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Do I HAVE to use unmodified thinset over a Ditra membrane?

Posted by raehelen (My Page) on
Fri, Apr 12, 13 at 21:52

Think I'm beginning to understand this thinset modified/unmodified thing. We are DIY'ing our Master BR.
Both floor and shower walls will be 12 X 24 porcelain tiles.
We will be using Ditra on the floor, and Hydroban on the walls. Total square footage is ~135 sq ft.

My problem, and maybe it's not really a problem, is that I need to use modified thin set everywhere, except after I've installed the Ditra using modified, I am supposed to use unmodified to apply the tile. This floor area will only be about 35 sq ft. Am I being a cheapskate not wanting to buy another bag of mortar for this area, instead of using the modified I will already have? I will be buying 2 50 lb bags of the modified.

Guess I'm sort of talking myself into buying the extra bag even as I type this, but now am wondering why one is not supposed to use modified over the Ditra?

Anyways, if someone knowledgeable can enlighten me, that would be awesome!


Follow-Up Postings:

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RE: Do I HAVE to use unmodified thinset over a Ditra membrane?

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.


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RE: Do I HAVE to use unmodified thinset over a Ditra membrane?

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.


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RE: Do I HAVE to use unmodified thinset over a Ditra membrane?

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.


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RE: Do I HAVE to use unmodified thinset over a Ditra membrane?

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


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RE: Do I HAVE to use unmodified thinset over a Ditra membrane?

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.


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