How to: Roofing Angles

Access a roof rafter on the side of the house or the attic. Attach a plumb bob to the underside of the rafter and steady the weight to prevent it from swinging.

Hold a carpenter's level in the horizontal position on the side of the plumb bob where the roof rafter is lower. Place the end of the carpenter's level that is opposite the plumb bob against the rafter.

Tilt the free end of the the carpenter's level as needed to make the level exactly horizontal. Mark the string of the plumb bob where it crosses where the upper edge of the carpenter's level.

Record the measurement on the carpenter's level where it crosses the plumb bob to obtain the run of the roof. Assume for this example that the run of the roof is 13 inches.

Measure the distance from the point where the plumb bob attaches to the rafter to the mark you made on the plumb bob's string. This distance is the rise of the roof. Assume for this example that the rise of the roof is 5 inches.

Divide the rise of the roof by the run of the roof to obtain the slope of the roof. The rise of the roof in this example is 5 inches, and the run of the roof is 13 inches, so the slope of the roof is 5 / 13 = 0.385.

Calculate the rafter angle as the inverse tangent of the roof's slope. The roof's slope in this example is 0.385, and the inverse tangent of 0.385 is 21.1 degrees. The rafter angle in this example is therefore 21.1 degrees.