How to minimize cupping in wide Merbau decking boards?

[coolnav]

Hi Bunnings Workshop members,

I really like the look of the Merbau 140x19 boards over the 90x19, but I’ve read in the workshop comments that wider boards have a higher risk of cupping, especially when the ground clearance is low.

In my case, the lowest clearance from the bottom of the joist to the ground will be 200mm. What steps can I take to reduce the risk of cupping?

Are there any best practices I should follow, and what specific factors should I consider to minimize potential issues?

Thanks in advance for your advice!

[EricL]

Hi @coolnav 

You're right that wider Merbau boards like the 140x19mm ones have a higher tendency to cup, especially in areas with low ground clearance where moisture levels can fluctuate. However, with 200mm of clearance, you're in a reasonable range, and there are steps you can take to minimize the risk.

First, proper ventilation is key. Make sure there’s good airflow under the deck to allow moisture to escape, reducing the risk of cupping and warping. If possible, install vents or leave gaps around the perimeter to improve circulation.

Next, sealing all sides of the boards before installation, including the ends, will help regulate moisture absorption and reduce movement. Using a quality decking oil or sealer will protect the timber and slow down moisture changes that cause cupping.

Spacing is another important factor. For 140mm boards, leave a slightly larger gap between them—around 5-6mm—to allow for expansion and contraction. Using stainless steel screws and fixing each board with at least two screws per joist (rather than just one) will help keep them securely in place and resist cupping. If you're using a screw and plug system or hidden fasteners, make sure they provide firm holding power.

Also, consider using kiln-dried Merbau, as it’s more stable than air-dried timber. If your deck is in full sun, keeping it well-oiled and shaded where possible will also reduce excessive drying, which can lead to cupping.

If you need further assistance, please let us know.

Eric

[coolnav]

Hi @EricL 

Thanks , I have decided to go with the 140x19 https://www.bunnings.com.au/140-x-19mm-5-7m-specrite-merbau-joined-set-length-pre-oiled-decking_p017... , what screws do you suggest for the board to joist and spacing of the screws will help me for sure, much appreciated?

-- Nav

[EricL]

Hello @coolnav 

I propose looking at Titan 10G x 65mm Wide Head Decking Screw 304 Stainless Steel - Pack of 500 or the Bremick Decking Screw 10g x 65mm SPIRAL RIB T25 SS304 -500 Pack. My best advice is to use a P&N 10G Quickbit TCT Cut Smart Countersink Drill Bit just to be on the safe side. This will prevent your decking panels from splitting and the countersink on the surface will stop the screw from pinching the timber thereby causing it to dimple.  

If you need further assistance, please let us know.

Eric