There have been a few threads on the forum about using ditra or similar uncoupling membranes and which situations to use them in.
As a majority of tiled floors involve the thin bed method. this means the tile is adhered directly to the substrate with an appropriate adhesive.
Cracks in the substrate will readily be transferred to the surface,causing the tiles to crack.
Where irregular hairline cracks in the screed or timber board joints are present,its not practical or possible to position a movement joint over these.
Stresses from drying shrinkage,deflection and moisture movement in the substrate,plus thermal and moisture changes affecting the flooring,can cause loss of adhesion,resulting in bulging or cracking of the floor....
In particular, deflections in suspended floors can induce high compressive stresses in rigid floor tiling, and may be the principal cause of hollowness in those situations.
The sheer stress,resulting in surfaces moving differently from each other,is often too great for the adhesive to hold...
This is where uncoupling membranes will separate the floor covering from the substrate thus guarenteing the tiles wont debond.....this really applys to larger floors not your average 5mtrs or so.........just in case you think it should used on every floor....
I have put in a link to explain the installation of ditra and movement joints on floors....
hope this clears up why and when uncoupling membranes should be used........dave..
http://www.schluter.com/p/content/FHB.pdf