Deck Board Spacing: The Right Gap Between Boards
Quick answer: leave about 1/8 inch between pressure-treated wood deck boards. Composites often need 1/4 inch for thermal expansion. Always check the board's own spec.
- The 1/8-inch rule
- Composite spacing
- Why gaps matter
- Worked example
- How to keep gaps even
- Common mistakes
The 1/8-inch rule
Pressure-treated wood deck boards are typically spaced about 1/8 inch apart. That's enough for water to drain and for the boards to expand without buckling. Too tight and boards cup and push together in humid weather; too wide and heels and small feet catch in the gaps.
Composite spacing
Most composite decking expands more with heat than wood, so manufacturers usually specify a 1/4-inch gap. Read the exact recommendation on the box — composite brands differ, and using the wrong gap voids the warranty. Some hidden-fastener systems set the gap automatically as you install.
Why gaps matter
The gap serves two jobs: drainage (water falls through instead of pooling against the joists) and expansion room (wood grows and shrinks with humidity). Boards butted tight will cup and buckle, and trapped moisture accelerates rot at the joists. A consistent gap is cheap insurance for the life of the deck.
Worked example
For a 12-ft wide deck with 6-in boards at 1/8-in gaps:
- Each board + gap covers about 6.125 in
- Rows ≈ 144 in ÷ 6.125 ≈ 23–24 boards
- Add waste and you're around 25–26 boards for the run
The deck calculator counts boards, joists, and cost from your exact dimensions, gap included.
How to keep gaps even
Use a spacer or a marked nail as a quick gauge, and keep it consistent board to board. For a bigger deck, a chalk line keeps the fastening straight. Even gaps look intentional; uneven ones read as rushed and catch debris.
Common mistakes
- Butting boards tight. They cup and buckle with expansion.
- Guessing the gap. Use a spacer; a glance isn't consistent.
- Same gap for composite and wood. Follow the board's own spec.
- Gaps too wide at the end. Waterproofing tape on top of joists still matters.
FAQ
Use about 1/8 inch between pressure-treated wood deck boards. Some composite boards specify a 1/4-inch gap for thermal expansion. Always follow the manufacturer's spec.
The gap lets water drain and gives wood room to expand and contract with the weather. Without it, boards can buckle, cup, and trap moisture against the joists.
Not for most wood — expansion will push boards together and cause cupping. A consistent small gap prevents that. Composite brands vary, so check their spec.
Yes. Most composites expand more with heat than wood, so manufacturers typically call for a 1/4-inch gap. Follow the board's own installation guide.