Hi, we're installing 1/4" slate tile and have an OSB subfloor. Should we do mortar + Ditra or mortar + CBU on top of the OSB (has 16" joists)? It has been mentioned in previous threads that mortar + CBU will make the OSB swell, but won't mortar + Ditra also make the OSB swell? Also, if you guys recommend CBU, can I avoid the plywood if using 1/2" CBU (I want to keep the tile 1/4" + CBU 1/2" = to 3/4" adjacent hardwood floors.) Thank you!

Is the OSB toung and grove or butt to butt seams? Also how thick is the OSB?
Detra does not provide any additional flex strengh to the floor only uncoupling to the substrate. Or for those who do not understand. As wood floors swell and shrink it pulls and pushes on the surface of the applied floor. This in turn cracks the grout as that is the weak spot on the floor. The Ditra allows this movement without effecting the grout and tiles as it takes up the movement without tranfering it to the tile mounted on it.
It does nothing to stop the up and down movement of the floor should any flex be present.
So if you suspect that there is movement up and down you need to either install additional material on top of the OSB or remove the OSB and then properly replace with thicker sub-floor material.
I would not be worried about swelling OSB. Not going to happen, and if it does not enough to cause any issues.
Either way its going to be a lot of work.

Concerning the joists, there's much more info needed than just the on center spacing, although that IS important. Go to the following site, and plug in the values from your joists-- Height and width of joists, on center spacing, and unsupported length. It'll then tell you if your joisting is suitable for stone as it sits right now:

http://www.johnbridge.com/vbulletin/deflecto.pl

Also, you need a minimum of 1 1/8" subfloor before the Ditra for a stone installation, and that doesn't mean cement board (which has no structural value whatsoever).

Thank you both for your information. The OSB is 3/4" thick and has butt to butt seams. I'm not sure about the other info about the joists but I'll see if I can find out. What would be the worst case scenario installing the flooring if the OSB is only 3/4" thick?? When we jump up and down on it, there is no squeak or bounce and the floors are level.
Also, would there be any advantage or disadvantage of one over the other (Ditra vs CBU) in this situation?
Thanks,
Annie

Alot of people when asked what the worst enemy is of a tile floor over wood frame, their answer is usually water. It's a good answer, but it's not the right one. The right answer is movement, and to dispel an old wives tale, the movement doesn't stop a year after construction. The SETTLING will usually stop by then, but for the life of the house, there will always be the seasonal expansion and contraction of all the materials in the structure of the house. In addition there are very stringient building codes to make sure that the floors don't bend too much for the flooring finishes that may be applied, when weight either gets placed on the floor (static load) or when weight is moved across the floor (live load). This bending in the floor is called "deflection". The deflection spec for natural stone is literally twice what it is for any other kind of flooring, including ceramic tile. What can happen is the grout and/ or tile could end up cracking, or the tile end up popping loose. The reason for the extra subfloor is because deflection has to be addressed BETWEEN the joists, as well as along them.

The choice is yours. I mean, you're not going to get a knock at the door from the tile police!