Composite Decking Expansion Gaps: Why Board Spacing Matters
- Joel Livesey
- Jul 19
- 8 min read
Of all the things that go wrong with a composite deck, the one we get called out to most often has nothing to do with the boards themselves. It's the gaps between them — or rather, the gaps that were never left. Composite decking expansion gaps are the least glamorous part of an installation and the single most common reason a deck that looked immaculate in April is lifting at the joints by July.
Composite boards move. Some more than others, but enough to matter. On a hot afternoon a board grows; on a cold night it shrinks back. If there's nowhere for that movement to go, something has to give — and usually it's the boards, lifting at the ends, waving down the middle of a run, or splitting around a fixing.
Here's what's actually happening inside the material, the gaps we leave on every job and why, and how to tell whether the deck you already have — or the quote sitting in your inbox — accounts for any of it.
Why Composite Boards Move in the First Place
Most wood-plastic composite decking is a mix of recycled wood fibre and HDPE plastic. The plastic is the part that matters here. Like every thermoplastic, it expands as it warms and contracts as it cools, and that's true of boards from Trex and NewTechWood alike.
Just how much movement we're talking about is not a guess — the manufacturers publish it. NewTechWood's own expansion table shows that a 3.6-metre board fitted at 20°C wants a gap of around 4.4mm, but if that same board is fitted on a cold day at 0°C, the gap needs to be nearer 7.3mm to leave room for it to grow. Put twenty boards side by side, butt them tight, and put a brick wall at one end, and all of that movement has only one direction left to go: up.
Millboard is an exception, and it's worth understanding why. It's built on a polyurethane resin mineral core with no wood fibre, so it's far more dimensionally stable than a wood-plastic composite — Millboard rates its acceptable movement at just 0.2%. That stability is one of the quiet things you're paying for, and it's why Millboard's gapping numbers are so much tighter than the others (more on that below). But no board on the market is completely static.
Two things are worth knowing. First, colour changes the numbers — a dark charcoal board in full sun runs considerably hotter than a light grey one and expands accordingly. Second, it's board temperature, not air temperature, that drives the movement, which is why the weather on the day of installation matters so much.
The Gaps We Leave on Every Job
This is where the detail lives, and where the three main systems we install genuinely differ. We work to the published spec for whichever board is going down, because that spec is also what the warranty is written against.
Between the long edges of the boards
This is the gap that lets water and debris drain through and gives the board room to grow across its width. Millboard calls for 4mm here. Trex asks for a minimum of 5mm side to side (rising to 10mm for docks and heavily shaded, tree-covered spots where drainage and airflow matter more). NewTechWood's standard clips set a 5mm gap for you, but they also offer a mini gap clip that brings the joint down to just 1mm for a tighter, near-seamless look — though a gap that narrow gives up some drainage and airflow, so it's a choice worth talking through. On a hidden-fastener system the clips often set this gap for you automatically, which is one of the reasons we use them.
Where boards meet end to end (butt joints).
This is the one that catches people out, because the numbers move around the most. Wherever we can, we design butt joints out altogether. The first move is to orientate the boards so they run the full length of the deck, leaving no end joint at all. Where the deck is too long for that, we look to introduce breaker boards — a board run across the joint line that gives each length a clean, deliberate edge to meet, rather than leaving a raw butt joint mid-run. Only when neither is practical do we fall back on a butt joint, and then the gap has to be right. With a stable board like Millboard, the end gap is tiny — just 1mm — because the board barely changes length. With a wood-plastic composite it's the opposite: Trex specifies 3mm at end joints above 5°C and 5mm below 5°C, and NewTechWood sizes the end gap straight off its temperature table depending on board length and the temperature on the day. Getting this wrong — closing it up on a composite board — is the fastest way to buckle a run.
Against walls, posts and anything fixed
Anything immovable is a hard stop, so this gap is generous and usually hidden behind a fascia or trim. Millboard leaves 10mm to any solid surface. NewTechWood allows up to 20mm at the barrier. Trex calls for 6mm to 13mm depending on the temperature at installation. A board that expands into solid brickwork has exactly one option available to it, and that's to lift off the joist.
The takeaway isn't the individual numbers — it's that a good installer knows they're different for each board and fits to the right one, rather than to a single rule of thumb.
