How can I make up a floor level by 55mm ready for tiling?

[rcflyers]

I want to tile the floor in my conservatory.
It was built 5 years ago and had a concrete base with expanded polystyrene insulation foam laid on it and then 18mm flooring grade T+G chipboard on top.
The foam went soft in places and the floor had dips in it as a result.
I have removed the chipboard and insulation and am now down to the bare concrete.
I need to make up the floor height by 55mm to match the existing floor level in the adjoining room ready for ceramic tiling.
Question is, how to make up that 55mm?
1. pour concrete in to a depth of 55mm (if it can be done without it cracking as normally concrete is laid at 100mm+)
2. fit joists on top of the concrete and board over the top
3. any other suggestions please?
Thanks.

 

[Notch7]

you cant use concrete.

you could use screed, but you will have no insulation.

I suppose you could use CLS 63 x 38 laid flat at 400 centres and 18mm plywood laid on top. The use ditra matting

for a diyer, timber joisting is an option, but you need to make sure all joints are fully supported and the plywood screwed at close intervals.

you could fit 30mm celetex in between.

I suppose the original floating floor could be replaced with another using celetex and T & G flooring -but that would not be suitable to tiling, I dont think.

 

[JP_]

According to https://www.speed-screed.com/knowledge-centre/minimum-screed-thickness-over-insulation/ you can have a 15mm screed using a "specialist calcium sulphate product" , so could have 40mm kingspan + thin screed.

You need some insulation or it'll be like walking on ice in winter.