The Ditra will provide protection from movement/expansion/contraction, not just waterproofing. It's an isolation membrane. I am not sure about the Redguard, I thought it was just for waterproofing.

My understanding is that Ditra is designed for just the circumstance you have- a concrete basement floor. It's not hard to install either, really! If you can do Kerdi, Ditra would be a piece of cake!

Not that much in the know, but I would ask more questions before putting tile directly onto concrete... maybe a pro will pop in on this thread ;-)

Grout line 1/8" unsanded grout.

My first reaction was nope-- use sanded. But if you mix the grout super thick, you might just get away with using unsanded. My recommendation would still be to use sanded with a joint that size, though.

As for the basement floor, you're taking a big chance. If you're going to do this at all, use the Ditra, but even then, I can't guarantee you there won't be problems. You say the crack is virtually flat. Either it is, or it isn't (not meaning to sound flip), and if it isn't, there's not a membrane in the world that'll protect against vertical movement, not even Ditra, which is considered by many (including myself) to be about the best in the world. THe ONLY way I'd consider tiling over that would be to do a mud job over a slipsheet (tar paper or polyethlene) and wire. That way, the floor would be completely independent of the structure. Anything short of that, though, I'd be very reluctant to warranty it.

Ok - I went downstairs and gave my basement floor crack the "lippage" test - I ran a jar lid over the crack and there was no lippage. I would call it "flat". Sounds like Ditra is the way to go.

Question: Our room is 7x11 - 77 square feet. Shower is about 16 square feet - and linen closet is 3 square feet - leaving 58 square feet. Ditra comes in rolls of 54 square feet. If I don't run the ditra under the vanity (8 square feet) I will have enough.

Would I just add an extra layer of thinset where the cabinet goes to make up for the height of the Ditra? We were planning to tile under the cabinet just to make it easier (18" tiles on diagonal would run under vanity). I hate to purchase a whole role of Ditra - almost $100) just for a few square feet!

Thanks again for your words of wisdom.

I would set the vanity first, and then Ditra and tile up to it. It'd be a whole lot easier.

Redguard seems a lot easier.

It is pretty simple, but you have to roll it on pretty thick, which can mean going over the surface multiple times to build it up. Time savings over super-simple Ditra: negligible.

Ditra will only add 1/8" to your floor height.