Deck Board Spacing: What Gap Should You Leave?

The gap between deck boards is not cosmetic — it controls drainage, allows expansion and prevents cupping. Get it wrong and you end up with a deck that holds water, warps or has uneven gaps after one season.

The standard 3/16 inch gap

For kiln-dried pressure-treated or cedar decking installed dry, leave a 3/16 inch gap between boards. This is roughly two nail widths and allows rainwater to run through without being wide enough to trap debris or pose a trip hazard.

Wet-installed pressure-treated lumber

Pressure-treated decking fresh from the lumber yard is wet and will shrink as it dries. If you install wet boards edge to edge (no gap), they will typically shrink to about a 1/4 inch gap over the first season. This is intentional — butting wet boards together lets the drying do the work. Check the moisture content of your boards; if they read above 19% on a moisture meter, butt them up tight.

Composite decking: follow the manufacturer

Composite decking expands with heat, not moisture. Most manufacturers specify gaps of 3/16 to 1/4 inch between boards end to end, and 3/16 inch edge to edge. The gap at the end of each board (the butt joint where two boards meet end-to-end over a joist) must match the manufacturer's spec — usually 1/4 inch — to allow thermal expansion without buckling.

Spacing at the house and fascia

Leave at least 1/2 inch between the deck boards and any adjacent house wall or fascia board so that moisture and debris don't accumulate in the gap. The IRC does not specify this clearance precisely, but most deck builders and manufacturers recommend at least 1/2 inch.