You mentioned Hydroban, so it sounds like you're doing a shower. If so, you don't use Ditra on the floor of the shower. If you're doing the floor outside of the shower, then no problem. You can't use modified thinset on top of the Ditra because modified needs air to dry. Ditra is impervious, and such large porcelain tiles are essentially impervious, so no air can get through with the tiles on top and the Ditra on the bottom. The thinset won't dry. Unmodified thinset cures, which is not the same as drying with air, so that's why you use it in between the Ditra and tile. You use modified between the plywood and Ditra because you need that to bond to the plywood, and the plywood will allow air in to dry the thinset. A bag of good unmodified thinset is not expensive.

Thanks catbuilder.

As I said above, even as I typed it, sort of felt silly, like just voicing the words, I basically answered my own question. But, you're right, I do remember reading why, because of the modified not drying, that makes sense. Ditra will just be for the BR floor, we are having a pan custom made for the shower, small detail I left out...

However, why is it OK to use modified on top of the Hydroban, same large format tiles? Is HB not also considered impervious? Guess that's why I'm still a little confused.

Because Laticrete says so. You always go with what the manufacturer specifies. Schluter used to say modified was ok, but they've changed their specifications.

Great question. I went through the same stuff, and forgot already, so thanks Catbuilder :)

I am, today, tearing out my next project, Yippy ;)

Raehelen, I enjoy your post and am looking forward to seeing the progress.

This post was edited by enduring on Sat, Apr 13, 13 at 8:10

Hydroban is a water barrier, but not a vapor barrier. Ditra is both. A technicality, but it is a difference if you want to get into the technicalities of vapor transmission.

It really comes down to different manufacturers having different specifications. For example, Nobel has a membranes similar to Schluter's membranes.

Look at Nobel's TS membrane, which is sort of like Kerdi. They both have similar "fleece" surfaces, but the core materials are different. Kerdi has a PE core, Nobel has a CPR core.

Though the core materials are different, they are both vapor barriers. Yet Nobel allows modified thinsets to be used with their product. Schluter does not.

I use a lot of Schluter. I use a lot of Nobel. I have a roll of NobelDeck that'll get used outside once the weather warms up a bit more. Both products are excellent products, and I've never had a problem with either manufacturer's products.