Governing Factors for Floor Joist Span
The **Maximum Allowable Span** for a floor joist is determined by structural limits set in building codes. The calculated span ensures the beam can handle the expected loads without failure (**Strength**) and without excessive movement (**Deflection**).
1. Deflection (Stiffness)
For residential floors, the span is usually limited by **deflection**, meaning the floor must be stiff enough to avoid noticeable bounce or sag. Building codes often limit deflection to **$L/360$** (the span divided by 360), which is the primary factor limiting the spans listed in the tables.
2. Key Span Table Inputs
| Factor | Effect |
|---|---|
| **Joist Depth (e.g., 2x10)** | Depth has the largest impact on span; increasing depth dramatically increases stiffness. |
| **Spacing (O.C.)** | Wider spacing (e.g., 24" O.C.) decreases the span, as each joist must carry more load. |
| **Wood Grade/Species** | Stronger, stiffer species (e.g., Douglas Fir-Larch) allow for longer spans than weaker ones. |
Critical Disclaimer:
This calculator uses simplified table data. For structures with heavier loads (e.g., stone tile floors, commercial use) or for lumber not explicitly listed, the design must be confirmed by a licensed **Structural Engineer** or verified against the specific load tables in your local building code.