Does Composite Decking Warp?

Choosing decking material can be a challenge. With so many factors to consider, you must do your homework.

Your budget will play a big part in the type of decking you eventually choose, but looking at the other benefits different materials offer will help you make the best decision.

Composite decking is a popular option in the UK because it is long-lasting and durable. Composite deck boards require significantly less upkeep than natural wood decking, and won’t typically split, splinter, or warp when installed correctly.

This guide will look at the main causes of warping in a new composite deck, how to minimise the risk, and how to resolve the warping.




The Causes of Warping in Composite Decks

Composite deck boards are made from reclaimed wood fibres and recycled plastic. The synthetic material in composite deck boards makes them more durable, and capped deck boards offer even more protection.

Natural wood decking will typically warp if it is exposed to moisture for a prolonged period. Composite decking does not suffer from this.

Composite decking won’t typically warp, but there are a few occasions where this can happen, including;


Incorrect Spacing

For many reasons, spacing between deck boards is an essential part of the installation process.

Boards placed against one another during installation will not have as effective a drainage system as spaced boards. Drainage is essential for all composite decking to minimise the risk of water pooling and creating a slippy surface.

Poor drainage can also promote the growth of mould and mildew, which can be slippy and stain your composite decking.

Spacing is also crucial for ventilation which also helps the surface of the deck boards to dry out properly.

A sealed deck also causes a humid atmosphere beneath the composite decking, compromising the subframe and joists. This can cause it to deteriorate more quickly and lead to an insecure platform.

Spacing is also essential to allow the natural thermal expansion and contraction of composite decking. If the deck boards aren’t correctly spaced, they can push against each other. With nowhere to go, the composite decking warps.

There should also be space allowed when placing deck boards against a wall for the same reason. The expansion will push the deck boards against the property walls, resulting in warping.


Screwing Composite Decking Boards in Place

Screwing the boards in place is an option that many people consider when installing composite decking without understanding the negative ramifications this action can have.

It is often seen as a convenient, cost-effective, and time-saving way to install deck boards.

The issue with screwing composite decking boards in place relates to the thermal expansion and contraction of the boards.

Screwing composite boards in place can keep them secure, but as boards expand, they can place unnecessary pressure around the screw. This will result in warped composite decking.


How to Prevent Composite Decking From Warping

The possibility of warped composite decking is relatively easy to avoid, and experienced professionals should advise you on the best ways to minimise the risk.

The following examples are the simplest solutions to help you avoid warped composite decking boards.


Space Appropriately

Spacing your boards properly is essential for drainage and ventilation and will also stop your composite deck boards from warping.

The following thermal expansion gap guide should be considered when installing Ecoscape composite deck boards near adjacent structures in the UK.

  • Temperatures of -10°C will require a 2.4 mm gap per 1 m length, a 5.9 mm gap per 2.4 m length and a 6.7 mm gap per 2.8 m length.
  • Temperatures of -5°C will require a 2.2 mm gap per 1 m length, a 5.4 mm gap per 2.44 m length, and a 6.2 mm gap per 2.8 m length.
  • Temperatures of 0°C will require a 2 mm gap per 1 m length, 4.9 mm length per 2.44 m length, and 5.6 mm gap per 2.8 m length.
  • Temperatures of 5°C will require a 1.8 gap per 1 m length, a 4.4 mm gap per 2.44 m length, and a 5 mm gap per 2.8 m length.
  • Temperatures of 10°C will require a 1.6 mm gap per 1 m length, 3.9 mm gap per 2.44 m length, and 4.5 mm gap per 2.8 m length.
  • Temperatures of 15°C will require a 1.4 mm gap per 1 m length, 3.4 mm gap per 2.44 m length and a 3.9 mm gap per 2.8 m length.
  • Temperatures of 20°C will require a 1.2 mm gap per 1 m length, a 2.9 mm gap per 2.44 m length and a 3.4 mm gap per 2.8 m length.
  • Temperatures of 25°C will require a 1mm gap per 1 m length, a 2.4 mm gap per 2.44 m length and a 2.8 mm gap per 2.8 m length.
  • Temperatures of 30°C will require a 0.8 mm gap per 1 m length, a 2 mm gap per 2.44 m length and a 2.2 mm gap per 2.8 m length.

A full table, up to 40°C in temperature and 5.4 meters in length, is available by visiting the Ecoscape UK Composite Decking Installation Guide.


Use Hidden Fasteners

An easy way to space your composite decking boards and avoid the issues caused by using screws during the installation process is by using hidden fasteners.

