Composite decking installation: best practices from the experts

Composite decking rewards precision. Movement, ventilation, and structural accuracy matter more than speed on these builds.

If you're installing capped composite boards such as Abbey Timber Modern Deck, the following best practices will help you deliver a stable finish, maintain warranty compliance and reduce post-install movement issues. Always refer to the manufacturer's installation instructions for your specific product before commencing work. 

Expert tip 1: Allow composite boards to acclimatise before fixing 

Composite decking expands and contracts with temperature changes. Boards should be acclimatised on site before installation, so they adjust to local conditions before being fixed.

With Modern Deck, boards should be laid out on the subframe for 48 hours prior to fixing. WPC boards can expand and contract lengthwise up to 1–1.5% depending on weather conditions, with boards over 5.4m capable of expanding up to 8mm in extreme heat.

On hot days, fixing early in the morning helps manage expansion and reduces the risk of excess gapping as temperatures drop. 

As with all composite products, treat expansion allowances as a guide only. Consider site temperature, exposure and board length before final fixing. 

Expert tip 2: Build a straight, drained and ventilated subframe 

Composite decking performs best over a level, square and well-drained structure. 

A fall of approximately 4mm per metre should be allowed to ensure water runs off the deck surface. Minimum ground clearance should allow ventilation beneath the deck. For Modern Deck installations, a clearance of 50mm over concrete with good drainage, and 300mm over soil, is recommended. 

Boards should not be fixed directly to solid surfaces such as concrete or tiles. 

Ventilation and drainage are critical. Poor airflow or trapped moisture can affect both the structure and the decking surface over time. 

Expert tip 3: Set joist spacing to suit exposure and use 

Composite decking span ratings are a starting point, not a blanket rule. 

Modern Deck 140x22 boards can span up to 450mm centres in residential settings. In areas exposed to strong direct sunlight, 400mm centres are recommended for improved stability. For commercial installations, a tighter spacing of 300mm to 350mm centres is suggested. 

Exposure also matters. A north-west facing deck receiving heavy sun may benefit from closer joist spacing, while a covered or south-east facing deck may not move as aggressively. 

Use these figures as a guide only and adjust for your environment and loading requirements. 

Expert tip 4: Fully support all butt joints 

Composite boards must be properly supported at every butt joint to control movement and maintain alignment. 

Install double joists under all butt joints, spaced approximately 30mm apart. Allow 10–15mm board overhang at the joint. Expansion gaps must be calculated based on board length and temperature at the time of installation. For example, at 20°C, a 5.4m board requires approximately 6.5mm at the butt joint.  If both ends of the deck are open with no obstructions, expansion allowances may be reduced accordingly. 

Always assess site temperature at the time of install and refer to the manufacturer's expansion table for your specific product. 

Expert tip 5: Use breaker boards on longer decks 

Long runs of composite decking require planning to manage movement cleanly. 

For decks exceeding 5.4m that require butt joins, incorporating breaker boards helps control layout and prevents scattered butt joints across the deck surface. 

Additional joists are required to support breaker boards and adjacent deck boards. Allow appropriate expansion gaps between deck boards and breaker boards as specified for the product. 

Breaker boards create symmetry and simplify expansion management across larger builds. 

Expert tip 6: Maintain correct clearances at all fixed points 

Composite decking must have consistent movement clearances at walls, posts and fixed penetrations. 

With Modern Deck installations, maintain: 

• Minimum 5mm spacing between boards 
• Minimum 10mm clearance where the board ends meet a wall 
• Minimum 5mm clearance around posts and fixed objects  
• Maximum 40mm board overhang past a joist  

Clearances should be treated as minimums. Consider local conditions and exposure when setting final gaps. 

Expert tip 7: Use the recommended fixing system and methods 

Composite decking should be installed using the manufacturer's approved clips and fixings to ensure correct spacing and secure engagement. 

Modern Deck uses stainless steel connector clips and starter/end clips for concealed fixing. Where face fixing is required, screws should be positioned 25mm in from the board edge, pre-drilled and spaced no more than 450mm apart. Face screwing into steel frames is not recommended. 

Warranty coverage applies only when boards are properly fitted in accordance with the manufacturer's recommendations. 

Always confirm fixing details against the installation instructions for the exact product being installed. 

In summary 

• Acclimatise boards before fixing 
• Build a level, ventilated and drained subframe 
• Adjust joist spacing based on exposure and application 
• Support butt joints with double joists 
• Allow for temperature-based expansion 
• Use breaker boards on longer decks 
• Maintain minimum clearances at walls and posts 
• Install using approved clips and fixing methods 

 

Abbey Timber has the largest range of decking timbers, composite decking, hardware, and tools, with fast delivery and a Sydney showroom where you can see the difference for yourself. 

If you would like more advice from one of the expert trade team at Abbey Timber, call or visit the showroom or chat to us online.