IRC Table R507.6 (40 PSF Live Load + 10 PSF Dead Load)
Deck Joist Spacing & Span Chart
Joist span defines the unsupported distance between the ledger board and the support beam, or between two beams. This engineering table details maximum allowable spans for No. 2 grade Southern Pine and Douglas Fir under standard residential building codes.
| Nominal Joist Size | 12″ On-Center Span | 16″ On-Center Span (Standard) | 24″ On-Center Span | Max Overhang / Cantilever | Application Scope |
|---|---|---|---|---|---|
2 × 6 | 9′-11″ (3.02 m) | 9′-0″ (2.74 m) | 7′-7″ (2.31 m) | 1′-3″ (0.38 m) | Low-profile ground-level decks, landings, short walkway spans |
2 × 8 | 13′-1″ (3.99 m) | 11′-10″ (3.61 m) | 9′-8″ (2.95 m) | 2′-1″ (0.63 m) | Standard residential backyard decks, mid-span patio platforms |
2 × 10 | 16′-2″ (4.93 m) | 14′-0″ (4.27 m) | 11′-5″ (3.48 m) | 2′-10″ (0.86 m) | Elevated multi-story decks, long unsupported clear spans |
2 × 12 | 18′-0″ (5.49 m) | 16′-6″ (5.03 m) | 13′-6″ (4.11 m) | 3′-5″ (1.04 m) | Heavy timber framing, commercial decks, spans exceeding 15 feet |
Decking Material & Angle Spacing Limits
| Decking Board Type | Board Angle | Maximum Joist Spacing | Engineering Requirement |
|---|---|---|---|
| 5/4 Wood Decking (Cedar, Treated Pine) | Perpendicular (90°) | 16″ on-center | Standard framing layout for residential pressure-treated pine. |
| 5/4 Wood Decking | Diagonal (45°) | 12″ on-center | Diagonal boards span longer hypouse distances between joists. |
| 2x4 / 2x6 Wood Decking | Perpendicular (90°) | 24″ on-center | Structural 2-inch lumber handles wider joist spacing with minimal deflection. |
| 2x4 / 2x6 Wood Decking | Diagonal (45°) | 16″ on-center | Stiff framing prevents cupping under diagonal traffic patterns. |
| Capped Composite / PVC Decking | Perpendicular (90°) | 16″ on-center (mfr spec) | Most brands (Trex, TimberTech) void warranty if spaced past 16″ O.C. |
| Capped Composite / PVC Decking | Diagonal (45°) | 12″ on-center | Mandatory 12″ spacing to prevent bouncy feel and board sagging in high heat. |
The L/4 Cantilever Rule
Under IRC R507.6, joist overhang past a support beam must not exceed one-fourth (1/4) of the actual adjacent joist span, or the maximum length listed in the IRC cantilever table (whichever is less).
Composite Board Sag Factor
Unlike wood, composite polymers soften slightly in high summer heat. If installing boards at a 45-degree angle or installing a picture-frame border, reduce framing spacing from 16″ down to 12″ O.C. to prevent bounce.