shower/ bathroom floor- on top of timber?

I am renovating an ensuite and have a few questions.....
After pulling up the existing floor tiles and underlying mortar, I'm left with a bare timber floor. I was going to fix compressed fibre cement sheets (cfcs) over the top, screed, waterproof then tile. However, I've been advised that the cfcs should not be fixed to timber- only floor joists. Plus its 15mm and that will creat quite a 'thick' floor- probably leading to a large step up into the bathroom.

I was then going to use 6mm JH fibre cement ceramic tile underlay over the timber floor but that is proving hard to get up my way. So I've been considering using cladding (something like Hardiflex 4.5 or 6.0mm) over the timber floor, then laying a screed over this, waterproof, tile.

Any suggestions whether this is a good idea or should I stick with the ceramic tile underlay? I can't see much difference between the two considering I don't have to worry about the structural strength with underlying timber floor.

You could cut out the floor boards and then the cfcs sheet will be on the joists and there won't be a thick floor.

Though I can't see that fixing it to floor boards instead of joists will be any worse, It would be an even stronger floor although thicker.

Would be interested in the reason why it "MUST be fixed to joists only!"

That's just the advice I received.
I'd really like to avoid cutting out the timber floor as most of the perimeter of the room doesn't line up with joists and I can see as alot of unnecessary work if I can simply lay somthing thinner over the top such as 4.5 or 6.0mm fc.

We renovated our bathroom about ten years ago and used an "undertile cement sheet" from Boral. It was about 6-8mm thick and came preprinted with dots. You just hammered a galvanised clout into each dot. Easy.

I also saturated the timber floor (Tas oak) with TBT fungicide as an extra guard against moisture and rot.

Cheers

Graeme

I could'nt see why you could not go straight over the timber if it's in good condition.the hardie flex sheets I would think would be fine,most addesive for fixing tiles has an waterproof addittive anyway or can be painted on.In Adelaide you can use a flexi mix addesive to go straight over timber and a flexi mix grout to allow for movment.Each State has it's on set of rules for the building code.peter.

I used 6mm cfc on a timber floor. I had some problems as a few tiles popped in the couple of weeks I left them ungrouted. This was probably because I used the wrong adhesive and the tiles popped up when the floor flexed with people walking on it - you need a good flexible adhesive. Anyway after I grouted the tiles the floor was stiffened up quite a lot and I never had any more problems. There is a step up of about 15mm from the floor in the hall.

Anyway I can't see any point in removing the floorboards (appart from the height which isn't that hard to accomodate), they only add stiffness.

I'm in the middle of doing a bathroom with the help of builder to do the waterproofing. Floor was hardwood T&G. Builder nailed tile underlay on the timber as descibed above by GraemeCook then waterproofed using a polyurethane waterproof (he reckons it's better than acrylic and worth the extra cost). The floor finished floor is raised about 40mm by the time the underlay, tiles and screed is all done.