Frequently Asked Questions
How is the maximum span calculated?
The tool reads a simplified IRC-style allowable span for your lumber size, spacing, and species, then adjusts it for loads above or below the 50 psf baseline using a square-root load factor. The result is the clear distance between supports.
Is this span code-compliant?
No. It is a planning estimate only. Allowable spans depend on grade, deflection limits, snow and wind zones, and connection details. Always verify the final span against your local building code and a licensed structural engineer or building official.
What load should I enter?
Residential floors typically use 40 psf live plus 10 psf dead, so 50 psf total is a common default. Decks, roofs, and snow regions use different values.
What is the difference between joists and rafters here?
Both are sized the same way for bending, but rafters carry sloped roof and snow loads while joists carry flat floor loads. Use the appropriate load and confirm rafter sizing against a roof framing table.
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Estimates for informational purposes only.
Important Disclaimer: Estimates for informational purposes only.
This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.