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Composite decking is known for its durability and resistance to rot. However, like any outdoor material, it can warp if installed incorrectly. The good news is that preventing this is straightforward with proper installation techniques.
The main causes of warping are thermal expansion, moisture buildup, and insufficient support. By addressing these factors during installation, you ensure a perfectly flat and stable deck for decades.
Causes of Composite Decking Warping
Thermal Expansion and Contraction
All outdoor materials expand in heat and contract in cold. Composite decking is no different. Without room to move, this natural process can cause boards to buckle.
Trapped Moisture and Poor Ventilation
While composite decking is waterproof, constant moisture trapped underneath can lead to problems. Without airflow, heat and moisture build up, increasing the risk of deformation.
Incorrect Joist Spacing
If the supporting joists are too far apart, the deck boards can sag or warp under weight over time. Proper support is non-negotiable.
How to Prevent Warping During Installation
1. Leave Proper Expansion Gaps
Expansion gaps are the most critical factor. They allow the decking to expand and contract naturally without buckling.
Gap between boards: 3-6 mm
Gap against walls/fixed objects: 10-15 mm
Always use spacers during installation to ensure consistent gaps.
2. Ensure Adequate Under-Deck Ventilation
A ventilated substructure allows heat and moisture to escape.
Install decking on a joist frame, not flat on a solid surface.
Ensure the ground below has a slight slope (1-2°) for drainage.
Do not enclose the sides of the deck completely; allow for air movement.
3. Follow Correct Joist Spacing Guidelines
Joist spacing depends on the type of composite board.
For hollow composite decking: Maximum 400 mm joist spacing.
For solid composite decking: Maximum 600 mm joist spacing.
Always check your supplier's specific recommendations.
4. Use Recommended Fasteners
Use the clips or screws designed for your specific decking. Improper fasteners can restrict natural movement and lead to problems.
The Simple Rule for a Perfect Deck
Preventing warping is not about the quality of the decking alone—it's about the quality of the installation. By following these simple steps for gapping, ventilation, and support, your composite deck will remain flat and beautiful for its entire lifespan.
Need reliable composite decking that performs when installed correctly? Explore Treslam's range of co-extruded WPC decking, engineered for stability and longevity.