The bathroom floor to be tiled is 5/8" subfloor with 1/4" ply ontop screwed every 6 inches.Iam trying to convince carpenter that this does not meet code, nor is it a proper base for tiling. I want him to put down hardibacker (1/4") but he has convinced home-owners not necessary. What do I do? :confused:

I would be asking for 1/2" hardibacker over 7/8" of wood...

What do you do? Make sure he's a licensed contractor... Get a warranty statement from him in writing and be prepared to call him when the grout cracks and starts falling out. I don't know where you're located or the building codes for your area, but in most states, I don't believe that flooring materials or methods are subject to building codes... they are usually determined by standard trade practices. No reputable contractor wants to get "call-backs" or leave a trail of dis-satisfied clients.