We recommend a 100 mm thick reinforced concrete pad for portable spa and 150 mm thick reinforced concrete pad for swim spa. Alternatively, a deck built to Australian building standards and certified by a qualified engineer that it can carry the full weight of your spa. Please consult with your product spec sheet for spa weight.
If the foundation is inadequate, it may shift or settle after the spa is in place, causing stress to the shell and cabinet.
Note: Steel SL52 will be adequate for a swim spa 150mm thick slab and F72 is also fine with 180mm thick slab.