Deck Board Gap & Spacing Calculator

Frequently asked questions

How much gap should I leave between deck boards?

It depends on the material and its moisture. Wet pressure-treated lumber straight from the yard goes down at about 1/16 inch, because it shrinks to roughly 1/8 as it dries. Already-dry treated lumber, cedar and composite go down at 3/16. PVC wants 1/4 inch because it moves the most with temperature. Tropical hardwood sits at 1/8.

Why do wet treated boards get a smaller gap?

Because the board does the work for you. Pressure-treated lumber leaves the yard saturated with treatment solution and loses a noticeable amount of width as it dries over the first few months. Lay those boards tight or nearly tight and the gap opens to about an eighth of an inch by itself. Lay them at 3/16 and you end up with quarter-inch gaps that swallow keys and coins.

What happens if deck board gaps are too small?

Debris packs into the joint, water sits there instead of draining, and the board edges stay wet — which is where rot starts on a wood deck and where mould grows on composite. With composite and PVC the bigger risk is expansion: boards with nowhere to go push against each other and buckle, and that is not covered by the warranty.

Why is my last deck board a narrow strip?

Because the deck width rarely divides evenly by the board width plus the gap. There are two clean fixes. Rip a little off both the first and last boards so the two edges match, which reads as deliberate. Or open the gap by a thirty-second of an inch across the whole run, which over twenty boards shifts the last one by more than half an inch and often absorbs the problem entirely.