Rafter Length Calculator

A rafter, its run and its rise form a right triangle, so the geometric rafter length is the hypotenuse.

These are the same equivalent right-triangle relationships used by current rafter calculators. A 12 ft run at 6:12 gives a 6 ft rise and a rafter of √(12² + 6²) ≈ 13.4164 ft, which is about 13 ft 5 in.

Rafter run is the horizontal distance from the wall or support line toward the ridge, for the roof section being calculated. It is never the sloped rafter length.

For a simple symmetrical gable roof, the run is about half the span before any ridge-thickness adjustment. For a single-slope shed roof, the run may equal the full horizontal span.

• Span — the full horizontal width between the supports.
• Run — the horizontal distance covered by one sloping rafter section.

The calculator never halves the span for a shed roof, because there may be no centered ridge condition.

The multiplier comes from the shared pitch engine, so it matches the Roof Pitch Calculator and the Roof Square Footage Calculator exactly.

If both dimensions are already known, no pitch conversion is needed first — the length is simply √(Rise² + Run²). A 5 ft rise over a 12 ft run is exactly 13 ft, which corresponds to a 5:12 pitch.

A horizontal overhang must be converted into sloped length — a 2 ft horizontal overhang is not 2 ft of rafter on a pitched roof.

If you already know the sloped tail length, enter it directly in Advanced Options instead, and it is used without conversion.

For a symmetrical roof, the centerline run may need adjusting when you want the geometric length to the side of a ridge member. When a ridge thickness is entered, BuildCalcWorks subtracts half its width from each half-span run.

Geometry adjustment only. Half the entered ridge thickness is removed from each half-span run. BuildCalcWorks does not determine whether a ridge board or structural ridge beam is required, or what size it should be.

Rafter length is geometry. Rafter size is structural design. A 14-foot-long rafter does not by itself tell you whether a 2×6, 2×8, 2×10 or another member is appropriate.

Structural sizing depends on loads, species, grade, spacing, supports and local requirements. BuildCalcWorks does not determine any of these, and this calculator does not calculate rafter spacing, loads, birdsmouth or cut dimensions, bearing or code compliance.

Roof work and direct roof measurement can involve fall hazards. Use plans, safe measurement methods, or qualified professionals where appropriate.

• Building plans and drawings
• Known building dimensions
• Attic or framing dimensions where they are safely accessible
• Ground-based measurements
• Existing construction documentation
• A qualified professional