When choosing between the different types of decking materials, there are many different factors to consider.
How long will the boards last? How much maintenance will they require? Are they susceptible to mould and rot?
These are all important things to think about whilst you make your decision, but there is another important question that can sometimes be overlooked.
How much does composite decking expand and contract?
In this article, we’ll take a look at how much composite decking expands and contracts, and what effect that can have on your decking boards. We’ll also examine the other types of decking material to see how they compare to composite when it comes to expansion and contraction.
What Is Composite Decking?
Composite decking is made using recycled wood fibres, plastic polymers, and a bonding agent. This mixture of composite decking materials is melted through a process of co-extrusion and then passed through a ‘die’ in order to shape it into composite deck boards.
Due to the materials used to make it, composite decking is far more durable than wood decking and is also very resistant to damp, rot, and insects. This gives it great longevity meaning that composite decking boards last a lot longer than wooden decking boards.
As they need to be replaced less often and are made with recycled materials, composite deck boards are also environmentally-friendly. Our composite decking products are made with up to 95% recycled materials and FSC-certified wood.
Composite also requires far less maintenance than traditional wooden boards and doesn’t need to be stained, painted, or sealed. This means that installing a composite deck will save you time and money in the long term.
Why Does Composite Decking Expand and Contract?
Composite decking expands and contracts as temperatures fluctuate. Both plastic and wood flex a certain amount when temperatures rise or drop. Wood expansion and contraction are also affected by how much moisture has been absorbed by the decking boards.
As composite decking boards are very water-resistant, they don’t tend to absorb moisture easily. This means that the main factor affecting contraction and expansion in composite deck boards will always be temperature.
Although uncapped composite boards are resistant to water absorption, capped decking boards are even more so. However, this doesn’t have much of an effect on the rate of expansion and contraction.
The small amount of water that can be absorbed by uncapped boards does not cause them to expand and contract more than capped boards.
What Causes Composite Decking To Expand?
When temperatures rise, composite decking experiences thermal expansion. This causes the boards to expand slightly during warm weather.
Quality composite decking is designed to have enough flexibility to withstand these expansions without the integrity of the boards being compromised. This means that periods of thermal expansion will not reduce the lifespan of composite decking or make it less durable.
Unlike wooden boards, composite deck boards don’t expand when they get wet. This is one of the many benefits of composite as a material when compared to natural wood or pressure-treated wood.
Although thermal expansion shouldn’t be much of an issue, there are some ways of mitigating it. For example, if your boards are a dark colour, they will absorb heat more quickly and expand slightly more than if they were a lighter colour.
What Causes Composite Decking To Contract?
Composite decking contracts when exposed to low temperatures. This is often referred to as shrinkage and is caused by the cells in the wood fibres and plastic pulling closer together.
Contraction occurs within a known tolerance, and composite boards like ours are manufactured to withstand contractions within these tolerances.
As composite decking is water-resistant it doesn’t experience the same rates of contraction as wooden boards do during low temperatures. Wood decking absorbs water which then freezes, causing it to contract much more significantly than composite does.
Does It Matter If Composite Decking Expands and Contracts?
So, we’ve established why and how composite decking expands and contracts. You may now be wondering if this actually matters.
As the rate of contraction and expansion in composite decking materials is fairly minimal when compared to other materials, it may seem like a trivial issue. However, there are still several things that will be affected by the slight contraction and expansion of composite boards.
Safety
One thing that may be a concern due to expansion and contraction is safety. If nails and screws are used to secure decking boards in place, then there is a risk they will be loosened by expansion and shrinkage.
This may cause damage to the boards and could also be a safety concern if the nails or screws are out of place. This may make the boards less secure and could also pose an injury risk.
If the screws and nails are fully intact and secure, this shouldn’t be much of an issue. However, the best way of removing this risk is to use grooved decking boards.
Grooved decking boards don’t require nails or screws to be secured in place. Instead, they are secured using hidden fasteners and clips. These are designed to withstand expansion and contraction and won’t pose any safety risks.
Installation
Another factor that may be affected by contraction and expansion is the installation process. A small amount of shrinkage won’t be an issue, but expansion can make the installation process difficult.
If the boards expand, they become harder to cut to the right size as the measurements will be slightly off. This means that it’s best to install composite boards when temperatures are lower, such as early in the morning.
If temperatures get too high whilst installing the boards, you should pause the process and start again the next morning.
Proper Deck Board Spacing
As composite boards expand and contract, proper deck board spacing is very important. Many people install decking with the boards touching each other as they think it looks more aesthetically pleasing.
