(1/4) = 14 degrees.