70m2 Floor, 2 substrates, Ditra and Hardie?

[clipboard2008]

The underfloor heating is already installed by the builder between the joists. It is set in a tray which is then filled with screed to transfer heat. It's quite popular as it allows you to put underfloor heating on second floors. This is obviously a ground floor.
Certainly can't fill underneath the caber flooring.
My plan is to hardie backer the floor then ditra matt it. I know it's a costly option but it's the only way I can see of ensure it's stable enough so it doesn't crack?

 

[Dumbo]

The underfloor heating is already installed by the builder between the joists. It is set in a tray which is then filled with screed to transfer heat. It's quite popular as it allows you to put underfloor heating on second floors. This is obviously a ground floor.
Certainly can't fill underneath the caber flooring.
My plan is to hardie backer the floor then ditra matt it. I know it's a costly option but it's the only way I can see of ensure it's stable enough so it doesn't crack?

I wonder how many builders get structural engineers calculations on that before loading the joists up with screed

 

[Spare Tool]

The problem you've got here is no...or should I say not many tilers on here will give you advise to just uncouple and tile over two substrates without following the joint directly through into the tiles, had this dilemma with customers before and tiles have cracked (I told em so ) then again some haven't.
I've used Silicon to adhere tiles across a joint before on a job that had cracked to remedy and it worked , used marmox boards plus a crackmatt only this year as a customer didn't want a dodgy shaped concrete to wood expansion joint in her floor and up to now its been fine, there is no defining answer, you won't get any guarantees off anyone unless you do it by the book, otherwise its a gamble and I'm afraid your stuck between a rock and a hard place with this one.

 

[Spare Tool]

@Tony73 what part of that do you disagree with?

 

[Bond]

The underfloor heating is already installed by the builder between the joists. It is set in a tray which is then filled with screed to transfer heat. It's quite popular as it allows you to put underfloor heating on second floors. This is obviously a ground floor.
Certainly can't fill underneath the caber flooring.
My plan is to hardie backer the floor then ditra matt it. I know it's a costly option but it's the only way I can see of ensure it's stable enough so it doesn't crack?

If there is access and enough space in the sub-floor area, you could add beams or other support to the floor joists.