Composite decking is increasingly popular as an alternative to traditional wood. This is due to its low-maintenance requirements—no treating or staining needed. However, like any building material, it can still expand and contract. Understanding when and why this happens will help you create a well-constructed deck that lasts you well into the future.

Warm teak composite decking with large Wendy house and raised wooden vegetable bed.
Our Warm Teak Brushed composite decking blends beautifully with the wooden elements of this design by Gardens of the Future. DesignBoard's clipping system ensure regular 4mm gaps along the long edges of the boards, allowing easy drainage.

Composite decking is a term that describes a multitude of options made from a variety of materials. London Stone offers three options:

  • DesignBoard is a blend 50% PVC and 50% rice husk fibres.
  • Grooved and Brushed replaces rice husks with bamboo fibres.
  • Millboard is blend of minerals and polymer resins with a Lastane finish.

All these are more resistant to the elements and less prone to warping and splitting than natural wood.

Why does composite decking expand and contract?

Temperature changes

Composite decking will expand as it gets hotter and contract as it cools. Space must be left for decking expansion, or boards may warp, crack or flake as they are forced up against each other. When the boards contract with a fall in temperature, uneven gaps may then appear.

DesignBoard and Brushed and Grooved will not expand widthways, but do expand along their length. A gap of 4mm should always be left between butt ends of both decking and fascia boards to allow for temperature fluctuations.

The construction of Millboard makes it particularly stable, with an expansion of a mere 0.2%. It should have a 1mm gap inserted between butt ends and 4mm between sides of boards.

Smoked Oak Millboard composite decking overlooks lawn and beds.
As for all Millboard boards, composite decking expansion is minimal, thanks to its resin-based formula. Here David Keegan Garden Design uses the Smoked Oak.

Moisture and humidity

Depending on their water absorbency, boards swell and expand when exposed to moisture, while dry conditions can cause them to shrink and contract. Even our DesignBoard and Brushed and Grooved ranges, with very low water-absorption levels of 0.48-0.51% will swell slightly. Millboard has 0% water absorption and should remain unaffected.

Minimise the effects of standing water on composite decking expansion by ensuring the installation has proper drainage.

It’s also very important to follow the installation guides, which give clear information on where gaps should be included, and their minimum width. 

Charcoal composite decking partly covers square pond in front of paved area with modular furniture.
The Charcoal DesignBoard composite decking keeps feet dry, raised above the water, in this bright, modern design by Limebok Landscaping.

Quality

Just how much any composite decking expands or contracts depends on the type and quality of its components. Low-quality decking that is not properly installed or maintained is more likely to warp, crack, and expand and contract excessively.

So long as it's correctly installed according to the supplied instructions, high-quality composite decking, like DesignBoard - so good for contemporary designs - and Millboard, will provide a long-lasting alternative to wood. Minimal composite decking expansion may occur but with correct installation it is no problem and the decking will enhance your outdoor space, retain its good looks and require minimum attention.

Browse our decking options for design ideas.

Post updated: November 2024