How to Determine the Length of Rafters Needed for a Roof

Measure the width of the roof with a tape measure to determine the roof span and divide it by two. Add half of the ridge's width. This gives you the roof's net run. The rafters stretch across half the width of the roof before meeting in the center once they're in place. Take the measurement in feet and inches.

Use the tape measure to determine the width of the roof's overhang in inches.

Calculate the roof's pitch by measuring straight out 12 inches from the top of the roof with a ruler. Then measure down from the tip of the ruler to the roof surface with a tape measure or a second ruler to calculate the severity of the roof's slope. For example, if the roof is 5 inches lower then the top after measuring out by 12 inches, the pitch is written as "5/12."

Enter the numbers for roof width (or run, depending on the rafter length calculator you're using), overhang and pitch into a rafter length calculator. The results give you the exact length of the rafters you need.