Composite Decking Expansion Gaps: The Installer's Rule

Short answer: Gaps exist because every board changes length with temperature, and the gap you leave depends on how warm the boards are on the day you install them. Cold install, largest gap in the range. Hot install, smallest. Side gaps are set by the hidden clip itself. Confirm every figure against Tarimatec's current installation manual for your board length and profile.

If you have landed here you are probably either about to build, or looking at a deck that has done something it should not, a board that has lifted at one end or a run that has crept tight against a wall since spring. Both problems live in the same place, which is the space between things.

We supply the Tarimatec decking range across Canada and we work alongside the crews who install it, so this is the conversation we have most often before a build. Here is the mechanism, the rule that governs the numbers, and the failure we see repeated every single spring.

Why the gap exists at all

Materials change size with temperature. A deck board that is warm is longer than the same board when it is cold, and across a full run of boards those small changes add up into something a screw and a joist cannot argue with.

The direction of that logic is what people get backwards, so hold on to this one sentence: a board is at its longest when it is hot. Everything else follows. A deck installed on a hot July afternoon has boards that are already close to their maximum length, so they have relatively little growing left to do and need the least room at the ends. A deck installed on a cold morning has boards at their shortest, with an entire summer of expansion still ahead of them, so they need the most.

Get that backwards and the deck tells you within six months. Boards with nowhere to go push against each other and against whatever is fixed at the end of the run. Sometimes that shows up as a run that visibly bows, sometimes as clips working loose, sometimes as a cracked board end where the pressure concentrated on one point. None of those are material failures. They are gap failures.

The rule, by install temperature

This is installation guidance derived from Tarimatec's installation manual, presented as a rule rather than as a set of millimetres. The actual figures depend on your board length and profile, and we are not going to publish numbers here that might not match the manual in front of your installer.

Board temperature at install	End-to-end gap	Board-to-fixed-object gap
Below 5 degrees Celsius, a cold install	Leave the largest gap in the published range. The boards are at their shortest and have the most expansion ahead of them	Largest in the range
5 to 15 degrees Celsius	Mid-range	Mid-range
Above 25 degrees Celsius, a hot install	Smallest gap in the published range. The boards are already near their longest	Smallest in the range

Two practical notes. Measure the boards, not the weather. A bundle that came off a cold trailer an hour ago is not at ambient temperature yet, and neither is a stack of dark boards that has been sitting in direct sun since nine in the morning. Let them settle on site first. And check the manual, every time, for the exact board length and profile you are installing, alongside our installation guidance.

A builder framing in Winnipeg in April

Picture the situation exactly. It is the second week of April, the frost is barely out, the crew is on site because the client wants the deck for May long weekend, and it is four degrees at eight in the morning. Every board on that pile is at the short end of its life.

That is the deck that most needs the largest gaps in the range, and it is also the deck where the gaps feel wrong to leave. They look generous. The client looks at them and asks why the deck is not tight. And then July arrives, thirty degrees with sun on the surface all afternoon, and every one of those gaps closes up to something that looks exactly right.

The temptation on a cold build is to tighten things up so the deck looks finished today. Resist it, and tell the client now rather than in July. A deck that looks slightly open in April and correct in July is a deck built properly. A deck that looks perfect in April is a deck that will be under compression all summer.

Side gaps are the clip's job, not yours

The spacing between boards along their length is a different question from the end gap, and on a hidden-clip system it is not a judgement call.

Tarimatec uses hidden clips in AISI 304L or 316L stainless at 25 clips per square metre. The clip body physically sets the spacing between adjacent boards as it seats, which means the side gap is consistent across the deck as long as every clip is seated properly. That is a genuine advantage over face screws, where side spacing depends on whoever is holding the spacer that morning. It also leaves no fastener head on the surface to catch a shovel in January, which our post on composite decking in a Canadian winter covers. Your responsibility on side gaps is quality of seating, not measurement, so check clips as you go rather than at the end of the run.

The substructure that the gaps depend on

Gapping only works if the frame under it is doing its job. Tarimatec specifies aluminium joists in EN AW 6005 T6 at 350 mm spacing, and both halves of that specification matter.

The spacing controls how the board is supported between fixings, and widening it because the joists happen to fall on a different centre is not a saving, it is a decision to change the way the board carries load. The aluminium matters because the substructure moves with temperature too, and an aluminium frame behaves predictably in a climate that swings as hard as ours, without the moisture-driven movement of a timber frame under a synthetic surface.

Then there is air. A deck is a system that has to dry out, and the gaps between boards are part of how it does that. Keep the space underneath ventilated, keep drainage running away from the structure, and do not seal the perimeter with solid skirting because it looks tidy. Blocked ventilation and standing meltwater cause more trouble over a decade than the boards or the fixings ever will. For the full build sequence, our guide on how to install composite decking in Canada walks through it in order.

The failure we see every spring

One mistake accounts for most of the calls, and it is always the same one: boards butted tight against something that will not move.

A wall. A post. A column. A stair stringer. A door threshold. The board end sits flush against it because flush looks like craftsmanship, and for four months it is. Then the boards expand into something that cannot yield, and the pressure has to go somewhere, so it goes into a bow in the run or a crack at the end of a board.

The rule to carry onto site is simple. Every fixed object is a board end. It needs the same clearance as a board end, sized to the temperature on install day, and if a cold install means the gap looks generous against the house wall, that is the gap doing its job. Trim, fascia and edge detail exist to make that clearance disappear visually without closing it physically.

Frequently asked questions

Why do composite deck boards need expansion gaps?

Because boards change length with temperature. Every decking material does it, and a deck built with no room to move will push against whatever is holding it, which is how you get buckled boards, popped clips and cracked ends. The gap is not sloppy workmanship, it is the part of the design that lets the deck survive a Canadian year.

What determines the size of the gap?

The temperature of the boards themselves, not the forecast. Boards are at their longest when hot and their shortest when cold, so a cold-day installation has the most expansion still to come and needs the largest gap in the range. A hot-day installation needs the smallest. Let the boards sit on site and reach ambient temperature before you measure anything.

What gap should I leave on a cold-weather install?

The largest gap in the manufacturer's published range, both end to end and against any fixed object. Those boards will be considerably longer in July than they are on the day you cut them, and the space you leave in April is the only room they will ever have.

How do I set the side gap between boards?

You do not set it by eye. Tarimatec uses hidden clips in AISI 304L or 316L stainless at 25 clips per square metre, and the clip body sets the side spacing between boards automatically. Your job is to seat every clip correctly rather than to measure the side gap, which is one of the practical arguments for a hidden-clip system.

What is the most common expansion mistake?

Butting boards tight against a wall, a post, a column or a stair stringer, usually during a cold-weather build when the boards are at their shortest. The deck looks immaculate in February and pushes hard against that wall in July. Treat every fixed object as a place that needs the same clearance as a board end.

Does the substructure affect any of this?

Tarimatec specifies aluminium joists in EN AW 6005 T6 at 350 mm spacing, and the gaps between boards are part of how the deck breathes. Keep air moving underneath and do not seal the perimeter with solid skirting, because trapped damp air and blocked drainage cause more problems over a decade than either the boards or the fixings will.

If you are pricing a build, get the gapping conversation into the quote rather than into the callback. Request a project quote in CAD and we will talk through board lengths, fixed objects and substructure with your installer, or order decking samples first, which are paid and ship from Vaughan, Ontario in 5 to 10 business days. Boards are import-to-order on an 8 to 12 week lead time, so a May deck is a February order. And if colour is still open, our post on whether composite decking gets hot is worth reading before you settle on a dark board.