Rafter Calculator
Calculate the geometric length of a common rafter before plumb cuts, seat cuts, and field adjustments. Start with either the full building span or a measured horizontal run to the ridge centerline.
How to use this calculator
Enter a span-or-run input mode, the matching horizontal dimension, roof rise per 12 inches, overhang, and ridge-board thickness.
Review whether dimensions are horizontal or on-slope, the ridge detail, birdsmouth limits, lumber grade, and local structural requirements.
Read the common-rafter line length, slope angle, and slope factor.
Formula
Centerline run is either building span ÷ 2 or the entered run. Calculated run = centerline run − ridge thickness ÷ 24 + overhang; rise = run × pitch ÷ 12; rafter length = √(run² + rise²).
Worked example
A 24 ft span, 6:12 pitch, zero overhang, and zero ridge thickness produce a 12 ft run and a 13.416 ft theoretical rafter length.
Assumptions and limits
- The result models a straight common rafter on a symmetrical planar roof and is not a structural span or member-sizing determination.
- Rafter Calculator treats the entered values as estimates and does not infer missing project, account, or personal details.
- Calculations retain working precision except where the cited method requires an intermediate rounding step; displayed values are then formatted separately. Source rules and dated rates must be rechecked when they change.
Sources
- International Code Council: International Residential CodeResidential roof framing terminology and structural context. Reviewed 2026-08-14.
- OpenStax: College Algebra 2eAlgebraic, ratio, radical, equation, and function formulas. Reviewed 2026-08-14.
- GAF: Pro Field Guide for Steep-Slope RoofsOfficial formula or method source used by this calculator. Reviewed 2026-08-14.
- National Institute of Standards and Technology: Circumference, Area and VolumeOfficial formula or method source used by this calculator. Reviewed 2026-08-14.
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Verification
How this calculator is checked
Trust comes from reproducible evidence, not model confidence. You can inspect the formula, assumptions, worked example, and cited sources on this page.
- Versioned calculationFormula logic is kept separate from the interface and covered by automated registry and behavior checks.
- Cited evidenceSources linked for independent checking: 4.
- Transparent AI useAI may assist drafting or adversarial review. Agreement between models is not proof, and no human expert review is claimed unless a named reviewer is shown.
Frequently asked questions
How does the Rafter Calculator work?
Centerline run is either building span ÷ 2 or the entered run. Calculated run = centerline run − ridge thickness ÷ 24 + overhang; rise = run × pitch ÷ 12; rafter length = √(run² + rise²).
What information should I enter in the Rafter Calculator?
Enter a span-or-run input mode, the matching horizontal dimension, roof rise per 12 inches, overhang, and ridge-board thickness, then verify whether dimensions are horizontal or on-slope, the ridge detail, birdsmouth limits, lumber grade, and local structural requirements.
What should I verify before using this Rafter Calculator result?
The result models a straight common rafter on a symmetrical planar roof and is not a structural span or member-sizing determination.
Last updated 2026-08-25 · span-or-run-v2 · content-2026-08-14 · Published by YunFanLabs