Deck Board Gap Spacing by Wood Type: Pressure-Treated, Cedar, Composite

We learned this the hard way. Built a 16-foot pressure-treated deck in July using a tight gap (1/8 inch) because that's what the YouTube video said. Two months later, the boards had shrunk to 5/16 inch gaps. The deck looked like a piano keyboard with a tooth missing.

The fix was to pull every board and re-space, except we couldn't because the screws had stripped out the joists. Ended up replacing the whole top course of joists too. A 2-hour gap-spacing decision turned into a weekend of rework.

Here's what each wood type actually needs.

Why gap spacing matters

Two reasons.

Drainage. Water that lands on the deck has to drain through the gaps between boards. If the gaps are sealed by swollen wood or filled with debris, water pools and rots the boards from above.

Wood movement. Wood expands when wet, contracts when dry. The gap you install on day one will be different by next summer depending on what species and what moisture content you started with.

Pressure-treated lumber straight from Home Depot is typically 20-30% moisture content (it's been soaked in preservative). Air-dry equilibrium in most US climates is 12-15%. So pressure-treated will shrink about 1/8 to 1/4 inch per 5.5-inch wide board as it dries over 6-12 months.

If you install at a 1/8 inch gap and the boards shrink 1/4 inch, your gap becomes 3/8 inch. Functional but ugly.

If you install at NO gap (boards touching) and the boards shrink 1/4 inch, your gap becomes a clean 1/4 inch. That's the right ending state.

The point: install gap depends on what the boards will shrink to, not what gap looks right today.

Pressure-treated, wet from the store

Most pressure-treated lumber at big-box stores is "wet." You can tell by:

• Visible moisture on the surface
• Heavier than expected for its size
• Greenish color in the wood (not just on the surface)
• Sometimes literally dripping when you pick it up

For wet pressure-treated, install boards touching each other (no gap) or with a maximum 1/16 inch gap.

The boards will dry over 3-6 months and the gaps will open up to 1/8 to 1/4 inch on their own. That's exactly where you want them.

If you install wet PT with a 1/4 inch gap, you'll end up with 3/8 to 1/2 inch gaps after drying. The deck will look gappy and lose drainage performance.

Method: butt the boards tight against each other. Drive screws while pressing the boards together with a knee or a board straightener.

Pressure-treated, kiln-dried (KDAT or "premium PT")

Some pressure-treated is kiln-dried after treatment (sometimes labeled KDAT, AC2 KDAT, or "premium" depending on the brand). This wood has been dried back to 15-19% moisture content at the factory.

For KDAT pressure-treated, install with a 1/8 inch gap.

KDAT costs about 15-25% more than standard wet PT. Worth it if you want predictable spacing and don't want to wait 6 months for the deck to look right.

Method: standard 16d nail or 1/8 inch spacer (a few 16d nails work as makeshift spacers).

Cedar and redwood

Both are typically sold dry, around 12-15% moisture content.

Install with a 1/8 inch gap.

Cedar and redwood shrink less than wet pressure-treated because they start dry and they're less prone to large moisture swings. The 1/8 inch gap on day one is roughly the gap you'll have forever, maybe drifting to 5/32 inch in dry winters.

Method: standard 16d nail as a spacer between boards.

Composite decking (Trex, TimberTech, AZEK, others)

This is where the manufacturer's spec matters more than any general rule. Get it wrong and you void the warranty.

• Trex Transcend, Enhance, Select: 3/16 inch gap between board ends, 1/4 inch gap when end-to-end butt joints occur, 1/4 inch gap on board sides.
• TimberTech AZEK: 1/4 inch gap on all sides for boards up to 16 feet, 5/16 inch for boards over 16 feet.
• TimberTech composite (not AZEK): 3/16 inch side gap, 1/4 inch end gap.
• Fiberon and Deckorators: check the specific product line, but typically 3/16 to 1/4 inch side gaps.

The reason composite needs more gap than wood: composite expands and contracts with temperature, not just moisture. A composite board on a hot south-facing deck can expand 1/4 inch over its length from a winter morning to a summer afternoon. The gap absorbs that movement.

Method: most composite installs use hidden fastener systems (Cortex, Camo, or proprietary) that automatically space boards correctly. If using face-screwing, use the manufacturer's recommended spacer or a precision spacer like a Bessey strap clamp.

End gaps (butt joints between two boards)

Wherever two boards meet end-to-end mid-deck, you also need a gap.

• Pressure-treated wet: 1/8 inch end gap
• Pressure-treated KDAT: 1/4 inch end gap
• Cedar/redwood: 1/4 inch end gap
• Composite: 1/4 to 5/16 inch end gap (manufacturer-specific)

Why the bigger gap on end joints: wood and composite both move more along the length of the board than across the width. End-to-end butt joints absorb the cumulative movement of two full board lengths.

If you skip the end gap, boards will buckle in summer and you'll get a tented seam in the middle of your deck.

Quick reference

Common mistakes

Installing wet PT with a 1/4 inch gap "to be safe." Result: gaps grow to 1/2 inch as wood dries. Looks bad and lets debris fall through into the joist system.

Installing kiln-dried PT touching. It won't shrink (much). Your deck will be permanent flush boards with no drainage between them. Water pools, wood rots.

Using a 16d nail spacer for composite. Most composite needs 3/16 to 1/4 inch, a 16d nail is about 5/32 inch. Slightly too small. Use the manufacturer's recommended spacer.

Ignoring end gaps. End joints will buckle into a tent within the first summer if installed tight.

Mixing wet and dry boards in the same install. Wet boards will shrink, dry boards won't. Your gap spacing will be inconsistent. Buy all boards from the same delivery, or wait for everything to equalize on the joists before screwing down.

How to test moisture content if you're not sure

Buy a moisture meter (about $25 at Home Depot, brand: General Tools MMD4E or similar).

Stick the pins into the side of a fresh pressure-treated board. The reading tells you:

• Above 20%: wet, install tight (no gap)
• 15-20%: damp, install 1/16 inch gap
• 12-15%: dry, install 1/8 inch gap
• Below 12%: rare for outdoor lumber, treat like KDAT or cedar

Worth doing on the first board of every delivery. Takes 5 seconds, saves you from gap-spacing regret later.

Last updated May 2026