Deck board gap: how far apart should deck boards be?
Gaps let water drain, debris fall through and boards move. The right gap depends on how wet the wood is when you install it, or on the composite manufacturer’s instructions.
Typical gaps by material
| Decking | Side-to-side gap | Why |
|---|---|---|
| Wet pressure-treated (fresh from the yard) | Tight, or about 1/16″ | It shrinks across the width as it dries, opening the gap. |
| Kiln-dried after treatment (KDAT), cedar | About 1/8″ | Already dry, so it won’t shrink much. |
| Tropical hardwood | Per supplier, commonly about 1/8″ | Dense wood moves with humidity. |
| Grooved composite / PVC | Set by the hidden clip, commonly about 3/16″ | The clip’s spacer tab sets the gap. |
Treat these as starting points. The manufacturer’s installation guide is the rule for composites, and some brands specify end gaps that change with the temperature at installation.
End-to-end gaps
Wood boards are usually butted tight at seams, because wood barely moves along its length. Composite and especially PVC boards expand and contract lengthwise with temperature, so manufacturers specify an end gap, and some vary it with the installation temperature and board length. Follow the chart in your product’s instructions.
The gap changes how many boards you buy
A larger gap means fewer rows. For a 12 ft deep deck with 5½″ boards:
- 1/16″ gap: 27 rows
- 1/8″ gap: 26 rows
- 3/16″ gap: 26 rows
- 1/4″ gap: 26 rows
One row more or less is one board per row-length, which can be several boards on a long deck. Enter your exact gap in the board calculator.
Gaps elsewhere
- At the house: leave a gap between the first board and the siding so water and debris can escape. The ledger flashing must stay exposed to drain.
- At posts: notch around railing posts with a small gap on each side.
- Under the deck: low decks need ventilation beneath them. Several composite brands publish a minimum clearance and open-area requirement.
Keep the gap consistent
Use spacers (a 16d nail is about 1/8″, and commercial spacer tools come in fixed sizes) and check every few rows that the remaining distance to the rim stays parallel. Small errors add up across 25 or more rows. Correct a few rows before the end, not on the last board.