Deck Board Spacing & Gap Guide (2026)

Leave about 1/8 inch between wet pressure-treated boards and 1/8–1/4 inch between dry, cedar or hardwood boards; composite and PVC follow the manufacturer's spec, usually 1/8–3/16 inch side-to-side. The variable that actually decides the number is whether the wood is wet or dry when you fasten it down. Wet pressure-treated lumber — the common kind, sold soaking — shrinks across its width as it dries, so it goes down nearly tight and opens to roughly 1/4 inch on its own over the following weeks. Dry, kiln-dried, cedar and redwood boards barely move, so you set the finished gap at install. Dense hardwoods like ipe want a slightly wider 3/16–1/4 inch to keep air moving underneath and prevent cupping. Whatever the material, hold the gap consistent with a spacer tool so every seam matches, and leave a separate end gap where boards butt.

Yes — always. The gap does four jobs: water drains through instead of pooling on the surface or sitting on the joists, air reaches the underside so boards dry rather than cupping and decaying, boards get room to swell with heat and moisture without buckling, and the deck stays safe and tidy rather than opening into wide uneven seams. A deck laid with no gap traps water on the framing and rots from below, usually starting on the joist tops where you cannot see it until the damage is done. The apparent exception proves the rule: wet pressure-treated boards go down nearly tight, but only because they shrink as they dry and open to roughly 1/4 inch by themselves. Too wide is its own failure — much past about 1/2 inch once the wood has dried becomes a heel-and-cane trip hazard.

Install them nearly tight — a very small gap, around 1/8 inch. Wet pressure-treated lumber is sold saturated with preservative and shrinks measurably across its width as it dries over the following weeks, so the gaps open on their own to roughly 1/4 inch, which is the target spacing. Install those same wet boards at a full 1/4 inch instead and they will dry to something nearer 3/8–1/2 inch — wide enough to look sloppy, drop tools and phones through, and read as a trip hazard. The test is simple: if a board feels heavy and damp and beads water on the surface, treat it as wet. Kiln-dried, cedar, redwood and hardwood boards are dimensionally stable and barely move, so with those you set the finished gap at install rather than letting drying do it for you.

Follow the manufacturer's specification exactly — typically 1/8–3/16 inch side-to-side. Composite and PVC move more with temperature than wood does, and they move along their length rather than mainly across their width, which is why the end gap matters more than the side gap and why makers publish a temperature chart: the colder the day you install, the wider the end gap has to be, because the boards will expand when the weather turns. Many hidden-fastener systems set the side gap for you — the clip that seats in the board groove is machined to the correct width, which is one reason composite decks are usually clipped rather than face-screwed. Deviating from the published spec is also one of the fastest ways to void a composite warranty, so check the specific product's instructions rather than assuming one brand's number carries to another.

Use two fasteners per board at every joist on a face-screwed deck. Two fasteners hold the board flat and resist cupping; a single screw lets the board rock and lift at its edges over time. Predrill the ends of dense hardwoods like ipe so they do not split, and use stainless or coated fasteners rated for the wood and its treatment — modern preservative chemistry corrodes uncoated steel quickly. Hidden-fastener systems work differently: clips seat in a groove milled into the board edge, so no screws show on the surface and many systems self-space the boards to the correct gap. Whichever method you use, every board end must land on a joist with support on both sides of the joint — usually a doubled joist or blocking — and butt joints should be staggered across the deck like brickwork so the seams never line up.