Spike's How To

What is the pitch of a roof

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.

And another way to do it involves a bit of math

first, measure the total length of the wall
measure the height of the roof from the top of the wall to the peak of the roof
measure the heel height of the truss or rafter (the height of the roof structure above the top plate of the wall)
subtract the heel height from the height measurement (this will give you the "rise" amount)
divide the length measurement in half (the "run" measurement)
divide the rise by the run

Example:

wall length = 20 feet   
roof height (measured from the top plate to peak) = 5 feet -  6 inches
heel height = 6 inches

Convert everything to the same units to simplify the multiplication;

length: 20 * 12 = 240 inches

roof height;  5 * 12 +6 = 66 inches

Divide the length in half;

240 ÷ 2 =120

The run is 120 inches

Subtract the heel height from the roof height;

66 - 6 = 60

The rise is 60 inches

Divide the rise by the run amount to get the amount of rise in 12 inches;

60 / 120 =  0.5

The rise in 12 inches of "run" is 0.5 inches
Convert the decimal 0.5 to inches by multiplying by 12;

0.5 * 12 = 6

The pitch of your roof is 6 : 12

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What is the pitch of a roof

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