1 Your measurements

Type feet and inches the way you'd write them, like 12' 6" or 30 3/8. Decimals work too.

ft
Outside of wall to outside of wall, across the gable.
6/12 means the roof rises 6 inches for every 12 inches it runs across. To measure, hold a level 12 inches long against the roof and measure straight down to it.
/12
in
Measured level from the outside of the wall to the end of the rafter.
in
1½" for a 2× ridge board.
in
3½" for 2×4 walls, 5½" for 2×6 walls.

2 Your rafter

Rafter length
14' 5 9/16"
6/12 pitch, overhang included
Roof angle
26.6°
6 in 12
Buy
16 ft boards
next stock length up
ridge center wall overhang 14' 5 9/16" (6/12)
Side view of one rafter, to scale, from the ridge to the eave. The notch over the wall is the birdsmouth.
Cut list for one common rafter
Run (ridge center to outside of wall)12' 0"
Total rise at the ridge center6' 0"
Rafter line length, ridge center to wall13' 5"
Less half the ridge board− 13/16" (3/4" × slope factor)
Plus the tail (overhang)+ 1' 1 7/16"
Rafter length, ridge cut to tail cut14' 5 9/16"
Plumb cut (ridge and tail)26.6° from square, or 6 on the common scale of a speed square
Seat cutsquare to the plumb cut, 3 1/2" long
Birdsmouth heel cut1 3/4" tall (plumb)
Notch depth into the rafter1 9/16", 22% of a 2×8's depth
Plumb cut depth at the ridge8 1/8" (the ridge board must be at least this deep, IRC R802.3)
Slope factor1.118 (rafter length per foot of run)

Rafter size and spacing depend on span, snow load and roof weight, which this calculator doesn't check. Use your code's rafter span tables or an engineer.

Powered by Nail the Math