Installing Slate floor tiles over a plywood floor

Can you lay slate floor tiles over a plywood floor or do you have to install cement board over the plywood?

You can lay on top of plywood as long as it is WBP (water boil proof) Marine Plywood, this has been treated & is much stronger than standard plywood.

It is also recommended that a minimum of 15mm marine plywood is used & that the floor does not flex, if it does then a more solid method would be to cross lay 9mm WBP Marine Plywood on a horizontal direction screwed down at every 300mm intervals & then overlay a second layer of 9mm WBP Marine Plywood on a vertical direction screwed down again at 300mm intervals, remember to use a bonding agent to prime the plywood & a flexible grout & adhesive.