I would absolutly follow Mongo's advise that he posted above on Fri, Mar 22, 13 at 22:31. Ditra is put under all kinds of stone and ceramic. As I understand it, it is a decoupling membrane that helps prevent shearing (horizontal) movements from cracking the tile. The subfloor and joist deflection issues help with preventing vertical movement that can also crack the tiles. How I understand it from the Ditra instructions is that, depending on the subfloor, you select the appropriate thinset to lay the Ditra. Then you lay the tile on the Ditra using appropriate recommended thinset. The orange will be completely covered. It will raise the floor a bit (1/4"?) more because the Ditra has a dimension to it. It is not recommended to be used for tiles smaller than 2" because of the waffle shapes. I bought the appropriate thinset that Schluter recommended at my HIGH END tile shop. It was expensive but a gem to work with. I have never used low end thinset and don't plan to. I have linked the site that makes Ditra. Look it over very well. There are great illustrations on how it works either on this site or others. When I did my floor (first time floor tiler) I used it and, while I took part of it up to get the lumps out, I thought it went well. The beautiful jobs of tiling by Bill V, and Mongct are all on top of Ditra, I believe. Here is my DH adding extra joists 2x8's to our under qualified 21"oc floor. Now it is 9-10" oc: Here is after I put down the (specific recommendation from Bill V.) underlayment of plywood onto the subfloor that was existing, and thick and newly re-enforsed. The only mistake I made was I snapped lines for the grid every 4" instead of the 6" that Bill stated. My mistake, but was not a problem for my DS to do:) Next came my warming floor system, stratigically laid out and planned to the hilt using drawing after altered drawing to get it to all work out. I am proud that I am only 8" short: Here is the floor with the self leveling compound being poured. It has a plastic lath that I put over the wires. I was recommended by the Moderate End tile store where I was going to buy my warming system. I changed my mind because I wanted 240v. But I had the lathe and I then read a tutorial over on the JB forum about using the lathe, so is used it in my installation. So: the BillV recommended underlayment, warming wire and 2 thermostats (1 is laying there not hooked up, as a backup down the road if needed), plastic lathe stapled closely, primer, self leveling compound: If you want to do self leveling compound read Staceyneil's thread on the topic from several years ago, I think it is in the bathroom forum. Here is my Ditra, set with the recommended thinset. After this picture was taken, I did take out some areas, about a foot square, in several places and reset them. I did this because there were large lumps, and I'm a 60yo lady with not as much strength as I used to have. So the thinset was uneven in a few spots. You can see it here. Not all of the lumps needed taking out but several were just too high. When I took it up and reset it it was fine! At this point you can seal the seams with Kerdi tape if you want to water proof it. I was going to do that :( but I forgot!!! If you are installing Ditra in a bathroom, don't forget:) Here I am setting my tile in the pattern I've chosen. Mongo was taken aback when I posted this because I turned the pattern on him :) He had initially helped me plan this out in a 90 degree angle. After it had been all laid and grouted with Spectralock epoxy grout: I post this to help you see the process and that it isn't difficult. The difficulty seems to be when people don't want to make adjustments with installations. There are a lot of people using this stuff. Edited to add: Please correct me if I am incorrect in anyway. This is how I installed my floor and I may have made an error either in the installation or my description. Here is a link that might be useful: The Schluter Site. This post was edited by enduring on Sat, Mar 23, 13 at 12:47

1) Definitely, Look into a product at Home Depot called Redgard. 2) Again, at Home Depot, look at Versabond. 3) You mention three different details-- plumb, level, and square. Level's pretty easy to accomplish. Plumb and square, unless you want to turn this into a mud job, it's next to impossible for a DIYer, and even most pros. No matter which one you're looking to address, both are pretty common problems, and the corner usually just gets cut in (in other words, make sure you don't have a full tile in the corner, so you can scribe to it).

@David Jensen - stained concrete is stunning! And it has a price tag. You do not just stain it and walk away. Concrete is porous (which is why it takes stains so nicely/easily...which is a draw back with red wine). It MUST receive some sort of finish. Please remember that commercial settings have NIGHT CLEANING STAFF that have RIDE-ON cleaning/polishing machines that get brought out several times per week. The concrete floors in Costco do NOT stay that "pretty" all by themselves. They take a crew of people riding along on those cleaning machines that clean at the front and leave polish behind out the back. Kinda like a tile/cement Zamboni. I see them in malls all over North America. Not many homeowners have that level of cleaning time/money/effort/machinery. It doesn't mean you CAN'T have stained conrete....it just means you will need to make some decisions. The FIRST decision is: How much MAINTENANCE are you willing to put up with? Can you handle a twice yearly polishing event (wax on, wax off grasshopper) that has to occur no matter how much you hate doing it? If you don't mind this idea, then you are one of the lucky one's. You get to have the "cheap and cheerful" stained concrete + urethane + wax finish. A single colour + urethane is in the $5-$7/sf range. If you do NOT want that type of hassle, then you have to step up your game and move into the epoxy finishes. A simple "clear/shiny" epoxy is $5-$7/sf. If you add the stain then you add another $2-$3/sf. That's where you get into the $10/sf range without much effort. If you want an exotic tri-colour stain then you get to $15/sf without blinking. If you want a picture of a beach at sunset then you will need to break out the big bucks = $25/sf! That brings us to your next decision: How much of a budget do you have? Again, the BASIC concrete floor (no stain) is $5/sf (that's some grinding, microtopping + polish + urethane finish). That one DEFINITELY will have serious amounts of maintenance. Lots of liquid polish+ lots of "stripping" old polish so that new stuff can be put down. Some of the most artistic floors are concrete. I think they are amazing. I also know the amount of artistry needed to get them into the "amazing" range is not cheap. I respect the process and - if I wanted a concrete floor - I would be more than willing to pay for that process. What often occurs is the homeowner thinks a finished concrete floor is a cheap way to get flooring. They think it will be bullet proof without ANY issues at all. How wrong they are. How very wrong and how bitterly disappointed they become when they realize an UNSEALED concrete floor (polished without finish) will stain and etch FASTER than marble. Again, Homeowner Know Thyself. Answer the questions: How much maintenance are you WILLING to do? How much budget do you have?

As Handymac suggests: remove the backer board and replace with one-side-good plywood of the same thickness. I can see OSB board in the photo. It is "OK" as a subfloor for staples, but sometimes the low-end OSB can have issues with squeaking or nails/staples working loose to create a noisy hardwood floor. For that reason, I'm suggestion a decent plywood in the thickness you need. Install properly (screwed into place as per "X" pattern on the plywood)....making sure not to screw into the joists. And yes, a large big box store will carry plywood. Take your measurements of the cement board (or even a chunk of it) with you and find the warehouseman in charge of the building supplies and get a replacement. Don't go cheap. Cheap isn't the way to go with floors. Go mid-range. You should be fine.