How to Calculate Rafter Length With a Framing Square

Measure the run, or horizontal distance of the rafter, from the center line of the building to the outer wall, using the tape measure.

Measure the rise, or vertical distance of the rafter, from the top of the ridge board to the top plate of the wall, using the tape measure. The top plate is the horizontal board that runs across the tops of the studs in the wall below the roof.

Calculate the pitch of the roof. Pitch is the number of inches of rise per 1 foot of run. A roof that rises 8 inches per foot has a pitch of 8.

Locate the rafter table on the front of the framing square blade. The blade is the wider and longer part of the framing square. The thinner, shorter part is called the tongue.

Read the top row of the rafter table to determine the rafter length per foot of run. If the roof has a pitch of 8, the table shows that the rafter should be 14.42 inches long per 1 foot of run.

Multiply the result from the table by the run of the rafter to calculate total rafter length.

Convert the decimal portion of the number to a fraction.

Multiply the numbers to the right of the decimal point by 8. The answer is the number of eighths of an inch that you should add to the rafter length.

As an example, assuming the roof has a pitch of 8, the table indicates that the rafter is 14.42 inches long per foot of run. If the rafter run is 15 feet, the rafter is -- 14.42 x 15 = 216.3 -- 216.3 inches long, or 18 feet and 0.3 inches long. Your tape measure probably does not have markings at tenths of an inch, so you have to covert 0.3 to a fraction. To convert 0.3 to a fraction, the equation is 0.3 x 8 = 2.4 you can read this as 2.4 eighths of an inch. Round it up slightly to 5/16, which is 2.5/8 of an inch.