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The gap to leave between deck boards, and the exact board count it produces.
With these inputs:
| Result | Value |
|---|---|
| Gap between boards | 1/16" |
| Boards across | 26 |
| Full-width boards | 25 |
| Last board rip | 4.94" wide |
| Distance to cover | 12.0 ft (144") |
| Board width | 5.5" |
| Board plus gap | 5.563" |
| Covered by full boards | 139.00" |
| Left over | 5.00" |
| Gap set by | wet treated lumber shrinks across its width as it dries, opening the gap to roughly 1/8 inch on its own |
Dry-lay the first three boards and measure before you fasten anything. Real boards vary by a sixteenth across a bundle, and the error compounds across a wide deck. Use a spacer of the exact thickness rather than eyeballing it — a deck screw shank, a plastic spacer or a scrap of the right material all work, and consistency reads far better than any particular gap size. Leave the same gap where the decking meets the house and any posts.
Gap size is not a style choice. Wet pressure-treated lumber shrinks as it dries and closes its own gap; kiln-dried wood and composite do not, so they need the gap built in. Get it wrong in either direction and you either trap debris or end up with boards touching. This gives the right gap for your material and moisture state, then works out the board count and the width of the final rip.
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.
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.
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.
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.
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