Deck Board Spacing: What the Manufacturers Actually Say

Trex says leave 3/16 inch between deck boards. TimberTech says 1/8 inch to 1/4 inch, depending on the product line and fastener. Both numbers are correct. They're just for different boards. With composite decking, the gap spec comes from the manufacturer of the boards in your hand, not from a general rule you saw somewhere else.

That's the first thing to get right on a deck build, and it's also the thing homeowners get wrong most often. A gap that's too tight buckles when the boards expand. A gap that's too wide looks sloppy and catches heels, dropped tools, and small dog toys.

This guide lays out what Trex and TimberTech each publish for their own boards, why gaps exist at all, how joist spacing factors in, and how to think about waste when you're ordering material. Run your own numbers through the deck board calculator once you know your gap spec.

Trex gap specs

Trex publishes three separate numbers depending on where the gap sits on the deck.

Side-to-side (the long gap running between two boards laid parallel to each other): 3/16 inch in all weather conditions. In heavily wooded areas, Trex recommends widening that to 3/8 inch so leaves and other debris can pass through instead of packing into the gap.

End-to-end (where one board butts up against another on the same run): this one depends on the temperature at install. On days warmer than 40°F, leave 1/8 inch. On days colder than 40°F, leave 3/16 inch.

Where decking meets a solid surface, such as a house wall: 1/4 inch above 40°F, 1/2 inch below 40°F. Trex is explicit that 1/2 inch is the ceiling. Don't exceed it under any circumstance.

Source: Trex, "How to Space Deck Boards"

TimberTech gap specs

TimberTech's numbers differ by product line, which is the detail that trips people up if they assume all composite decking behaves the same.

AZEK (TimberTech's capped polymer line): side-to-side gap of 1/8 inch to 1/4 inch, with the exact figure depending on which fastener you're using. AZEK does not require an end-to-end gap at all. TimberTech says the material doesn't buckle with a tight installation, closer to how solid wood behaves than how other composites behave.

PRO and EDGE (TimberTech's capped composite lines): side-to-side gap of 1/8 inch minimum up to 7/32 inch maximum, again depending on fastener type. The end-to-end gap for these two lines is temperature-dependent, but TimberTech publishes the exact figures in a chart in its installation guide rather than on the general public page, so check that guide for your specific product before cutting boards.

Source: TimberTech, "Deck Board Spacing: What You Need to Know"

Why the two brands don't match

Trex's 3/16 inch side gap and TimberTech's 1/8 inch to 7/32 inch range overlap but aren't the same number, and neither company frames its figure as an industry standard. Each is a spec for its own material. The practical takeaway: don't borrow a gap width from a forum post, a neighbor's deck, or a different brand's spec sheet. Pull the number from the install guide for the exact boards you bought. If you're mixing brands on one project, which most builders don't recommend, follow each board's own manufacturer spec for the boards it applies to.

Both companies agree on the reason gaps exist in the first place, even where they land on different numbers: drainage and material movement. Gaps let water run through the deck surface instead of pooling on top of it, and they give the boards room to expand and contract without buckling as temperature changes. That's also why several of the specs above are temperature-dependent. A board installed tight on a cold day has more room to expand once summer heat hits it, so the gap needs to be wider going in.

Joist spacing underneath

The gap between boards gets the attention, but the framing underneath sets real limits on what you can do with decking pattern and board choice.

16 inches on center is the standard, most common joist spacing for a residential deck. Some situations call for tighter framing at 12 inches on center: heavier loads, decking installed at a 45-degree angle to the joists, or certain composite products that require the tighter spacing to stay within their rated span. 24 inches on center is rarely used for decks, because most decking materials, wood or composite, are only rated to span up to 16 inches without excessive flex.

This isn't a preference call. The governing code reference is 2021 IRC Section R507.6, and the joist-span tables in that section are built around three specific spacing columns: 12, 16, and 24 inches on center. Confirm your framing plan, and any deviation from 16 inches on center, with your local permit office before you build.

Waste factor: order more than the math says

Once you've settled on a gap and a joist layout, the board-count math from the deck board calculator gives you a base number. Don't order exactly that number.

TimberTech's own guidance is to add 10% to 15% extra boards for a standard, straight-run layout, to cover cutting mistakes, waste, and off-cuts. For a complex pattern, diagonal or herringbone, TimberTech says to bump that up to roughly 30% waste, since angled cuts throw away more of each board.

Decks.com backs the same direction: diagonal, chevron, herringbone, and other intricate layouts add another 10% to 15% waste on top of your normal allowance. Read that as stacking on top of your base waste factor, not replacing it, though Decks.com's own page doesn't spell out the exact combined total.

The pattern you pick isn't only a style decision. It changes how many boards you need to buy.

Value retention: why spacing shows up in resale

A deck is one of the few home projects where the finish work is judged at eye level, up close, by anyone who walks across it. Uneven gaps are visible in a way that a slightly-off paint color or a mismatched light fixture isn't. Buyers and appraisers walk decks. A board that's buckled from a too-tight installation, or a gap that widens and narrows down the run because it wasn't set consistently, reads as an amateur build even if the framing underneath is solid.

Composite decking is marketed on longevity, and the gap spec is part of what makes that promise hold. A deck installed to the manufacturer's spacing sheds water instead of trapping it, moves the way it's supposed to as temperatures swing, and doesn't develop the visible buckling or cupping that comes from ignoring the spec. That's the difference between a deck that still looks square in year 10 and one that already looks tired by year 3.

FAQ

What's the standard gap between deck boards?

There isn't one universal standard. Trex specifies 3/16 inch side-to-side in all weather, widening to 3/8 inch in wooded areas. TimberTech specifies 1/8 inch to 1/4 inch for AZEK and 1/8 inch to 7/32 inch for PRO and EDGE, depending on fastener type. Use the spec published for the exact boards you bought, not a generic number.

Do wood deck boards need the same gap as composite?

The sourced data for this guide covers Trex and TimberTech composite specs specifically. Both brands tie their gap recommendations to temperature and drainage, which are considerations that apply to any decking material that expands, contracts, and needs to shed water. For wood-specific gap figures, check the install guidance from the lumber supplier or treatment manufacturer for the boards you're using.

Why does the end-to-end gap change with temperature?

Boards installed on a cold day have more room to expand once the weather warms, so both Trex and TimberTech (for the PRO and EDGE lines) call for a wider end-to-end gap below 40°F than above it. AZEK is the exception. TimberTech says it doesn't require an end-to-end gap at all, in any temperature.

How much extra decking should I order for a diagonal pattern?

TimberTech recommends roughly 30% waste allowance for diagonal or herringbone layouts, up from 10% to 15% for a standard straight run. Decks.com separately notes that diagonal, chevron, herringbone, and other intricate layouts add another 10% to 15% waste on top of your normal allowance. Run your dimensions through the deck board calculator to get a base board count, then apply the waste factor that matches your pattern.

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