Rafter Length Calculator

Find the geometric line length from ridge to wall and fascia while keeping half-span, ridge deduction, pitch, and overhang visible.

Common-rafter line lengthThe result separates ridge-to-wall length from the sloped tail beyond the wall.

14.465 ftrafter line length to fascia

Horizontal run to ridge: 11.938 ft
Rafter length to wall line: 13.347 ft
Sloped tail length: 1.118 ft
Total line length: 14.465 ft
Slope multiplier: 1.118

Line length is geometry only. It does not size lumber, locate cuts, or verify span, bearing, loads, connections, or code requirements.

Show calculation

[(span ÷ 2 − ridge thickness ÷ 2) + overhang] × √[1 + (6 ÷ 12)²] = 14.465 ft

Transparent method and editable product assumptions. Verify current labels and project requirements.

How this calculator works

For a symmetrical gable, the horizontal run of one common rafter starts at half the building span. Half the ridge-board thickness is deducted because the opposing roof planes meet at the ridge faces rather than the centerline.

The x-in-12 pitch becomes a slope multiplier using the Pythagorean theorem. Multiplying horizontal run by this factor gives the straight line from the ridge face to the wall line. The horizontal overhang receives the same factor to produce the sloped tail length.

The result is a geometric line, not a cut list. Plumb cuts, birdsmouth geometry, seat depth, fascia details, lumber stock length, field tolerances, structural span, loading, bearing, connections, and code compliance remain outside this calculator.

Formula

Run to ridge = span ÷ 2 − ridge thickness ÷ 2; line length = (run + horizontal overhang) × √[1 + (pitch rise ÷ 12)²]

The live substitution appears with your result.

Control example: 24 ft span at 6:12

Half the span is 12 ft. A 1.5 in ridge board deducts 0.0625 ft, leaving an 11.9375 ft run. The 6:12 multiplier is 1.1180.

The ridge-to-wall line is 13.347 ft. A 1 ft horizontal overhang adds 1.118 ft, for a 14.465 ft total line length.

Assumptions and limits

The roof is a symmetrical gable with one constant pitch.
Building span and overhang are horizontal projections.
Ridge thickness is measured perpendicular to its faces.
The result follows the theoretical top-edge line before field cuts.

Common measuring mistakes

Using the full span as the run of one rafter.
Adding a horizontal overhang directly to an already-sloped length.
Subtracting the full ridge thickness from each rafter run.
Treating geometric length as approval of lumber size or structural span.

Sources

Product-specific values remain editable. These sources support the method and stated examples; they are not a universal substitute for your supplier’s current label.

OSHA Construction Glossary Supports vertical-to-horizontal x-in-12 roof-slope notation.
American Wood Council span calculator help Shows that structural rafter span depends on loads, species, grade, spacing, bending, deflection, shear, and bearing—items this geometry tool does not select.

Questions people ask

How do I calculate common-rafter length?

Find the horizontal run, multiply it by √[1 + (pitch rise ÷ 12)²], then add the overhang using the same slope factor.

Is rafter run half the building span?

For a symmetrical gable it begins with half the span, then half the ridge-board thickness is deducted from each side.

Is overhang measured horizontally or along the rafter?

This calculator expects horizontal overhang and converts it to sloped tail length.

Does the result include birdsmouth and plumb cuts?

No. It reports theoretical line length. Cut geometry and stock selection require a detailed framing layout.

Does this size the rafter lumber?

No. Species, grade, spacing, loads, span, bearing, connections, and local code must be checked separately.