Which brings us back to the classic (and in my experience - increasingly regular) dilemma. Customer has bought 12mm thick travertine for their bathroom walls and wants you to fit them direct onto the plaster skimmed surface..... Do you point out the dangers, give a range of safe options, and then tile straight onto the plaster cos customer says they want that done without the fuss and expense of expensive over boarding? Or do you walk away from the job (knowing that another tiler with less scruples will take on the job)? I have to be honest and say I have accepted one such job (customer had recently skimmed walls did not want over boarding) and walked away from two others in the last two years (old walls customers did not want over boarding). However - I have found customers are increasingly willing to accept additional preparation costs to get best results (e.g. Ditra on floors and waterproof boarding options on walls).