Rafter Calculator

Inputs

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Outside-wall to outside-wall building width for a centered gable roof.
ft
Rise per 12 units of horizontal run: 6 means 6:12.
rise/12
Horizontal projection beyond the outside wall, not sloped tail length.
in
Subtracts half the ridge thickness from each common-rafter run. Use 0 if you do not want a ridge deduction.
in
Length along the ridge, used only for rafter count and stock length.
ft
On-center spacing along the ridge. Count assumes a rafter pair at each end.
in

Results

Default result
Edit inputs
Rafter cut length(L_cut)
14.465ft
Pass

Common rafter length plus the sloped overhang tail.

Common length plus sloped overhang tail.

Also computed

Common rafter length(L_r)13.347ft

Run to ridge multiplied by the slope factor.

Wall plate to ridge, before overhang.

Run to ridge(run)11.938ft

Half the building span minus half the ridge board thickness.

Roof rise(rise)5.9688ft

Run to ridge multiplied by pitch / 12.

Sloped tail length(L_tail)13.416in

Horizontal overhang multiplied by the slope factor.

Roof angle(theta)Pass26.57°

theta = atan(pitch / 12).

Angle above level from the entered pitch.

Plumb line to rafter(90-theta)63.43°

Angle between a plumb cut line and the rafter edge.

Method notes 3 notes
  • Gable roof geometry only: common rafters, centered ridge, symmetric pitch and straight overhang.
  • Rafter count assumes one pair at each end, then one additional pair at each on-center spacing interval along the ridge.
  • This does not check structural span, member size, birdsmouth limits, uplift, snow/wind load, bracing, ties, bearing, code or local inspection requirements.

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