Joist sizing basics
Joist size is selected based on span, spacing, species/grade, and loads. Our calculators provide planning estimates; always verify with official span tables.
Species comparison
| Species | Relative strength | Notes |
|---|---|---|
| Southern Pine | High | Common for structural framing |
| Douglas Fir-Larch | High | Good stiffness |
| Hem-Fir | Moderate | May require shorter spans |
Load calculations (overview)
Residential decks typically use 40 psf live load + 10 psf dead load. Increasing loads reduce allowable spans.
Installation tips
- Use corrosion-resistant fasteners and connectors.
- Install proper flashing at ledger.
- Add blocking at midspan for longer joists and at transitions.
Code requirements by region
Many jurisdictions adopt the IRC with local amendments. Check your local building department.
Resources
Legal Notice & Disclaimer
Planning tool only. Results and charts on this site are simplified planning aids based on common assumptions. They are not a substitute for stamped engineering, permits, or inspections. Always confirm with local building officials and product manufacturers.
No warranties. This site is provided “as is,” without warranties of any kind. We disclaim liability for any loss or damage arising from its use.