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Hi all,
So I'm going to try build a deck which is 6.4m by 2.5m.
Just wondering what the correct spacing is for the support posts across both lengths?
Also reading other posts that suggest 300mm deep. 3/4s will be dug straight into dirt but one length are going to be 1m exposed before it goes into the ground, are these okay to also be 300mm deep or do they need to be deeper with the exposed section.
Thanks in advance
Welcome to the Bunnings Workshop community @jono2. It's wonderful to have you join us, and many thanks for your question about building a deck.
The spacing of your support posts is driven entirely by the size of the bearers and joists you choose. The span tables for your timber will tell you exactly how far you can run before a post is required. As an example, if you were using 90 x 45, you would need a support roughly every metre because that size cannot span far without deflecting. As the calibre increases, the distance between posts increases as well, so the design really starts with your bearer size rather than a fixed post spacing.
There is also no single depth that suits every post hole. The correct footing size depends on your soil type and ground conditions. For most standard soil types, you would usually be looking at something around a 300 mm diameter and 450 mm deep concrete footing as a minimum for a low deck. In your case, you have one side where the posts will be exposed by about a metre before they enter the ground. Those posts should sit in a larger and deeper footing, something closer to 400 mm by 400 mm by around 600 mm deep, although the exact dimensions do depend on local soil behaviour. The more height a post has above ground, the more leverage it has on the footing, so the footing needs to counter that movement.
It would be worth contacting your local council, partly to check whether your deck requires approval, and partly to ask whether they have guidance for footing requirements in your area based on soil reactivity. You can also refer to these guides: How to understand deck span tables and How to build a deck.
Please let me know if you have any questions.
Mitchell
