Deck board spacing guide: gap, expansion, and fasteners
Updated April 26, 2026
Quick answer
Gap by board material
| Material | Gap when installing | Why |
|---|---|---|
| Wet pressure-treated | ¼ in (tight) | Boards shrink as they dry |
| Kiln-dried PT (KDAT) | ⅛ in | Already shrunken, gap is final |
| Cedar / redwood | ⅛ in | Stable once acclimated |
| Composite (Trex, TimberTech) | ⅛–¼ in (per spec) | Expands with heat |
| PVC | 1/16–⅛ in (per spec) | Less thermal movement than composite |
| Ipe / hardwood | 3/16 in | Stable; gap is for drainage |
| Bamboo composite | ⅛ in | Dimensionally stable |
Always check the manufacturer's spec sheet for composites, gap requirements vary by brand and ambient temperature at install.
Joist spacing by board direction
| Boards run | Wood (5/4 or 2x6) | Composite | Hardwood |
|---|---|---|---|
| Perpendicular to joists | 16 in OC | 16 in OC | 16 in OC |
| Diagonal (45°) | 12 in OC | 12 in OC | 12 in OC |
| Picture frame border | Add blocking | Add blocking | Add blocking |
Formula
Boards needed by row
board_width_with_gap = nominal_width_in + gap_in rows_needed = ceil( deck_width_in / board_width_with_gap ) Example: 12 ft (144 in) wide deck, 5.5 in boards, ⅛ in gap = ceil(144 / 5.625) = 26 rows
Worked example
12 × 16 ft deck, composite, ⅛ in gap
Boards run 16 ft, perpendicular to 12 ft joists at 16 in OC.
- 1. Deck width144 in
- 2. Board + gap = 5.5 + 0.1255.625 in
- 3. Rows = 144 / 5.62525.6 → 26
- 4. Boards × 16 ft length26 boards
- 5. + 10% waste29 boards
→ 29 × 16 ft composite boards. 26 cover the field; 3 are insurance.
Why the gap matters
- Drainage: water needs somewhere to go. Without gaps, leaves and debris pack between boards and trap moisture against the wood.
- Expansion: wood swells when wet, composite swells when hot. No gap means cupped, buckled, or split boards within a year.
- Air flow: gaps let the underside of boards dry out, critical for longevity.
End-to-end gaps (butt joints)
Most composite manufacturers require ⅛–¼ in end-to-end gaps between boards that meet over a joist. Wood butt joints can be tighter (1/16 in) but never zero, wood expands lengthwise too, just less than sideways. Always center butt joints over a joist and offset them at least 4 ft between adjacent rows.
Screw spacing and pattern
Two screws per joist, placed about ¾ inch from each edge. Pre-drill at board ends to prevent splitting (within 4 inches of the cut). Use composite-specific screws on composite boards, standard deck screws strip out the plug system on most brands.
| Deck size | Joists | Boards | Screws (no waste) |
|---|---|---|---|
| 8 × 10 | 7 | 18 | 252 |
| 10 × 12 | 9 | 22 | 396 |
| 12 × 16 | 10 | 26 | 520 |
| 16 × 20 | 13 | 35 | 910 |
| 20 × 20 | 16 | 44 | 1,408 |
Buy a 5 lb tub of 2.5 in deck screws, about 350 screws. Add tubs as needed. Always have 20% extra; you'll strip a few.
Common mistakes (and how to avoid them)
✗ Installing wet PT lumber with a ⅛ in gap.
Fix: Wet PT shrinks 5–10% as it dries. Install at ¼ in tight; the gap will close to ⅛ in over the first season. ⅛ in install on wet boards = no gap in 6 months.
✗ Using deck screws on composite boards.
Fix: Composites have a plug or hidden-fastener system specific to the brand. Generic deck screws strip out, dimple the surface, and void the warranty.
✗ Skipping end-to-end gaps over joist butt joints.
Fix: Composite boards expand lengthwise with heat. Tight butt joints buckle in summer. Use the manufacturer's spec, usually ⅛ in per 8 ft of board.
✗ 16 in OC joists for diagonal composite boards.
Fix: Diagonal layouts need 12 in OC for any board type, the diagonal span is √2 longer than the perpendicular span. 16 in OC under diagonal composite voids most warranties.