Rafter Length Calculator

Your details

Switch between imperial feet-and-inches and metric metres.
Choose whether you know the full building width or just the horizontal run (half the width).
The total inside width of the building, measured from wall to wall.
ft
The roof pitch format you want to enter.
Vertical rise in inches for every 12 inches of horizontal run. Common values: 4, 6, 8, 12.
/ 12
The horizontal projection of the eave beyond the outside face of the wall. Typical range: 12-24 in (300-600 mm).
in
Actual thickness of the ridge board. Standard lumber: 1.5 in (38 mm) for a 2x, 3.5 in (89 mm) for a 4x. Half this value is deducted from the run.
in
Width of the wall top plate. Used to calculate birdsmouth seat-cut depth. Standard 2x4 wall: 3.5 in (89 mm).
in
Rafter lengthModerate slope
14.465

Total rafter length including overhang, minus ridge deduction.

Rafter length14 ft 5 9/16 in
Roof angle26.57deg
Slope factor1.118
Hip / valley factor1.5
Pitch (rise in 12)6/ 12
Birdsmouth seat depth0.261
Birdsmouth seat depth3 2/16 in

Rafter length is 14.465 ft (14 ft 5 9/16 in).

  • A 6.00/12 pitch equals a 26.6-degree slope, which is classified as moderate.
  • The slope factor is 1.1180: every foot of run requires 1.1180 ft of rafter material.
  • Birdsmouth seat-cut depth is 3.13 in. The maximum allowable seat cut is typically one-third of the rafter depth per code.

Next stepAdd 5-10% waste when ordering lumber. Double-check your ridge board length and verify your on-center spacing before cutting.

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