Rafter calculator

Job sheet

Plumb rafter job sheet

6 in 12, span 24 ft.

  • Rafter length13.42 ftHorizontal run to the ridge. Add overhang yourself.
  • Rafters3216 each side, including the end rafters.
Plumb rafter job sheet
6 in 12, span 24 ft.

Rafter length: 13.42 ft (Horizontal run to the ridge. Add overhang yourself.)
Rafters: 32 (16 each side, including the end rafters.)

Order the board longer than the number on the sheet. The plumb cut and the seat cut eat the end, and the tail past the wall is extra. Tally the sticks on board feet. Shingles on that roof are roofing. Sheathing is plywood.

FAQ

How long is the rafter?

Half the span is the run. Rise is run times pitch over 12. Length is the hypotenuse. A 24 foot span at 6/12 has a 12 foot run and a 6 foot rise. The rafter is 13.42 feet to the ridge, with no tail.

How many rafters?

Building length divided by spacing, rounded up, plus the rafter at the end. Then double it for the other side of a gable. A 20 foot building at 16 inches on center is 16 rafters a side, 32 total.

Sources

  1. NIST Handbook 44: The foot used in the run, rise, and hypotenuse.