Explain outcomes to the customer of tanking vs not tanking. If they choose not to have it done then write it into your invoice that this was the case. So if you do get the call back, you are covered to a degree. What you must remember is that putting this in the invoice is not an excuse for bad workmanship though
Tanking is rumoured to become part of building regs on new builds and renovations in the very near future by the way
Chase Tiling Academy