Lumber & framing calculator
Rafter calculator
Preliminary paired-rafter count and rafter length from roof plan dimensions, pitch and spacing.
Calculator result and formula
Rafter pairs to plan: 18 pairs
Formula: Rafter length = √((width ÷ 2)² + ((width ÷ 2) × rise ÷ 12)²)
Formula ID: LUM-RAFTER-001 · Version 1.0.0.
Inputs
- Building length: Ridge direction used for rafter spacing Default: 30.
- Building width: Total horizontal building width Default: 20.
- Pitch rise: Vertical rise for every 12 units of horizontal run Default: 6.
- Rafter spacing: On-center spacing; verify design requirements Default: 24.
- Waste factor: Allowance for cuts and selected stock lengths Default: 10.
How this calculator works
This calculator uses roof width, rise-per-12 pitch and stated spacing to produce a preliminary paired-rafter count and sloped rafter length.
- Measure building length and total horizontal building width.
- Enter roof rise per 12 and spacing specified by the project.
- Verify rafter size, birdsmouth, overhang, ridge detail, spans and connection requirements independently.
Worked example: Example: a 30 ft building length at 24 in spacing has 16 rafter pairs. A 20 ft building width at 6:12 creates a 11.18 ft run per rafter before waste allowance.
Assumptions and limitations
- Rafter length uses one half of a symmetrical roof's building width and the stated rise-per-12 pitch.
- Overhang, ridge-board deductions, birdsmouth, valley/hip geometry and stock-length splices are excluded.
- This tool is not a rafter-sizing or structural-design calculator.
Common questions
- Does this design a rafter?
- No. It is a geometric quantity estimate only and cannot determine structural rafter sizing or connections.
- Why show rafter pairs?
- A simple symmetrical roof uses two rafters per spacing location; complex roofs must be taken off by their actual geometry.
Reference notes
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