I am refinishing my bathroom, and a near-future task is flooring. I'm going to be using the Schluter Ditra product rather than hardiboard and I've built up the floor and subfloor for a combined thickness of 1-1/8" (or so). There is a small dip toward the middle of the floor of roughly 1/2" that mostly levels out by the bathroom door. Should that dip be leveled out with a leveling compound before applying the Ditra, or will using thinset to compensate be sufficient? The span of the dip is around 5' long by about 3' wide (width of the wall to the tub), with the deepest part being about 1/2".
If you have a good solid base, you can use a modified thinset, the same material to lay the Ditra into. Use a straight edge to screed the thinset over the low spot of the floor, do not sweat the "rake" marks left by the screed. If any "tags" occur from using the screed, they can be scraped off and vacuumed the next day. Then set the Ditra. The Ditra otherwise may not sink into the depression of the floor and get a good bond to the substrate. Also, the flatter you get you floor to start, the easier it will be to get the tile to lay in plane with the surrounding tile.