The roof's pitch is the height (rise) it obtains after a certain distance (run). Typically the pitch of a roof is expressed as the amount of rise measured in inches in a one-foot run (12 inches). E.g. A 6 : 12, for every 12 inches of horizontal length, the roof height increases by 6 inches.
There are several ways of getting the pitch, and the easiest way is using a framing square.
L-shaped framing square has a body (wider part) and the tongue(skinny part).
Place the ladder where you can safely reach the roof on the gable side. Set the square's body at the 12-inch mark level across from the roof's edge. Read the measurement on the square's tongue, where it is even with the top of the roof. Take both these measurements from the same sides, e.g. if you are using the bottom of square horizontal, then read the tongue measurement on the outside. If you are using the top of the body, read the tongue measurement on the inside of the square.
Suppose you don't have a square, draw an imaginary level line (a) inward from the roof's edge. @ 12 inches measure vertically upwards from this line to the edge of the roof for the rise measurement. The gable overhang will make this a bit harder to get an accurate measurement.
wall length = 20 feet roof height (measured from the top plate to peak) = 5 feet - 6 inches heel height = 6 inchesConvert everything to the same units to simplify the multiplication;
length: 20 * 12 = 240 inches
roof height; 5 * 12 +6 = 66 inches
240 ÷ 2 =120
The run is 120 inches
66 - 6 = 60
The rise is 60 inches
60 / 120 = 0.5The rise in 12 inches of "run" is 0.5 inches
0.5 * 12 = 6
The pitch of your roof is 6 : 12