Roofers and Solar Installers don’t always talk the same language when it comes to explaining roof angle.  Solar Installers are always told to install panels at an angle – given in degrees (usually based on latitude position).  Roofers, on the other hand, use construction terms to define a roof angle. A  roofer describe this angle as “pitch”. For example a “5:12 Pitch”  means for every 12” of run there is a 5’ rise.  A Solar Installer can easily find the pitch of an existing roof  using a simple L-Square and a small level.

Hold the level against the L-Square and make sure the bubble is level. The “run” should be at the 12” mark. Check the rise. In this case we can see that there is 5 inches or rise for the 12 inches of run.  As you can see, we have created a triangle between the L-Square and the roof.

Since the purpose of this article is to get to the answer, we will avoid deep trigonometric functions and other math terms that make grown men weep – or slip into a coma while standing on a ladder. All we are looking for is angle “a”, as shown in the right triangle below.

We are looking for angle (a)

Now, your calculator must have an inverse tangent function. Sometimes these are labeled atan, arctan, or tan^-1 button. Often you must hit the 2^nd function key of the tan button to get the inverse function. Also, make sure your calculator is set up to give answers in degrees instead of radians.

To calculate the angle of a 5:12 roof enter;

tan^-1 (5/12) = 22.619 degrees

This works with any roof rise. For example;

tan^-1 (1/12) =    4.763  degrees

tan^-1 (2/12) =    9.462  degrees

tan^-1 (3/12) =   14.036 degrees

tan^-1 (4/12) =   18.434 degrees

tan^-1 (5/12) =   22.619 degrees

tan^-1 (6/12) =   26.565 degrees

tan^-1 (7/12) =   30.256 degrees

tan^-1 (8/12) =   33.690 degrees

tan^-1 (9/12) =   36.869 degrees

tan^-1 (10/12) = 39.805 degrees

tan^-1 (11/12) = 42.510 degrees

tan^-1 (12/12) = 45         degrees

Tom Cawley – PV Installation Instructor