Outdoor Build

The Gap Between Deck Boards Is Doing More Than Draining Water

Installing deck boards edge to edge in dry weather routinely leads to buckling once the wood absorbs moisture and expands.

Wood is hygroscopic — it absorbs and releases moisture with the seasons, expanding across its width as it does. Pressure-treated lumber installed wet (as most of it is, straight from the treatment process) shrinks as it dries, opening gaps naturally. Installed bone dry with boards butted tight, the same lumber later swells with humidity and has nowhere to go, buckling or cupping.

The standard spacing is about 1/8 to 3/16 inch for treated lumber installed wet, a bit less for composite decking which moves far less than wood. Composite manufacturers publish exact gap specs per product because thermal expansion, not moisture, drives their movement instead.

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