Building & structure

Deck Board Spacing & Gap Guide (2026)

Gap too tight and the deck buckles and pools water; too wide and it's a trip hazard. Here's how much space to leave between deck boards — and why.

7 min read·Updated 2026-07-19·structural

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As a rule of thumb, leave about a 1/8-inch gap between wet pressure-treated boards (they shrink as they dry, opening to ~1/4 inch), and a full 1/8–1/4 inch between dry lumber, cedar and hardwood. Composite and PVC follow the manufacturer's spec exactly — usually 1/8–3/16 inch side-to-side and a larger end gap that changes with temperature. Gaps let water drain and boards expand; here's how to get them right.

Recommended gap by material

Board	Side-to-side gap	Notes
Wet pressure-treated	~1/8 in	Shrinks as it dries → opens to ~1/4 in
Dry / kiln-dried lumber	1/8–1/4 in	Won't shrink much — set the final gap now
Cedar / redwood	1/8–1/4 in	Stable; gap for drainage + airflow
Ipe / hardwood	3/16–1/4 in	Dense; needs airflow to avoid cupping
Composite / PVC	1/8–3/16 in	Follow the maker's spec exactly

The single most important variable is whether wood is wet or dry when you install it. Wet pressure-treated lumber (the common kind, sold soaking) shrinks across its width as it dries, so you fasten it nearly tight and let drying open the gap. Dry, cedar and hardwood boards barely move — set the gap you want at install. Get an exact board count for your layout with the deck material calculator.

Close-up of evenly spaced deck boards with a consistent narrow gap between each board — A consistent 1/8–1/4 inch gap lets water drain and air circulate — the difference between a deck that lasts and one that rots from below. · Photo: Unsplash / Unsplash

Why the gap matters

Drainage: water must fall through, not pool on the surface or sit on the joists. No gap = standing water = rot.
Airflow: gaps let the underside dry, preventing cupping and decay.
Expansion: boards swell with moisture and heat (composite especially) — the gap gives them room to move without buckling.
Safety & looks: too wide (over ~1/2 inch by the time wood dries) becomes a heel-and-cane trip hazard and looks sloppy.

A wooden deck surface and steps with an even gap between every board — Even gaps between the boards let water drain and air move underneath — wet pressure-treated wood goes down tighter and opens to this spacing as it dries. · Photo: Unsplash / Unsplash

End gaps and butt joints

Boards move along their length too, so leave an end gap where a board butts another or meets a wall — typically 1/8 inch for wood and per-spec for composite (composite end gaps grow in cold weather; makers publish a temperature chart). Stagger butt joints across the deck like brickwork so seams don't line up, and make sure every board end lands on a joist with support on both sides of the joint (often a doubled joist or blocking).

A finished wooden deck bordered by green foliage, its boards laid in continuous runs — On a run this long the butt joints get staggered like brickwork, and every board end lands on a joist — never cantilevered past its framing. · Photo: Unsplash / Unsplash

Fastening: spacing and method

Two fasteners per board at every joist for face-screwed decks — this holds the board flat and resists cupping.
Hidden fasteners (clips in the board groove) give a clean, screw-free surface and self-space many composite systems automatically.
Predrill hardwood ends to prevent splitting; use stainless or coated fasteners rated for the wood/treatment.
Keep gaps consistent with a spacer tool (or a framing nail) so every seam matches.

Board spacing works hand-in-hand with the framing below it — boards can only span so far between joists. Check the joist spacing guide so your board direction and joist spacing match, and see the full build sequence in how to build a deck.

Deck screws and hidden fastener clips used to secure deck boards to the joists — Two fasteners per joist for face-screwed boards, or hidden clips for a clean surface — either way, consistent fastening keeps boards flat. · Photo: Unsplash / Unsplash

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Calculate exact decking boards, joists, beams, posts, footings, and fasteners for your deck — IRC 2021 compliant.

Frequently asked questions

How much gap should I leave between deck boards?

Leave about 1/8 inch between wet pressure-treated boards (they shrink to ~1/4 inch as they dry) 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.

Should deck boards be installed with a gap?

Yes — always. Gaps let water drain, air circulate under the boards, and boards expand without buckling. A deck installed with no gap traps water and rots; the only exception is wet pressure-treated wood, which goes down nearly tight and opens as it dries.

Do wet pressure-treated deck boards need a gap?

Install them nearly tight (a very small gap). Because wet pressure-treated lumber shrinks across its width as it dries over the following weeks, the gaps open on their own to roughly 1/4 inch — the target spacing.

What gap do composite deck boards need?

Follow the manufacturer's specification exactly — typically 1/8–3/16 inch side-to-side, with a larger end gap that increases in cold weather per the maker's temperature chart. Composite expands and contracts more than wood, so correct gapping is critical.

How many screws per deck board?

Use two fasteners per board at every joist for face-screwed decks — this holds boards flat and resists cupping. Hidden-fastener systems use clips in the board grooves instead, giving a clean surface and often self-spacing the boards.