Replacing bathroom floor

[tomton]

Hi,

I'm about to start bathroom refurbishment, to do so I have to replace part of the old chipboard floor because it's got big "hill" right in the centre of the bathroom (probably there was not enough space left for expansion).

Bathroom floor is 180x180cm.
At the moment there is 22mm thick chipboard and I'm planning to partially replace it with 2x9mm External Grade Ply, first layer screwed to beams and the second screwed to the first layer 25-30cm grid.
Is this a good idea? I want to keep new floor as thin as possible as it's gonna be tiled and I'm trying avoid massive step into the bathroom.

Plan B is 12mm chipboard and 9mm Ply on top.

All comments and suggestions are welcome.

Here is a image showing what I want to do.

 

[mattysupra]

confused? Why two lots of 9mm ply and not one sheet of 18mm? If tiling i would recommed thicker than 18mm too!

 

[tomton]

Thanks for your reply.

Thickest Ply i could find is 18mm (B&Q, WICKES).
I thought two layers of 9mm Ply screwed together will be less susceptible for deformation.

Will single 18mm Ply be enough or should go safe and put 12mm chipboard topped with 9mm ply?

Will 5mm gap around new floor be enough for expansion (2.5 m2)?

Can I put some standard attic insulation underneath new floor as there are recessed entrance door right underneath the bathroom and it would definitely help to keep warmth.