What Actually Goes Wrong When the Gaps Are Wrong
Too tight is the failure we see most, and it's almost always at the butt joints on a wood-plastic composite. The ends creep up off the joist and you get a raised lip you can catch a toe on. Sometimes it shows as a slow wave down a long run, most visible in low evening light. On screwed-down boards it can split the board around the fixing, because the board is trying to move and the screw won't let it.
Too wide is less dramatic but still a problem. Heels and chair legs catch in an oversized gap, leaves and grit collect in it, and a 10mm joint simply looks wrong next to a 4mm one three boards over. Trex is explicit that no gap should ever exceed 13mm. Inconsistency is what the eye picks up, more than the size itself.
Then there's the warranty, which is the part homeowners rarely think about until they need it. Almost every composite manufacturer excludes cover where the board wasn't installed to their spacing specification, and installation error is a leading reason claims get turned down. A deck fitted hard up against a wall isn't just at risk of buckling in the next hot spell. It's uninsured.
The Weather on the Day of Install Changes the Gap
This is the bit that separates a good installer from a mediocre one. If we're laying boards on a cold February morning, the material is already contracted and will only grow from there, so the gaps go in at the generous end of the range. Fit the same deck during a heatwave and the boards are already near their maximum, so the gaps come in tighter — or you end up with permanent chasms in winter. Both NewTechWood and Trex build this directly into their instructions, with different figures above and below roughly 5°C.
Boards should also be given time to acclimatise on site before anything is fixed down. NewTechWood and Trex both recommend at least a couple of days laid out on the frame; Millboard, being far more stable, suggests laying out around a day before to blend colour and settle to temperature. It matters most when the boards have come straight off a cold lorry into a sunny garden — fixing a stack of cold boards down on a warm day is a reliable way to build a problem in from the start.
A Word on NewTechWood's Locking Clips
If your deck is a NewTechWood system, there's one more piece of the puzzle that quietly does a lot of the work: the clips.
NewTechWood boards are fixed with hidden clips that sit in the groove on the side of each board and screw down to the joist. Most of these are what they call T-clips — they hold the board down and set the gap, but they deliberately allow the board to slide a little as it expands and contracts. That sliding is the whole point. The board needs to be free to grow and shrink along its length, and the T-clips let it.
But if every clip let the board float, the whole deck could gradually creep in one direction over the seasons. So NewTechWood uses one special locking clip per board, fitted in the middle. This one pins the board firmly at its centre point. Everything either side of it is still free to move — which means the board expands evenly outwards in both directions from that fixed middle point, rather than shunting to one end.
It's a clever, simple idea, and it's why we follow the system to the letter: one locking clip, dead centre, on every board, with sliding clips everywhere else. Fit a locking clip in the wrong place — or fit two — and you defeat the entire expansion strategy, which brings you right back to lifting ends and buckled runs. Done properly, you never see any of it. The deck just stays flat, year after year.
This is also true of NewTechWood's mini gap clips mentioned earlier — the ones that give the tighter 1mm joint. They work on exactly the same principle, with a sliding clip along the board and one locking clip per board at the centre, so even with a narrower gap the board still expands evenly from a fixed middle point. The tight finish only holds up because the locking clip is still doing its job underneath.
How to Tell Whether Your Deck Was Gapped Properly
You do not need tools for a first look. Crouch down and sight along the length of a run at eye level. A properly fitted deck reads as flat and the joints read as consistent. Any board end sitting proud of its neighbour is telling you something.
Then check the perimeter. Look at where the boards meet the house wall or a fixed step. If a full-length board is butted straight into brickwork with no visible allowance at all, that is a problem waiting for the next warm spell, even if the deck looks perfect today.
Finally, look underneath if you can. Movement problems and subframe problems often travel together, because a frame with joists spaced too far apart lets boards flex as well as expand. We went into that in detail in our guide to what sits under your decking, and it is worth a read if you are having any of these symptoms.
Getting Composite Decking Expansion Gaps Right From the Start
None of this is difficult. It is just detail, applied consistently, on a job where nobody will ever see the thing you got right. Composite decking expansion gaps only become visible when they are missing, which is exactly why they are the first thing to get skipped when a deck is being fitted quickly or cheaply.
If your deck is already showing lifted ends or a wave in a run, it is usually fixable, and it does not normally mean starting again. Boards come up individually. If you are still at the planning stage, this is one of the things worth asking any installer about directly before you sign anything. Have a look through our completed projects to see how we detail it, or get in touch and we will talk through what you have got.
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