How to Measure a Gable Roof

Set an extension ladder against each side of the roof's gable end. Have a partner grab one end of the tape measure. Stretch the tape measure between the ladders. Climb one ladder while your partner climbs the other. Measure the distance between the ends of the roof's rafters to determine the roof's span. Note the measurement with paper and pencil.

Move one ladder to the center of the gable end, and leave the other in position. Use the tape measure and pencil to mark the center point of the span on the gable end's surface. Move the ladder from the end of the gable to one of the roof's eaves. Climb onto the roof with the tape measure. Have your partner climb the ladder at the center of the gable.

Extend the tape measure's body to your partner. Hold the tape measure's clip on the peak of the gable end. Roughly plumb the tape. Have your partner determine the distance from the peak to the mark that represents the center point of the span. This distance is the roof's rise. Note the rise on paper.

Divide the span by two with a calculator or long division. The result of the calculation is the roof's run. The proportion of rise to run is called slope. Building professionals describe slope in units of 12 inches. For example, a roof with a rise of 48 inches and run of 144 inches has a "4 in 12" slope, which means that the rafters rise 4 inches for every foot of run.

Add the square of the rise to the square of the run to determine the square of the rafters' length. This equation is called the Pythagorean Theorem. Because the rise and run meet at a right angle, the equation allows you to determine the square of the remaining side, called the hypotenuse. The hypotenuse of a right triangle corresponds to a roof's rafters. Use the calculator's square root function to determine the square root of the hypotenuse's square. The result represents the length of each rafter.