Hidden fasteners provide a secure and aesthetically pleasing way to install composite decking without risking warping.

Hidden fasteners are clips that can be screwed directly into joists and clipped onto decking boards. Suppliers can provide fasteners and clips designed specifically for their product and guarantee a secure fit.

Rather than screwing through the decking board, they hold it in place, allowing for expansion and contraction. The boards can move without being limited by screws.

Screwing directly through a board gives it nowhere to go during expansion. This causes other parts of the board to move more freely compared to the area secured by the screw.

T-clips also have spacers that allow your boards to be fitted securely, allowing space for expansion and promoting good drainage and ventilation.


How to Fix Warped Composite Decking

Depending on the level of warping your decking has suffered, it may be possible to carry out repairs by replacing individual sections.

If the warping is severe, contacting a professional installer for further advice may be necessary.

Sometimes, the decking may be beyond repair and should be replaced. This is why it is important to carry out the installation process correctly.

You must first check the full deck to identify the affected areas.

Measure the area, and take the trim board depth into account. The affected deck board should be cut and pried free of the decking.

You should then be able to put your replacement board in. If you are using hidden fasteners, you need access below the deck to secure it.

Clamping a warped board against a straight wooden plank is another option and can save your existing deck board. Another option is to take out the warped composite deck board and lay it flat on hard standing, with the bow touching the floor, the deck board may go back to its original shape.

This is a more complicated process and can be done by removing the affected board carefully or screwing a flattening board on top.



Why does composite decking sag?

The most common reason that composite decking sags is poor joist spacing.

A wood deck is considerably lighter than composite decking. Because of this, joists can be spaced further apart. If the same measurements are used when building a subframe for composite decking, it can result in sagging.

Spacing the joists closer together and double joisting the end of each composite plank will provide the support necessary for composite materials.

It is also very importing to make sure the joist is supported correctly. Our plastic 50x50mm joists can span 600mm, aluminium 40x40mm joist can span 600mm and the 125x50mm plastic bearers span 1500mm. If you are suing C16 or C24 tantalised timber please refer to a span specification table. Joists that are not supported correctly will sag causing the composite decking board to fail.


How long should composite decking last?

Properly maintained composite decking can last anywhere up to 25 to 30 years.

Ecoscape Clarity composite decking boards have a 20-year warranty, while the Forma range offers a 25-year warranty.

For warranties to stand, the correct installation process must be followed, and the accessories provided must be used.

Other terms and conditions, such as the products used to clean your composite decking, can also affect the validity of your warranty.


Can you power wash composite decking?

Yes, you can power wash composite decking. You should check the supplier guidelines for maximum pressure and hold the spray safely away from the decking.

Rather than sweeping all over the deck, you should go with the grain down each deck board individually; this will minimise the risk of marks.


What is the best subframe material for composite decking?

Timber subframes are relatively low-priced options but don’t typically last as long as more durable materials.

Aluminium is a great, long-lasting, strong option, which can offer a similar performance to composite decking.

Plastic subframes are competitively priced and are easy to work with. Plastic does last a long time and won’t be affected by moisture. Plastic joists are made from 100% recycled plastic which would otherwise end up as landfill.


Does composite decking fade in the sun?

Composite decking won’t warp purely because of the sun. However, it can fade depending on the type you choose.

Not all composite is the same. Different suppliers will have different qualities with individual benefits.

Ecoscape’s Forma range is capped with a durable outer layer that improves performance, increases scratch and stain resistance, and comes with a colour guarantee to protect against fading.


Final Thoughts

Warped boards on a composite deck are nowhere near as common as timber decking, but this does not mean it can’t happen.

Prevention is the best course when dealing with a warped composite deck, and it is relatively simple. Ensuring the installation guide provided by the supplier is followed should help with this.

You must use the components and accessories provided by the supplier to adhere to the terms and conditions of the warranty. You may not be covered if you suffer warped deck boards and have used alternative methods and equipment.

Properly spaced decking and avoiding screwing composite deck boards directly to the joists will considerably minimise the risk of issues. Composite decking can warp if it does not allow the space for thermal expansion and contraction.

Gary is the founder and CEO of Ecoscape UK. Gary graduated from Liverpool JM University in 2007 with a 2:1 in Economics. A passion for innovation, design and sustainability, Gary has put to market numerous wpc products/systems, some of which have UK and European registered designs. Outside of work Gary enjoys spending time with his family, the great outdoors and watching Manchester United.