I'm installing 12X12 ceramic tiles on a concrete slab floor. Do I need to put cement backer board, plywood, or Ditra over the slab, or can I just use thinset mortar directly to install the tiles? I'm going to use Versabond fortified thin-set mortar for this application.

As long as the slab is at least 28 days old and has no sealers, paint or adhesive residues you can go right over it with the Versabond. If it's a new slab, then Ditra is good insurance against shrinkage cracks which may or may not occur. IF it is sealed or painted or has old adhesive residue on it, then it needs to be mechanicly removed, unless it's got black adhesive residue, it which case, use hot water and a 4" scraper to get teh slab as clean as possible and then use FLexbond (usually on a pallet next to the versabond) to set your tile.

Thanks for your help. The slab's definitely old. Removed 2 layers of linoleum tile & the slab looks yellowed from the adhesive from the 1st layer of linoleum tile. How do I mechanically remove the adhesive residue? Though, the floor doesn't feel sticky like it has residue.

Boil some water and pour it on the slab in a spot where you want to start. Let it sit for a while and then pour some more water on it and go to town with a 4" scraper. Suck up the slurry with a shop vac. As long as the slab will absorb water, you will be fine.

Thank you!

Bad idea. If you lay floor tile directly over a concrete slab and someday the concrete cracks and heaves, so will your tiles, I've seen it happen to my sisters sunroom floor. And let me tell you where the tiles cracked and heaved up slightly is one nasty sharp edge. I'm not sure what the answer is. Maybe like you said before, concrete board then tile. But after seeing that I'd never lay tile directly on a slab.

If your concrete floor heaves that much you have much bigger issues than cracked tiles.

You stated that your floor was old, so it should have already done all the shifting that it is going to do.

After you get done cleaning everything off, you should moisture test it by placing saran-wrap or alum-foil 12" square over part of the concrete and duct tape down the edges. Wait 24 hours and if you have any condensation on the plastic after you lift it up, you have a moisture issue.

Hope this helps!

My question is similar. Is it possible to tile over pre-existing (and fairly ancient) tile? the floor is in good shape, but I am concerned about where the floor slopes slightly to accommodate a drain in the floor.