How to Calculate the Pitch of a Roof
Pitch is a measurement of the relative steepness of a roof's slope, and finding the pitch of an existing roof requires only basic tools and simple math.
Roof pitch is a ratio of a roof's vertical rise compared to its horizontal span. It is an expression of the angle of the roof, and the pitch measurement gives an idea of the relative steepness or shallowness of the roof.
Pitch Notation
Roof pitch is written as a proportion reflecting the amount of vertical distance that the roof rises over a given vertical distance, or run. In standard pitch notation on building plans, the run is assumed to be 12 inches, and the rise is the amount of vertical rise over each 12 horizontal inches. For example, a roof that rises 6 vertical inches over each 12 inches of horizontal span has a pitch of 6/12.
Other less common ways of noting pitch include "6:12," "6 in 12" and "6 over 12."
Measuring Roof Pitch

Measure 12 inches from one end of the level and make a pencil mark on the level at that point.

Place the end of the level against the bottom of a roof rafter and hold the level horizontally so that it's perfectly horizontal.

Measure straight up vertically from the pencil mark on the level to the bottom of the rafter. The resulting measurement is the roof's rise. Its pitch is the rise measurement over a 12inch run.
Converting Pitch to Angle
To convert roof pitch to an angle measurement, divide the rise by the run, making sure that you are using the same measurement units. The result is the tangent of the roof's angle. To determine the angle, use a scientific calculator to find the inverse tangent of the rise divided by the run.
For example, in a roof with a 6/12 pitch, the tangent is 0.5, and the inverse tangent is 26.57. Therefore, the angle of the roof is 26.57 degrees.
As a shortcut to finding a roof's angle, most rafter squares are marked with the angle of common roof pitches. You can also use a roof angle calculator or table of common angles.
Things You Will Need
 Carpenter's level, at least 18 inches long
 Tape measure