Bathroom Sub floors

[Digger1]

We are putting 22mm chipboard floor board on the first floor.  In the bathrooms there will be UFH that sits between the joists on alu trays.  The UFH company says put 18mm ply down on top.  The tiler says either 18mm ply or 22mm chipboard + ditra + tiles, and the carpenter says you can do either but the 22mm chipboard will give no flex and the ply may do.

 

The finished bathroom floors will have tiles and ditra mat then run into carpet at the thresholds.  Obviously i would like the transition to be as level as possible between rooms.

 

Is there a preferred option?  The plumber mentioned that the underfloor heating having to work through 18mm ply or 22mm chipboard doesn't seem optimal!  

Is there a thinner cement board that can used on the floor?

Can you get shower trays to sit on the joists rather than the sub floor so they can be flush with the tiles?

 

[ProDave]

Isn't Ditra a decoupling mat?

 

We have tiles straight onto the 22mm chipboard with a (different) decoupling mat.

[markc]

I’m confused as to why the chippie is thinking about flex between 18mm ply and 22mm chip? 
using thinner cement board would cause problems with flex.

[ProDave]

Don't seek advice from the chippie, seek advice from the tiler.

[Simplysimon]

and knowing te joist centres would help