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Welcome to the Bunnings Workshop community @chrisizms. It's great to have you join us, and many thanks for your question about constructing a deck.
With a space of 150mm minus 19mm for decking, that leaves you with 131mm, so you can use 90 x 45mm H3 for your frame. That will leave you with a 40mm air gap under the frame, and you could either attach it to the concrete with angle brackets, like many of our members have, or adjustable feet are another option.
Check out this guide on How to build a low-level deck.
Please let me know if you have any questions.
Mitchell
Hi @chrisizms,
You can do a joist over bearer system, but typically low-level decks are constructed from a single layer of joists. I can't see much benefit in using a bearer in your application, especially if the bearer and joist are only 70mm, and you're effectively reducing the air gap under the timber by doing so. Using a single 90mm joist with a larger air gap underneath would be the more effective method. Having adequate air flow under a low-level deck is quite important and helps prevent the early onset of decay.
Any reason why you'd like to go with the joist over bearer arrangement?
The minimum height for the adjustable feet would be 25mm with the mini-foot.
Mitchell
The spacing on the feet is determined by the load-bearing characteristics of the joist/bearer, @chrisizms. For instance, if you do go with 90 x 45mm MGP10 framing, I'd typically advise that it needs to be supported every 1000mm, but it can span a fair distance further than that. Placing the adjustable feet every metre would be a safe bet.
Mitchell
