Rafter Calculator

Inputs

Building span (W)

Outside-wall to outside-wall building width for a centered gable roof.

Roof pitch (p)

Rise per 12 units of horizontal run: 6 means 6:12.

rise/12

Horizontal overhang (e)

Horizontal projection beyond the outside wall, not sloped tail length.

Ridge board thickness (t_r)

Subtracts half the ridge thickness from each common-rafter run. Use 0 if you do not want a ridge deduction.

Building length (L_b)

Length along the ridge, used only for rafter count and stock length.

Rafter spacing (s)

On-center spacing along the ridge. Count assumes a rafter pair at each end.

Default result

Rafter cut length(L_cut)

4.4088m

Pass

Common rafter length plus the sloped overhang tail.

Common length plus sloped overhang tail.

Also computed

Common rafter length(L_r)4.068m

Run to ridge multiplied by the slope factor.

Wall plate to ridge, before overhang.

Run to ridge(run)3.6386m

Half the building span minus half the ridge board thickness.

Roof rise(rise)1.8193m

Run to ridge multiplied by pitch / 12.

Sloped tail length(L_tail)340.78mm

Horizontal overhang multiplied by the slope factor.

Roof angle(theta)26.57°

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.