:confused: Thank you for your answers;Clear-Mind-Web &Kstar4u, I can't seem to get my hands on the BC building code. But US code states 1" subfloor. A new problem has arisen about tiling over plywood... I forgot to mention this is also a heated floor which would create a lot more flex with the heating cooling. Can some-one give me the Standard Trade practices for this type of installation. Is this even a fire-safe installation without the hardibacker? In 6 years of tiling I have never come across this situation, usually when I walk in the contractor has the site ready, of course, with hardibacker? Should I get the homeowners to sign a release?

Absolutely if you are just foing over plywood. It will not hold up using thin set. You might try some glue on the tiles then do the joints after they dry. Not a good situation.

Use Your Experience Here, Its Your Name On The Line. In My Business I Do It My Way Cause Im Liable. First The Wood Floor Will Move And Crack That Tile, Second You Need Some Type Of Hardi Board Or Durock To Lay First,
Hope This Helps , Nichols Trim And Carpentry