How to Stop Decking Boards from Cupping

Q: Hi Bill, I have been called back to a 130x19mm Spotted Gum decking job to find that the 4mm deck spacing I used has all but gone. There is also some cupping on the surface of the decking boards. The decking is mostly protected by a roof area above. Is this a com

mon problem with Spotted Gum? (Ron, Rythdale).

A: Hi Ron, This is the time of the year when decking issues often surface. This is because of the higher humidity associated with the winter period. For decking, winter is when it would be at its highest moisture content causing it to expand. These decking problems are not related to selecting Spotted Gum. From the picture provided it is very clear that this deck has virtually no ventilation. This is the main issue of many decking problems I have inspected.

The lack of ventilation causes the ground under the decking to become very damp allowing it to be absorbed by the underside of the decking boards. Compared to the top side of the decking this high moisture content results in cupping as well as expansion. I can see that this decking has been designed to be at the same level as the surrounding lawn. While this looks seamless, this type of design restricts a cross flow of air in the subdeck space. Dense vegetation – such as lawns – can block air from entering and escaping. Ventilation is what will help dry out the subdeck space avoiding the decking from becoming affected. The sides of this deck should be free of any perimeter baseboards or include vents for some airflow.

The other important factor here is drainage, water should not be able to pool under the decking. Moisture should be directed away with aggie pipes or a graded fall in the ground away from the deck. The suggested Bowens installation guidelines state that decking wider than 90mm should be installed with a 6mm gap spacing. This allows for the natural seasonal expansion of timber and still allows moisture to pass through the decking gap. The 4mm gap spacing is not sufficient for 130×19 decking, board expansion will cause the decking edges to meet each other under pressure. This can shear fixing and buckle boards upwards. As a rule of thumb, the amount of expansion can vary between decking species, though common ones like Spotted Gum and Blackbutt tend to expand approx. 0.3mm over a 90mm width for every 1% increase in moisture content. Merbau decking is certainly one of the most stable in comparison.

It’s great that the decking is protected by a roof covering and has an oil coating as rain and UV exposure are deteriorating elements for timber. Ideally, the coating should be applied on all 4 sides before installing and to all cut ends when installing. Ron, on your next decking job, consider having the decking supplied with an all-around precoat of Cutek. Subsequent final coats and ongoing maintenance coats also need to be applied.

If you require further assistance, please speak to your sales representative or visit your nearest Bowens store. Good luck!

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