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Rafter calculator

Give it the pitch and building span to get the common rafter length, overhang tail, stock length and plumb-cut angle.

Pitch — rise per 12 in of run
Ridge

Result

13 ft 4 3/16 in
plate to ridge face · run 11 ft 11 1/4 in × 13.42 in/ft
Overhang tail (+12 in level)1 ft 1 7/16 in
Total along the rafter14 ft 5 9/16 in
Stock to buy16 ft
Plumb cut (speed square)26.6° · 6/12
Ridge rise above plate6 ft
The gable, to scale — this is the real pitch
13 ft 4 3/16 inrise 6 ft126span 24 ftplumb cut 26.6° · 6/12 pitch
How is this calculated?

Using the tool

The framing-square rafter table, with the picture: give it the pitch and the building span and it hands you the common rafter cut — length to the ridge face with the half-ridge deduction already taken, the overhang tail, the stock length to buy, and the plumb-cut angle to set on the speed square.

The geometry

Every common rafter is the same right triangle: 12 in of run, the pitch in rise. The framing-square number — length of rafter per foot of run — is just the hypotenuse: √(12² + rise²), so a 6/12 roof uses 13.42 in of rafter for every 12 in it travels. Multiply by the run and you have the length along the slope.

Two field details do the rest: the run is measured to the face of the ridge, so half the ridge thickness comes off the half-span (horizontally — the ridge doesn't change the height math); and the overhang rides the same slope, so a 12 in level projection adds more than 12 in of tail. The plumb cutat the ridge and the vertical side of the bird's-mouth are the pitch angle; the seat that lands on the plate is its complement — one speed-square setting cuts both.

Formula sources: Framing-square common rafter table — length per foot of run = √(12² + rise²), Standard roof-framing practice: half-ridge deduction, level overhang projection

This sizes the cut, not the member: rafter depth, species, grade and spacing come from the span tables in your code (IRC R802) and your local amendments. Check bird's-mouth seat depth limits against the rafter size before cutting.