If tiling on to a solid t & g chipboard floor in a small bathroom & en-suite new build, would it be ok to overlay with 6mm hardi fixed with flex adh & screwed. Then tile with flex adhesive & grout with gt1?
will the flex adh adhere to the chipboard or is it just to fill the voids behind the hardi & allow for expansion?
Just that the customer wants to keep the height down.
Thanks.

preferably i would use 12mm ply on this,by the time you stick the hardie down with adhesive you will almost be at the same level,could you not just screw down 6mm cement boards? that would keep the height down!

In my opinion, the use of hardie in this situation depends entirely on the size of the floor area. if small as you indicate it will be fine. The adhesive bed only needs to be applied using a 6mm notch trowel and is really just to fill the voids but obviously it will stick the board to the floor as well. Personally I would prefer Hardie to ply in this situation.

Thanks for your replys guys.

The rooms are only 3 & 4m2 each.
No thresholds fitted or other flooring in yet either.

The Hardie is looking favourable i feel, but i've heard of other tilers using Ditra also, is this a good practice?

Not forgetting tiling straight to the chipboard with 2part flex also.

What would be the pro's & cons of using ditra & 2part flex in this situation?

Don't quote me on this but I thought schluter did a vinyl adhesive for sticking ditra down to certain floors???????

Ditra will do nothing to help with any deflection. It is designed to provide protection form lateral movement created by contraction/expansion of the substrate. The general problem with chipboard is that it is unstable in a number of ways. 1 of them being the potentional for the "chips" to break away from the board. Using ditra only will still not protect for these pieces breaking away. Hardie on the other hand is mechanically fixed with screws as well.

You can tile straight onto chipboard with appropriate adhesive and I have done it myself, but my preferred method now is to overlay with 6mm Hadiebacker 250, glued and screwed.

Cool....
I think i'll stick with my original plan then.
Thanks for your inputs, much appreciated.

I know it will bump up the cost to the customer but you could always rip up the chipboard and lay 18mm ply then tile straight on = no increase in height