Rafter Size Calculator
Rafter size means the lumber dimension — 2x4 through 2x12 — needed to span your roof without excessive deflection. This calculator returns rafter length and angle from your span and pitch; the reference table below shows typical dimension ranges by span.
Calculate Your Rafter Length
Enter your building specifications below. Results and the roof diagram update live as you type.
Live Roof Diagram
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Estimate Your Project Budget
Automatically calculated from your inputs above in the calculator.
Estimates only. Actual costs vary by region, supplier, and site conditions.
Matching Lumber Size to Rafter Span
Rafter size is driven by span, spacing, species/grade, and roof load — a longer unsupported span or heavier snow load needs a deeper member (more resistance to bending) or closer spacing to compensate. Common residential rafters range from 2x6 for short spans to 2x10 or 2x12 for wide, heavily loaded spans.
Rafter depth resists bending (deflection) under load, while width mainly affects nailing surface and stability — this is why increasing depth (going from a 2x8 to a 2x10) has a much bigger effect on allowable span than increasing width.
The American Wood Council publishes span tables cross-referencing species, grade, spacing, and load against maximum allowable rafter span for each dimension — always confirm your exact rafter size against the current span table or a stamped design for your load and grade.
Frequently Asked Questions
What size rafters do I need for a 20 ft span?
A 20 ft rafter span commonly needs a 2x10 or 2x12 at standard 16"-24" spacing, depending on species, grade, and snow load — always verify against your local span table or an engineer's design before final sizing.
Does rafter depth or width matter more for strength?
Depth matters far more — a deeper rafter (like a 2x10 vs a 2x8) resists bending much more effectively than a wider one, since bending resistance increases with the square of the depth.