Composite decking expansion gaps are a crucial aspect of the installation process, playing a key role in the long-term performance and durability of your composite decking. These gaps are spaces left between boards during installation, allowing for natural expansion and contraction of the material due to temperature changes and moisture fluctuations.

One of the primary reasons for incorporating expansion gaps in composite decking is to accommodate thermal expansion. Unlike traditional wood decking, composite materials are more susceptible to temperature changes. As temperatures rise, composite boards expand, and as they cool down, they contract. Without proper gaps, the boards may warp, buckle, or become damaged over time. This is due to the composition of the board, with plastic fibres that are more susceptible to temperature changes.

Moisture is another factor that can influence the size of expansion gaps. Composite decking is resistant to moisture, but it is not entirely impervious. Changes in humidity levels can cause the material to swell or shrink. By leaving appropriate gaps, the decking can expand and contract without being damaged, preventing issues like warping, bowing, or cracking.

What Expansion Gaps Do I Need?

The amount of expansion a composite decking may experience varies depending on the specific material and the temperature fluctuations in the local climate. Manufacturers typically provide guidelines regarding the recommended gap size for their products. At Composite Warehouse, we suggest a 6 mm gap end to end of the boards and a 25 mm gap away from fixed objects such as your property or exterior walls. This allows for enough room for expansion without compromising the structural integrity of the decking. Our composite decking hidden fasteners provide a 3 mm gap, and we also have a decking clip that provides a larger gap of 4-6mm.

When installing your own composite decking, you should pay special attention to the ends of the composite boards when creating expansion gaps. It is recommended to leave a gap at the butt joints as well as the sides of the deck. Neglecting these areas can result in pressure build-up, leading to potential damage during expansion. Always check with your supplier what gaps are required to prevent damage and stick within your warranty guidelines. When it comes to thermal expansion, it is also recommended to allow your boards to climatise for at least 24 hours outside before fitting them.

Gap to add

Gaps Will also Allow for Ventilation and Drainage.

Proper spacing also facilitates drainage and ventilation beneath your decking. Adequate gaps between boards enable rainwater to flow through, reducing the risk of water pooling and potential moisture-related problems such as mould or rot. Additionally, ventilation is crucial for the longevity of the decking structure, as it helps prevent the accumulation of moisture beneath the boards. Of course, when it comes to drainage, there are other necessary installation techniques, such as a gradient for runoff.

While expansion gaps are vital, it’s essential to maintain a balance. Too small of a gap may not provide sufficient room for expansion, leading to problems over time. On the other hand, overly large gaps may be aesthetically unpleasing and could create safety concerns. Following the manufacturer’s guidelines and considering the local climate conditions are crucial for achieving the right balance.


In conclusion, composite decking expansion gaps are a critical element in the installation process, ensuring the long-term performance and durability of your decking. By allowing for natural expansion and contraction due to temperature changes and moisture fluctuations, these gaps prevent issues such as warping, buckling, and cracking. Proper spacing also aids in drainage, ventilation, and overall decking stability. Following manufacturer guidelines and considering local climate conditions are key to achieving the right balance for expansion gaps in composite decking installations.