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Common Gable Rafter Length Calculator

Calculate common rafter length from building width and roof pitch, including the overhang tail.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
26.25 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Total rafter length (incl. overhang)

16.14 ft

High confidence
Rafter length to ridge (no overhang)
14.67 ft
Overhang tail length
1.47 ft

For the dimensions entered, expect a total rafter length (incl. overhang) of 4.92 m. Of the working steps, rafter length to ridge (no overhang) dominates at 4.47 m. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard carpentry rafter-length formula: rafter length = run × √(1 + (rise/12)²), where run is the horizontal distance from wall plate to ridge

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Frequently asked questions

Why is the run half the building width?
For a standard gable roof, the ridge sits at the centerline of the building, so each common rafter spans horizontally from the exterior wall plate to the ridge — a run equal to half the overall building width.
Does this length include the rafter tail (overhang)?
Yes — the total result adds the overhang tail's sloped length (overhang × slope factor) to the rafter length from wall plate to ridge, so it reflects the actual board length needed including the eave overhang.
Does a steeper pitch increase both the rafter length and the overhang length?
Yes — the same slope factor, √(1 + (rise/12)²), applies to both the run and the overhang, since the rafter tail follows the same roof slope as the rest of the rafter.