Composite decking: does it expand and contract?

If you’re a homeowner or business owner looking to upgrade your patio area, composite decking has quickly become the top choice.

It’s low maintenance, weather resistant, and built to last — but one question we hear time and time again is, ‘Does composite decking expand and contract?’.

It’s natural to wonder how your decking will hold up in the British environment, so we’re here to tell you that it does, in fact, expand and contract under certain conditions. Today, we’ll walk through what you can expect from your purchase, how much it actually moves, and why it happens.

If you’re looking for premium composite decking that is resistant to warping, buckling, and fading, then browse our products at WPS Handrails. Choosing the right supplier makes all the difference, and we’re here to provide expert advice and guidance.

What, exactly, is expansion and contraction?

During expansion and contraction, the plastic polymers (usually polyethene or PVC) react to heat.

These plastic polymers expand slightly in response, making the boards grow longer by a few millimetres. While you can’t see this happening with the naked eye, it does affect the overall length of the deck during this process.

As temperatures lower, the plastic pulls closer together, shrinking back to their natural state. Contraction usually happens overnight when the environment is cooler.

Does composite decking expand and contract?

Yes, composite decking does expand and contract. But don’t worry—this is completely normal and actually helps keep the boards in great condition for longer.

Since composite decking is made of wood fibres and recycled plastic, it can withstand harsh weather conditions and handle anything the outdoors throws at it.

However, this does mean it’s sensitive to hot and cold temperatures. When temperatures fluctuate, the composite boards adjust to cope. While this movement is minimal, this process needs to be accounted for when installing the boards to prevent problems down the line.

While composite is made up of wood fibres, it doesn't absorb moisture the same way timber decking does.

Expansion and contraction can’t be avoided, but as long as the composite is installed correctly, they won't cause cracking or warping. Choosing a reputable supplier, like WPS Handrails, or opting for professional installation will help you save time and money.

Factors influencing movement

A few factors can influence how much the composite boards expand and contract.

1. Changes in temperature

As we all know, the UK weather can switch at the drop of a hat. With unpredictable rain one minute and sunshine the next, your decking needs to be able to keep up.

Composite boards deal with these temperature fluctuations by expanding slightly when heat rises and contracting as temperatures plummet. The plastic content in the boards causes this—a good-quality composite should remain flexible for expansion and contraction to avoid becoming weak.

You’ll notice the most shifting during:

• Prolonged exposure to sunlight 

• Really cold weather during winter 

• Abrupt weather changes during the day and night

2. Board colour

You may be thinking, ‘How does the colour of my boards affect expansion and contraction?’, but it matters more than you think.

Darker boards, like charcoal and graphite, absorb more sunlight. What this means is that the boards will heat up more quickly and expand slightly more than lighter coloured boards.

But don’t worry; you don’t have to avoid darker colours. You’ll just need to make sure you space the boards properly, to achieve this, we recommend using spacers to set the gaps for your deck boards during installation to ensure fixing clips are installed consistently and correctly. If they're in direct sunlight, it might also be beneficial to invest in a pergola for some shade.

3. Board type and construction

Not all decking boards are the same—some are solid, while others are hollow, and some are capped with an outer protective layer.

While uncapped boards are resistant to moisture, capped boards offer even more protection, providing excellent protection from water damage (although this doesn’t affect expansion and contraction, it is beneficial for areas prone to pooling).

Solid boards are heavier, so they hold slightly more heat and are less prone to flexing. Hollow boards, on the other hand, are lighter, and movement can occur more quickly.

At WPS Handrails, we offer free samples of our boards, so you can find the best option for you before buying.

4. Composite board spacing

Composite decking will expand and contract along its length primarily due to changes in temperature, therefore adequate spacing needs to be provided in order to preserve the integrity of your deck installation, spacing recommendations can be found in our installation guide which are also summarised below.

Spacing your composite boards correctly will prevent them from pressing together as they expand, which would lead to warping and lifting over time.

While it may look more appealing to place the boards tightly next to each other, butting them together will just cause them to push up and buckle at the ends. So, it may look good for the time being, but your wallet will be paying for it later.

We recommended leaving a 3-6mm gap for side-to-side boards and 3-6mm for end-to-end boards to account for expanding and contracting. This will give your boards enough space to move without colliding.

However, if you’re installing your boards next to a fence or wall, leave an expansion gap of around 6mm to 13mm to prevent any pressure build-up.

Using cross spacers to install your deck boards is recommended to ensure consistent gaps between boards before fixing the clips. This helps mitigate the risk of movement or warping caused by temperature expansion and contraction. The size of the cross spacer should be matched to the width-to-width gap. If the temperature is above 5℃ during installation, use 3mm cross spacers. If it is below 5℃, use 6mm spacers instead.

You can read more about installing composite decking on our blog. 

Secure long-lasting decking with WPS Handrails

At WPS Handrails, we supply premium composite decking that has been tested in real UK gardens and commercial builds. 

With a range of technologically advanced boards to choose from, we have something for every style and budget. You can expect stain and mould resistance, as well as protection from scratching and UV damage. 

Lasting between 15 and 25 years, our composite decking is great for both residential and commercial projects. Click on our options below to get started on your upgrade: 

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