Discuss Upstairs ensuite floor... which solution? in the DIY Tiling Forum area at TilersForums. The USA and UK Tiling Forum (Also now Aus, Canada, ROI, and more)

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I've started refurbishing my ensuite and I'm a bit overwhelmed by the number of options available for floor preparation. Have done a lot of googling and not found any threads on here which completely address the situation I have, so any advice would be really appreciated!

The floor was previously 8mm porcelain tiles directly onto 22mm chipboard. A couple of them were cracked, and they lifted cleanly in about 2 mins with no adhesive remaining on the floor. Obviously not good so I want to do it properly.

I'm planning to use 10mm rectified porcelain 60x30 tiles but not directly onto chipboard. The problem I have is that although the floor is solid and has minimal deflection (tried the glass of water test), there is a sag towards the centre of the room of about 8mm across a 1m spirit level. Ideally I'd like to have a perfectly flat/level surface to tile onto.

The low point is on top of a joist. I did consider taking up the 22mm chipboard completely and re-laying 18mm structural ply (packing the joist), but the advice seems to be that ply should only be used for over-boarding and not for the floor itself.

I was originally planning to use Hardiebacker with an S1 adhesive under and over it (priming the chipboard first), with the tiles on top, but now think I need to use some kind of self-levelling compound to level the floor. What brand of self-levelling can I use over the hardiebacker? Can I fit tiles to the self-levelling directly?

I also read that Mapei renovation screed can be used to level chipboard. Could I lay ditra mat over that?
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If your floor is solid and sound (no movement) then I would overboard with a 6mm backer board (Bal board is my go to as it is solid but can be cut with a knife and screwed using normal screws) also put a layer of adhesive under to fill any voids (glue and screw)

Then you can tile direct to the backer board using any powder adhesive
Or level over the boards then add a decoupling mat and tile straight to that to help if any lateral movement

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