Deck board spacing: how big a gap?
The gap between deck boards drains water, lets the wood breathe and allows for movement. The right size depends on what the boards are made of — and how wet they are today.
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Why decks have gaps
- Drainage: water and debris fall through instead of sitting on the surface.
- Drying: air movement helps wood dry after rain, slowing rot and mildew.
- Movement: wood swells and shrinks with moisture; composites expand and contract with temperature.
Typical side-to-side gaps
| Decking | Typical gap when installed | Why |
|---|---|---|
| Pressure-treated lumber that's still wet | Tight or about ⅛ in | It shrinks as it dries, opening the gaps |
| Kiln-dried or dry wood | About ⅛ to 3/16 in | It will swell a little when wet |
| Composite and PVC | Per the manufacturer — often around 3/16 in, set by hidden fasteners | Mostly temperature-driven movement |
Wet pressure-treated boards feel heavy and cool and may be sold with a "wet" or "treated" label; kiln-dried-after-treatment (KDAT) lumber is dry and stable. When in doubt, check a board's moisture or ask the lumberyard.
End-to-end gaps
Where boards butt end to end, composites need expansion gaps that depend on the board length and the temperature on installation day — the manufacturer's installation guide has a table. Wood boards are usually butted tight over a joist (ideally a doubled joist or blocking) because they barely move lengthwise.
Joist spacing
Decking also limits how far apart the joists can be. The International Residential Code allows 16 in on center for 5/4 (1¼ in) wood decking and 24 in for 2 in wood decking when boards run square to the joists — less for diagonal layouts. Composites follow their own ratings, usually 16 in or less. The deck calculator flags spacing that's too wide.
How gaps change the board count
A 5.5 in board with a 3/16 in gap covers 5.6875 in of deck width, so a 12 ft wide deck takes 26 rows; with no gap it would take 27. The difference is small, but it's why the calculator asks for the gap.
Tips for even gaps
- Use spacers — nails, purpose-made gap tools or the hidden fasteners themselves — rather than eyeballing.
- Straighten bowed boards as you fasten, working from one end.
- Check every few rows with a tape that you're still parallel to the house, and adjust gradually.
- Leave a gap between the decking and the house siding so water can drain and air can circulate.
Frequently asked questions
Should pressure-treated deck boards be gapped?
If they're still wet from treatment, many builders install them tight or nearly tight because they'll shrink. If they're dry, leave about ⅛ to 3/16 in.
Can the gap be too big?
Yes — gaps much over about ¼ in can catch heels and chair legs and let small items fall through.
Do hidden fasteners set the gap?
Many hidden fastener systems for grooved boards create a fixed gap automatically; use the ones the decking manufacturer specifies.
Sources
- International Residential Code, Table R507.7 — maximum joist spacing for decking.