Expansion gaps in composite decking: why the end gap matters most

A composite board grows and shrinks along its length as it warms and cools. Across its width the same rate adds up to far less, because a deck board is dozens of times longer than it is wide. The side gap between boards is set by the clip. The gap at the end of each board, at butt joins and against walls, is the one that moves, and it is the one you set by hand. Get it right and nobody notices the deck moving. Get it wrong and the boards push against the wall in summer or open up wider than you set them in winter.

How much does composite decking expand?

The makers publish the figures, and they are bigger than you might expect from something that looks like timber. One Australian manual gives roughly 1 to 1.5 mm of movement per metre of board, and up to 8 mm across a full 5.4 m length between a cold morning and full sun on a hot day¹. Another gives a coefficient of 0.04 mm per metre per degree, which is the same order: a 5 m board that swings 40 degrees moves about 8 mm⁴. A third puts its own board at about 3 mm over a 40-degree swing on a 5 m length, which is less³.

Those are movements in the board, and the board's temperature is not the air temperature. One maker says its boards sitting in full sun on a warm to hot day run about 20 to 25 degrees warmer than the air². A deck fixed on a mild day in the shade will spend its summer afternoons far hotter than the day it went down, and every board on it will be a few millimetres longer. My guide to whether composite decking gets hot covers the heat side.

PVC boards follow their own tables. One maker's table gives its plain PVC boards a coefficient of 0.070 mm per metre per degree, against 0.040 to 0.045 for its polyethylene composite boards, and puts its glass-fibre reinforced PVC boards at half the plain PVC figure⁸. If you are laying PVC, read its own guide rather than assuming the composite numbers carry over. My guide to what PVC decking is and who it suits has more on how it behaves.

Why the end gap depends on the day

The gap you leave is not a fixed number. It is the room the board needs to grow from today's length to its hottest length, and one guide's formula works it out from exactly that. Most of the guides give the gap against the temperature on the day for that reason.

The tables follow a consistent pattern. One manual's table gives a full-length board 4 mm at each end when it is fixed at 10 degrees, falling to 1 mm at 40 degrees¹. Another switches from a 3 mm gap to a 5 mm gap once the installation temperature falls below about 4.5 degrees⁵. A third tells you to halve its table value when the board has a gap at both ends rather than one, because the board shares its growth between the two⁴.

The line I'd underline in one manual is about touch. If the board feels warm or has been sitting in full sun, it is already partly expanded, and the manual says to reduce the gap¹. A board fixed on a cold morning with a tight gap has already contracted, and it will expand into a gap that is not there. One maker says to fix boards when the air is under 25 degrees and the boards are out of full sun³. Another gives an ideal range of 5 to 25 degrees at ground level, with no installs in frost or heat⁴. Leave the packs in the shade and open them as you go.

Where the gaps go

• Board ends at a butt join. Both ends sit on their own joist, with a sister joist added beside the original so each end has timber to fix into, and the gap between the ends comes from the table. One manual multiplies its table gap by one and a half at a butt join¹, and another sets a minimum of 2 mm².
• Board ends at a wall, post or house. The guides ask for more here than at a butt join, because the wall will not give. One guide's minimum is 6 mm², and a second guide asks for 10 mm⁷.
• The side gap. The clip sets it, at 3 mm or 6 mm in one system¹ and 3 mm or 5.5 mm in another⁴. One guide explains the side gap in terms of dirt and safety: under about 4 mm traps dirt and stops water draining, and much over 6 mm can catch high heels and keys⁷.
• Mitres on a picture frame. A mitre opens on both faces as the boards shrink, and one manual tells you to reduce the table gap at a mitre by 2 mm to make that less likely¹. The framing for that is in picture-frame borders on a composite deck.
• Fascia boards. A fascia runs along the deck edge and moves like any other board. One guide has it fixed to a rim joist through slightly oversized or routed holes, never straight onto the joists or the decking⁴.

Butt joins are the weak spot. One maker advises against butt joins on any size of deck and recommends breaker boards once a deck is longer than one board¹. Another recommends a breaker board whenever there is more than one butt join⁴. A breaker board turns many end gaps into two, and both are against a board you can see and control.

What owners report when the gaps are wrong

Movement at the board ends is one of the complaints in my guide to composite decking problems and disadvantages, and the forum and review accounts show how it plays out.

One Whirlpool account shows the extreme. A member, kensvay, described in April 2015 repairing a large area of composite at a school overseas where the boards had gone down with the wrong clips and no expansion gaps. Boards were "buckling at some joints up to 400mm high", and the ends had pushed through steel balustrades¹². The movement kensvay reported, about 4 mm a day on a 3 m board, is in line with the 1 to 1.5 mm per metre one Australian manual says to expect¹². The boards were moving normally. The missing gaps did the damage.

Milder versions turn up in owner reviews. A NewTechWood owner called Warren wrote on ProductReview in January 2026 that "the movement between joints in length is incedible", in a review that also complains of static and fading on a deck laid a few years earlier⁹.

A ModWood owner in New Zealand wrote on ProductReview in March 2023 that on the evening the builders finished, some of the butt-join gaps were far wider than the instructions described¹¹. The spacers were loose and the boards had slid along the clips before the adhesive set, and the owner pushed them back and wedged the ends. That one was not seasonal movement. A gap is only as good as whatever holds the board until the fixings take hold.

The account I'd want anyone lifting boards to read is from an Ekodeck owner in south-east Queensland who wrote on ProductReview in April 2025. After lifting a section of boards by the front door to replace a soft support underneath, the owner could not get them back in the same order, because the board that used to fit against the door no longer fitted the space left¹⁰. The owner concluded the boards had expanded, re-laid that section starting at the door and extended the frame for an extra half board. That sounds like movement across the boards rather than along them, and it shows how small changes add up over a whole deck. If you ever lift composite boards, plan to re-gap them rather than drop them back where they were.

A Whirlpool member wrote in October 2016 that a carpenter had told them the boards expand so much in the sun that 5 mm gaps were being left at the ends¹³. Another, HouseGuy, wrote in May 2017 of a partly covered composite deck with no problems, adding that the install rules deal with expansion if you read them¹⁴.

Checks you can do yourself

Walk the deck on the hottest afternoon of the first summer and look along each row of board ends. The butt-join gaps should be nearly closed but not touching, and no board should be touching the wall. Then look again on the coldest morning. The same gaps will be open. A gap much wider than the others probably belongs to a board that was fixed hot. On a clipped deck, a board that has crept along the joists shows as one end gap closed and the other open. One manual's single locking clip per board, placed nearest its centre, is there to stop that.

Before you fix a board

Read the temperature table for your boards and keep a thermometer in the shade on the frame, not in the sun. Cut both ends square and fresh so the gap is the gap. One guide asks for a clean cut of at least 5 mm off each end⁶, and an owner on ProductReview noted that supplied lengths can vary by up to 5 mm¹⁰. Use spacers at every end, and check the clips are setting the side gap you bought. Joist centres and fixings are covered in joist spacing for composite decking and hidden clips vs face screws, and the order of work in how to install composite decking. The deck designer will show where the butt joins fall for the board length you choose, so you can plan a breaker board instead.