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can anyone advise the most appropriate backing board , must withstand the element and be suitable for stone wall cladding to be applied to. approx area 3000 x 1800. I plan on concreting in three steel posts for the boards to attach to .. along the lines of photos supplied. Thx 
Welcome to the Bunnings Workshop community @markhor. It's fantastic to have you join us, and many thanks for your question about external cladding.
You'll need to consult with the cladding manufacturer about what would be a suitable substrate for their product. The weight of the product per square meter will dictate what substrate is suitable. They'll also advise on what adhesive to use on that substrate. It would be best to cross-referenced that with available external backing boards and appropriate ways of fixing it to your steel posts. A timber frame might need to be built. You're talking about a significant amount of weight here, so everything needs to work together. The cladding and glue needs to be suitable for the backing board. The backing board needs to be suitable to hold the weight of the cladding. The fixings need to be appropriate to support the board and total load. Finally, the steel posts and footers need to be able to bear the weight of everything.
You'll likely be using Hume Prima 2440 x 1200 x 7.5mm Blueboard, but as I've mentioned, it needs to be adequately fixed to support the weight it is bearing. This is usually done with a timber frame so the board can be fixed at the required intervals. I'm not quite sure how it would work with your three steel post method. I presume you'd need to attach an appropriate amount of timber framing onto your steel posts and then fix the boards to the timber.
If you're going with one of our stacked stone cladding suppliers, I'd be happy to contact them after the weekend to determine what a suitable exterior substrate would be. In the meantime, you should find this step-by-step guide useful: How to lay a stacked stone feature wall
Please let me know if you have any questions.
Mitchell
I'm sorry to hear that, @markhor.
After further investigation with our supplier, it appears that cement sheeting is not particularly suitable for tiles in external applications. "Do not use HardiTex Blue Board as a substrate for stone installation. The manufacturer does not recommend it as it will not support the weight, especially if it is subject to any moisture." "Fibre cement sheeting generally is not recommended as a substrate for external walls to receive tile finishes, and we recommend the sheet manufacturers be consulted where the Panel Stone is to be fixed in external applications." They only recommend sheet lining for internal application. If you've spoken to James Hardie and they weren't prepared to give details of a specific backing board, I would suggest it's because their products are unsuitable.
If you were going with our Decor8 Panel stone, it is less than 27kg per square meter, so it's suitable for adhesive fixing to most substrates. Unfortunately, in the absence of an appropriate backing board for external use, they would only be suitable for application over concrete, bricks or blockwork. Here's Decor8's installation guide.
If you were to go with render, then you'll be using Hume Prima 2440 x 1200 x 7.5mm Blueboard.
Mitchell
