Structural Reference
Load Capacity & Span Charts
Floor joist maximum spans, design load requirements by occupancy, and nominal vs. actual lumber dimensions. Based on IRC 2021 and IBC 2021. Structural decisions always require a licensed engineer.
⚠ Critical Warning — Structural load calculations must be performed by a licensed structural or civil engineer for any load-bearing application. Span tables are simplified general references that do not account for species variations, grade, moisture content, local snow/seismic loads, or cumulative loading. Never make structural decisions based on this table alone. This is reference data only, not engineering advice.
Design Load Requirements by Occupancy
| Application | Live Load (PSF) | Dead Load (PSF) | Total Design Load | Code Reference |
|---|---|---|---|---|
| Residential floor (sleeping areas) | 30 PSF | 10 PSF | 40 PSF | IRC Table R301.5 |
| Residential floor (all other areas) | 40 PSF | 10 PSF | 50 PSF | IRC Table R301.5 |
| Residential deck (attached) | 40 PSF | 15 PSF | 55 PSF | IRC Table R301.5 |
| Residential attic (storage) | 20 PSF | 10 PSF | 30 PSF | IRC Table R301.5 |
| Residential attic (no storage) | 10 PSF | 5 PSF | 15 PSF | IRC Table R301.5 |
| Roof (slope < 4:12) | 20 PSF | 15 PSF | 35 PSF | Verify local snow load requirements |
| Light commercial floor | 50 PSF | 20 PSF | 70 PSF | IBC; verify with structural engineer |
| Office space | 50 PSF | 15 PSF | 65 PSF | IBC Table 1607.1 |
| Retail/commercial | 100 PSF | 20 PSF | 120 PSF | IBC Table 1607.1; varies by occupancy |
Floor Joist Maximum Spans — Douglas Fir #2
At 40 PSF live load + 10 PSF dead load (standard residential). Longer spans require larger members or engineered lumber.
| Joist Size | 12" O.C. Spacing | 16" O.C. Spacing | 24" O.C. Spacing | Application |
|---|---|---|---|---|
| 2x6 | 9'9" | 8'10" | 7'8" | Floor joist · Douglas Fir #2 |
| 2x8 | 13'1" | 11'10" | 10'4" | Floor joist · Douglas Fir #2 |
| 2x10 | 16'5" | 14'11" | 13'0" | Floor joist · Douglas Fir #2 |
| 2x12 | 19'9" | 17'11" | 15'7" | Floor joist · Douglas Fir #2 |
O.C. Spacing Matters
16" on-center is the most common floor joist spacing. Closer spacing (12" O.C.) allows longer spans. Wider spacing (24" O.C.) saves material but reduces span capacity significantly.
Engineered Lumber (LVL, I-Joist)
For spans exceeding sawn lumber limits, use LVL beams or manufactured I-joists. These can span significantly longer with less deflection. Always size per manufacturer span tables, not generic wood tables.
Nominal vs. Actual Lumber Dimensions
| Nominal Size | Actual Dimensions | Common Uses |
|---|---|---|
| 2x4 | 1.5 x 3.5 | Interior walls, framing, blocking |
| 2x6 | 1.5 x 5.5 | Exterior walls (2x6 framing), floor joists (short spans) |
| 2x8 | 1.5 x 7.25 | Floor joists, rafters, headers |
| 2x10 | 1.5 x 9.25 | Floor joists, rafters, long-span headers |
| 2x12 | 1.5 x 11.25 | Floor joists (long spans), stair stringers, beams |
| 4x4 | 3.5 x 3.5 | Posts, columns, fence posts (non-structural) |
| 4x6 | 3.5 x 5.5 | Beams, posts under moderate loads |
| 6x6 | 5.5 x 5.5 | Structural posts, deck posts, columns |
| 4x8 | 3.5 x 7.25 | Beams, headers for medium spans |
Sources: IRC 2021 Table R802.4 · IBC 2021 Table 1607.1 · AWC Span Calculator Data reviewed Q1 2026 — always use a licensed structural engineer for load-bearing design