I'm going to take delivery soon of a steel bath, and need to make a decision soon on overboarding.
I'm going for ceramic tiles and this is my first tiling attempt.
Joist centres are 400mm with joist width 50mm and joist depth 225mm. Bathroom floor area (excl. bath) is tiny - just 2M X 1.15M. Existing floor is 18mm T&G. I say T&G, but some boards have been replaced with ply and so therefore the floor is also partly composed of square edge 'boards'.
I've dismissed removing all the boards and fitting nogged 25mm WBP. Reason is due to the stud walls making it difficult to remove some of the existing boards at the floor perimeter.
So, I'm torn between hardie and ply. I've done a lot of searching on this excellent site and it appears that the choice of substrate largely depends on the deflection/rigidity of the existing floor and the tiler's preference.
The existing ply in the bathroom was installed 18 months ago by a CORGI/GSR installer after running pipework under the bathroom floor as part of a new combi install. The ply now flexes like billy-o - it really flexes a lot! ! The installer did not nog at the ply board edges. I'll post a pic if necessary.
I'll replace the ply with square edge softwood (e.g. pine) boards, screw down existing boards and make good. Then I'll overboard. I'd like a sheet material that has the dimensional stability of cement backerboard but the thickness/rigidity of 12 (18?) mm ply. I don't think I have confidence in a 6mm backer board...
The thickness of this backerboard looks ideal:
Fitting 12mm ply and 6mm hardie on top seems an unusual approach, but I'm thinking about that.
I realise there will be a height differential with the landing, but hey, if there's gonna be a step - might as well make it 18mm rather than 12mm (+ tile/adhesive).
Cheers for any advice!