Ply under tiles

[Bonzer]

Bathroom floor, about 3 sq metres. I want to tile but don't want a huge step at the door.

I intended to remove the boards and lay 19 or 22mm ply instead and tile (& undertile heat cable) on that.
The floorboards( about 19-20mm) go under a couple of walls, architraves etc so I'm going off the idea. I'd have to cut them a couple of inches short of the walls and put 2x2 or similar underneath the joins.

What thickness of ply would I need on top of the boards if I left them there?

The boards are in quite good condition and pretty flat, T & G though a couple have been up.

 

[ChrisR]

bump
____________________

moderator

enough said

 

[masona]

The floorboards( about 19-20mm) go under a couple of walls, architraves etc so I'm going off the idea.

Are you saying the floorboards thickness is about 19-20mm or the floorboards goes right under wall and the wall rest on the floorboards? Is the wall timber studding or blocks/bricks?