Pressure-Treated Deck Board Spacing: Wet vs Dry
Pressure-treated lumber can change width as it dries, so the starting gap depends heavily on how wet the boards are when installed.
Open deck spacing calculator →Do not use one spacing number for every pressure-treated board
Fresh pressure-treated boards are often still wet and can shrink across their width as they dry. That is why wet boards are commonly installed tighter than dry or acclimated boards. Dry material needs an intentional drainage and movement gap from the start.
The safest workflow is to identify the actual moisture condition, check the lumber supplier or installation guidance, and then use that side-to-side gap in a layout calculation. The Leftover deck spacing calculator includes separate planning presets for fresh/wet pressure-treated lumber and dry/acclimated wood, while still letting you override the value.
What changes when the gap changes?
The gap affects how many board rows fit across a fixed deck width. Small changes repeated across twenty or thirty rows can change the final edge condition and sometimes the total board count. Use the calculator before fastening the first course so you can see the row count and coverage.
This is layout guidance only. Structural framing, joist spacing, fasteners and end gaps are separate requirements.
Pressure-Treated Deck Board Spacing FAQ
Should wet pressure-treated deck boards be installed tight?
Fresh wet treated boards are commonly installed tight or very close together because they shrink as they dry, but supplier guidance for the actual lumber controls.
What gap should dry pressure-treated boards have?
A small side-to-side gap such as about 1/8 inch is a common planning starting point for dry or acclimated wood; verify the actual lumber guidance.
Does this page calculate joist spacing?
No. Joist spacing is structural and separate from the visible side-to-side gap between deck boards.