Space deck boards with a gap of 1/8 to 1/4 inch between them. That gap lets rainwater drain through, gives the boards room to expand and contract, and keeps debris from packing into a solid mat that traps moisture against the wood.
Getting the gap right also affects how many boards you need, because the spacing changes how many rows fit. Run your numbers through the deck board calculator and it handles the math for you.
The spacing math
Your number of rows is simple:
Rows = deck length ÷ (board width + gap)
A standard board is 5-1/2 inches actual width. Add your chosen gap, divide your deck's run by that figure, and you have your row count. Because the gap is part of the equation, even a small change in spacing shifts the total board count on a large deck, which is exactly why eyeballing it leads to a short order.
Pressure-treated vs. composite gaps
The right gap depends on your material, because the two behave very differently after installation.
- Pressure-treated lumber: Treated boards usually arrive wet and shrink as they dry out. Install them tighter, around 1/8 inch, because the gap will naturally widen as the wood loses moisture over its first season.
- Composite decking: Composite is dimensionally stable, so it won't shrink the way wet wood does. Follow the manufacturer's exact spacing specification, which accounts for thermal expansion along the board's length as well as side to side. Manufacturers often call for end gaps too, especially in hot climates.
That difference matters. Gap wet treated lumber too wide and you'll end up with cavernous, debris-catching gaps once it dries. Gap composite by feel instead of by the spec sheet and you risk buckling on a hot day. If you're still choosing materials, our pressure-treated vs. composite cost breakdown covers the trade-offs.
How to keep the gap consistent
A consistent gap is what separates a clean deck from a sloppy one. A few proven methods:
- Spacers. Slip a uniform object between boards as you fasten each one. A 16d common nail gives roughly 1/8 inch; a deck board spacing tool gives you a repeatable, dialed-in gap.
- A spacing jig. Drop two spacers at each end of the board, fasten, then move them to the next seam. A deck board spacing tool speeds this up and frees both hands.
- String or chalk line. Snap a reference line every few boards to confirm you haven't drifted. Small errors compound across a wide deck.
Work off a single reference edge and recheck your spacing every several rows. If you do drift, correct it gradually across a few boards rather than all at once, so the change isn't obvious.
Common spacing mistakes
- Forcing crooked boards to a fixed gap. Most lumber has a slight bow. Pull the crown toward your spacer and fasten from one end to the other, letting the gap absorb the curve.
- Ignoring end gaps. Where two boards meet end to end over a joist, leave a small gap there too. Butting them tight invites rot and buckling.
- Skipping the spec sheet on composite. Manufacturer spacing isn't a suggestion. It's tied to the warranty.
- Using a gap so tight water can't drain. Boards installed with no gap trap water and debris, and that's the fastest route to a failing deck.
Fastening at the right gap
Once your spacing is set, fasten consistently. Plan on two screws at every joist crossing, driven straight so the board doesn't shift your carefully set gap. A good impact driver sinks screws cleanly without splitting board ends. For the full count, see how many deck screws per board.
The bottom line
Aim for 1/8 to 1/4 inch: tighter for wet pressure-treated lumber that will shrink, and exactly per spec for stable composite. Keep the gap uniform with spacers or a jig, leave end gaps where boards butt, and let water drain freely. Confirm your row count and total boards with the deck board calculator before you buy, and your deck should look intentional from the first board to the last.