Joist spacing and board count guide
Updated April 26, 2026
Quick answer
Joist spacing by application
| Boards | Direction | Min joist OC |
|---|---|---|
| 5/4 wood (PT, cedar) | Perpendicular | 16 in |
| 5/4 wood | 45° diagonal | 12 in |
| 2× wood (2×6) | Perpendicular | 24 in (16 preferred) |
| Composite (Trex, TimberTech) | Perpendicular | 16 in |
| Composite | 45° diagonal | 12 in |
| PVC | Perpendicular | 16 in (some 12) |
| Hardwood (ipe, cumaru) | Perpendicular | 16 in |
Always confirm with the board manufacturer, many composite brands explicitly void the warranty for 24 in OC.
Joist count by deck size (16 in OC)
| Deck size | Joist length | Joists @ 16 in OC | Joists @ 12 in OC |
|---|---|---|---|
| 8 × 10 | 10 ft | 7 | 9 |
| 10 × 12 | 12 ft | 9 | 11 |
| 12 × 12 | 12 ft | 10 | 13 |
| 12 × 16 | 16 ft | 10 | 13 |
| 12 × 20 | 20 ft | 10 | 13 |
| 14 × 18 | 18 ft | 12 | 15 |
| 16 × 20 | 20 ft | 13 | 17 |
| 20 × 20 | 20 ft | 16 | 21 |
Formula
Joist count
joists_at_OC = ceil( (deck_width_in - 1.5) / OC_in ) + 1 Where OC = 12 or 16 inches. The -1.5 accounts for the rim joist at one end; +1 adds the joist at the start. Always add 2 rim joists running the perpendicular direction.
Worked example
12 × 16 deck, composite boards, 16 in OC
Standard backyard deck, boards run the 16-ft direction.
- 1. Joists span the 12-ft direction (144 in),
- 2. Joists = ceil((144 - 1.5)/16) + 110
- 3. + 2 rim joists12 total
- 4. 2×8 SYP @ 16 OC max span11'-10", OK for 12 ft
→ 12 × 2×8 PT joists @ 16 in OC. 2×10 if you want extra stiffness.
IRC max joist spans (residential, 40 psf live load)
| Joist size | 12 in OC | 16 in OC | 24 in OC |
|---|---|---|---|
| 2×6 | 9'-11" | 9'-0" | 7'-7" |
| 2×8 | 13'-1" | 11'-10" | 9'-8" |
| 2×10 | 16'-2" | 14'-0" | 11'-5" |
| 2×12 | 18'-0" | 16'-6" | 13'-6" |
Rough guide. Always verify with your local code official, adjustments apply for cantilevers, pools, hot tubs, or any concentrated load.
How joist spacing changes the deck feel
12 in OC feels noticeably stiffer underfoot than 16 in OC, even with the same boards. If you want a "no bounce" deck for a roof terrace or an outdoor kitchen, drop to 12 in OC even if the boards don't require it. The extra joists cost ~$80 on a 12×16 deck, cheap upgrade.
Cantilevers and overhangs
Joists can cantilever past the supporting beam by up to 1/4 of the unsupported span (so a 12-ft span allows up to 3 ft of overhang). This is the cleanest way to hide the beam behind the rim joist for a flush look. Don't cantilever beyond ¼, code violation and the deck will bounce.
Common mistakes (and how to avoid them)
✗ Specifying 24 in OC under composite boards.
Fix: Most composite brands void warranty above 16 in OC. Stick to 16 in OC perpendicular, 12 in OC diagonal, always confirm with the brand spec sheet.
✗ Using 2×6 joists for a 12-ft span.
Fix: 2×6 maxes out at 9 ft @ 16 OC. A 12-ft span needs 2×8 minimum, 2×10 for stiffness. The lumber cost difference is $40, worth it.
✗ Skipping blocking between joists on long runs.
Fix: Joists over 8 ft long need mid-span blocking (solid 2× pieces between joists). Without blocking, joists twist under load and the deck bounces.
✗ Cantilevering joists past the beam more than 1/4 of the span.
Fix: Code limit is 1/4 of the supported span. A 12-ft joist can cantilever up to 3 ft. Beyond that, the deck bounces and the IRC says no.