However, failure to leave the right amount of space in between them can have several negative consequences.
Contraction
If the boards are too far apart, the gaps will become even bigger when they contract during low temperatures. This can be a safety hazard for people walking across the deck.
Expansion
If the boards are too close together, friction could damage the boards when they expand during high temperatures. In some extreme cases, this could lead to the boards warping, twisting, splitting, cracking or even rising away from the substructure.
The correct amount of space that should be left in between the boards during installation is between 3mm and 6mm. This will ensure that your boards can withstand the pressures of contraction and expansion, which will help to maintain their durability and longevity. Please refer to our composite decking installation guide, on page 34 you will find an expansion and contraction table. Simply pick the correct length of composite deck board and the temperature at the time of installation, the table will suggest a gap to leave between the boards or away from a wall. For example, a 3000mm board installed when its 10 degrees should allow for a 4.8mm gap.
How Does Composite Decking Compare To Other Decking Materials?
Although composite decking is prone to expansion and contraction, it performs better when it comes to this metric than many other types of decking.
How much a particular type of decking expands and contracts may not be one of the first things you consider when choosing decking, but it is important.
Let’s take a look at the three other most popular types of decking, which are wood, UPVC and aluminium, to see how they compare to composite when it comes to contracting and expanding.
Wood
Wood is the decking material that is most affected by contraction and expansion. The main reason for this is that it absorbs a lot of water. The only way of mitigating this is to seal and re-seal wooden boards annually to try and limit their rate of water absorption.
During low temperatures, the water inside wooden boards freezes, causing them to expand. This leads to splintering and cracking. When temperatures are high, the water inside the boards quickly evaporates, which causes them to contract.
This is one of the reasons that wooden decking has such poor longevity. After a few years of hot summers and cold winters, the boards will most likely be splintered, cracked and warped. This means they will either need to be repaired or replaced, which is costly and time-consuming.
If the boards are left in this condition, they will quickly become an eyesore and will also start to be hazardous. Split and splintered wooden decking can result in sharp edges which pose a safety risk.
UPVC
UPVC decking is made of 100% plastic which means it contracts and expands far less than wood decking. However, as with composite boards, UPVC boards will still expand and contract slightly depending on the temperature.
The main method of accounting for this in UPVC decking is the same as it is for composite. The right amount of spacing in between the boards is vital to ensure that they don’t become damaged or unsafe when they expand and contract.
Lighter-coloured boards will absorb less heat during warm weather. Divider or splitter boards can also be placed in between the boards to prevent any friction from occurring when they change size.
Aluminium
Aluminium decking doesn’t experience much contraction or expansion in response to temperature changes. For this reason, it is used in the construction of structures where warping and bending cannot be risked.
Unlike a composite decking board, an aluminium board doesn’t need to have a space between it and the next board to avoid damage. Aluminium decking boards can safely be in contact with each other even during high temperatures.
This is one of the reasons that aluminium decking has such longevity and rarely needs to be replaced. However, there are many other downsides to aluminium as a decking material when compared to composite.
FAQs
What happens if I install composite boards without leaving a gap between them?
If you install composite boards without leaving any gaps between them, they can be damaged when they expand and pose a safety risk when they contract. Improper deck board spacing will also mean that water can’t drain away from the boards properly.
Do I need to leave a gap between a composite deck and my house?
When installing a composite deck, you should leave a gap of between 5mm and 15mm between the edge of the deck and your house. This will allow proper water drainage to occur during wet weather, as well as proper drainage of meltwater from snow.
Do composite deck boards warp?
It’s highly unlikely that composite boards will warp as they are very water-resistant, unlike wood decking. However, they can sag or buckle if the right amount of space isn’t left between them during installation.
How long does a composite deck last?
Generally, composite decks should last between 25 and 30 years. There is no maintenance required in order to maintain their longevity, and the lifespan won’t be affected by the minimal amount of shrinkage and expansion that occurs due to weather conditions.
Final Thoughts
Although composite decking does expand and contract, it shouldn’t be much of an issue as long as the boards are installed properly with the right amount of spacing.
When compared to other decking materials such as wood, composite has a clear advantage due to its water resistance. This means that shrinkage and expansion are fairly limited and won’t reduce the longevity of the boards.
In climates where temperatures fluctuate constantly, such as the UK, this is an important factor to consider. Clearly, composite decking is one of the best choices when it comes to contraction and expansion.
