Deck board spacing: how big a gap?

Why deck boards need gaps

The gaps do three jobs. They let rain and debris fall through instead of sitting on the deck. They let air reach the edges of the boards, so the boards dry out after rain. And on composite decks they leave room for each board to grow and shrink in length as the temperature changes.

That last job is why the makers tie the end gap to the temperature when you install. A board laid on a cold day will grow as it warms, so its ends need more room. A board laid on a hot day has already grown, so they need less. In each maker’s chart below, the end gap gets smaller as the temperature goes up.

Debris is also why the side gap can be wider under trees: Trex allows 3/8 in in wooded areas where debris collects.

So there are three kinds of gap to get right: the side gap between neighboring boards, the end gap where two boards meet end to end, and the gap where a board stops against the house, a post or another board.

Side gaps between boards

The side gap runs along the long edges of the boards. Trex’s guide sets it at 3/16 in “in all weather.” The other makers give a range or a minimum:

On TimberTech composite, the CONCEALoc clips set the gap at 3/16 in, the wide end of its range. Whichever figure applies, settle it before you plan the layout, because the gaps add up across the deck:

Across 25 boards there are 24 gaps. Going from 1/8 in to 3/16 in gaps adds 1/16 in to each, so the boards cover 24 × 1/16 = 1½ in more, and the last board lands 1½ in farther out.

End gaps change with the temperature

Where two boards meet end to end at a butt joint, composite boards need a gap sized for the temperature when you install them. The makers don’t agree, so go by your own board’s row:

Side by side on a few install days, the differences show:

Trex’s chart has two temperature bands, split at 40 °F. TimberTech’s has three and Fiberon’s five. The gap goes by the temperature when each board goes down, so if the day warms past a band while you work, the gap changes with it. The deck board calculator uses each maker’s gaps for the day’s temperature.

Fiberon’s figures are from its fastening instructions, FIB-0354-LIT 1/22. The 2/20 revision gave 1/32 in at 110 °F and up, where the 1/22 edition gives none.

Gaps at the house, posts and board ends

Boards also need room where they stop against something that doesn’t move, such as the house or a post, and where the end of one board meets the side of another, as where the main boards end against a border board.

• Trex: 1/4 in at the house, posts and other fixed objects above 40 °F, and 1/2 in below. The same 1/4 in or 1/2 in where a board end meets another board’s side. Picture-frame miters get 1/8 in above 40 °F and 3/16 in below.
• TimberTech: at least 3/16 in to walls and other fixed objects.
• Fiberon: starter clips at least 1/4 in from the wall. Where a board end meets another board’s side, 1/8 in at 70 °F and below and 1/16 in at 71 °F and above.

As with end gaps, Trex and Fiberon ask for more room on colder days: Trex doubles its wall gap from 1/4 in to 1/2 in below 40 °F.

Put together for Trex boards on a 50 °F day: 3/16 in between boards, 1/8 in at butt joints, and 1/4 in at the house, at posts and where board ends meet a border board. On a 35 °F day it’s 3/16 in, 3/16 in and 1/2 in.

Hidden clips set the side gap

With hidden clips you don’t measure the side gap at all: each clip sets it as the next board goes down.

• TimberTech CONCEALoc clips: 3/16 in.
• Fiberon Phantom Universal clips: 3/16 in.
• CAMO edge clips: 3/16 in.
• Trex Hideaway clips set the side gap on Trex boards.

Clips space only the long edges, so the end gaps and wall gaps above are still yours to measure. The hidden deck fastener calculator counts the clips, start clips and screws for your deck, with the boxes to buy.

Joists, overhang and extra boards

The same guides set a few layout rules that go with the gaps:

• Trex boards must span at least three joists. On joists 16 in on center, the first and third of those joists are 2 × 16 = 32 in apart, center to center.
• TimberTech composite boards need joists no more than 16 in on center, or 12 in when the boards are laid at an angle to the joists.
• TimberTech lets boards overhang the rim by up to 1 in, and Fiberon also allows up to 1 in of overhang.

When you order, add extra for waste. Trex adds 5% to its board count. TimberTech suggests 10–15% for boards laid straight and 30% for diagonal or herringbone layouts. The deck board calculator counts rows, board lengths and butt joints, then adds the extra.

Wood decking

For wood decks, the American Wood Council’s DCA 6 deck guide says to space decking boards about 1/8 in apart. TimberTech’s spacing article adds a note on pressure-treated lumber: freshly treated wood is installed tight, because it shrinks as it dries. The gaps open up as the boards dry.

If you plan to stain or seal a wood deck, the deck stain calculator works out the gallons from each product’s published coverage per coat.

Sources for the numbers on this page

1. Trex — Decking installation guide, 2026 edition (gaps, spans, fastening, stairs) · checked October 3, 2026
2. Trex — How to space deck boards · checked October 3, 2026
3. Trex — How many deck boards do I need? · checked October 3, 2026
4. TimberTech — Composite decking installation guide, REV 09/23 · checked October 3, 2026
5. TimberTech — Deck board spacing · checked October 3, 2026
6. TimberTech — How to calculate how much decking I need · checked October 3, 2026
7. TimberTech — Advanced PVC decking installation with TOPLoc, REV 09/23 (PDF) · checked October 3, 2026
8. TimberTech — CONCEALoc hidden fasteners (coverage) · checked October 3, 2026
9. Fiberon — PE decking fastening instructions, FIB-0354-LIT 1/22 (distributor copy, PDF) · checked October 3, 2026
10. Fiberon — Phantom Universal hidden fasteners (3/16 in gap) · checked October 3, 2026
11. CAMO — Hidden deck clips (starter clip coverage) · checked October 3, 2026
12. American Wood Council — DCA 6: Prescriptive residential wood deck construction guide (2015 IRC edition) · checked October 3, 2026

Corrections and updates are logged on the sources page.