I am in the planning stage for an outdoor deck and am wondering what size bearers and joists to use. The area is 5.5 x 4.1 and I need to excavate soil first so need to know bearer and joist sizes to work out how much soil to remove. The decking on top will be 23mm thick too. I have looked at various timber standards pages but not sure if they relate to WA as there's soft/ hardwoods and different formulas so any advice would be most appreciated. The decking boards will run along the long length, so 5.5 metre length.
The required size of your bearers is determined by how far they need to span between supports. The required size of your joists is determined by how far they need to span between bearers. So, for a low-level deck, you could use 90 x 45mm Pine timber for your bearers or joists if they are supported every 1000mm. If you wanted the supports further apart, then you'd need to increase the calibre of the timber as per span tables. Alternatively, swapping to hardwood will typically extend the distance that the calibre of timber can span.
You should find most timber span tables relate back to the National Construction Code, but it would be worth checking with your local council to determine whether there are any local regulations you must adhere to. For example, if you are in a bushfire-prone area there might be a BAL rating your structure must adhere to.