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Discussion Starter · #1 ·
Ditra:can I make up 3/16" difference between old & new floors w/ thinset under Ditra?

Have 3/16" difference between small area of old subfloor (lower) and new subfloor.
Home Depot guy (I know ... that's why I'm checking) says it should be ok - but use 3/8" notched where I'm trying to build up floor)..
The plan is granite tile; I've got a decent subfloor & underlayment. Tiles will span the old & new subfloor (next to tub & toilet). Floor otherwise is pretty level. I'm guessing I want to feather out modified thinset very gradually away from the place where the two floors meet? (at least 2 tile widths (12 x 2)?).

Didn't find anything in the Ditra handbook which expressly prohibits this. What do you with experience working with Ditra think?
 

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There are 2 thicknesses of Ditra' HD usuallly carries only the original. But no, Ditra can't be 'floated' over thick thinset, it's meant to be pressed firmly onto the subflooring with a float.
 

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Discussion Starter · #5 ·
you can skim coat the thinset to gain some height. let it cure. Then do the nomal Ditra installation.
Thanks.
Do I use the latex-modified thin set for this skim coat then? (Schluter handbook says latex-modified thin set for placing Ditra in, then unmodified thin-set above the Ditra for setting the tile in).

If I really should be using self-leveling compound I can, but I have a 50 sq. ft. bathroom and I already have far more thin set (both modified & unmodified) than I need, so if I can get away with the thin set build-up, it would be nice. But I don't want cracking, etc. under my tiles down the road!
 

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The choice of modified or unmodified thinset to place Ditra has more to do with what the subfloor is made of than the Ditra itself.

On the other hand, you use unmodified to set the tiles on the Ditra - that's another story..
 

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if you are concerned about cracking then use the modified which will be more flexible. But the cracking will not transmit into the tile since you are using the Ditra which isolates these cracks from the tile. (a decoupling membrane)
 

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Discussion Starter · #8 · (Edited)
The choice of modified or unmodified thinset to place Ditra has more to do with what the subfloor is made of than the Ditra itself.

On the other hand, you use unmodified to set the tiles on the Ditra - that's another story..
Underlayment is 1/2 inch plywood. Schluter says to use unmodified thinset for under Ditra.

I was just wondering about whether it's fine to use unmodified thinset for *building up the floor* (to make up the 3/16" difference between old & new floors). Based on Bob's answer, it seems it's ok.

Sorry for all the questions, but I'm new to this and I'd rather ask questions than do it wrong.

By the way, since the plywood has 1/8" gaps where one sheet joins another, I'm guessing it's fine if that gets filled with thinset?? The gaps are there for expansion so that's why I'm wondering - do the pros avoid the gaps, or is it not a problem for thinset to get in them.
 

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Schluter says to use unmodified thinset for under Ditra.
NO IT DOESN'T - Read it again.

ALSO, did you know that granite tile is thicker than most ceramic tile? Granite tile will be 3/8" thick. Personally I don't think you will have as much variance as you say you will.:)

Use a cheap caulk to fill the plywood gaps.
 

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No, no; in your case, if you are laying the Ditra on top of plywood, then you use modified thinset to do so BUT you use regular, unmodifed to lay the tiles onto the Ditra.

I thought we were clear on this, but obviously not. Look don't mess around with thinsets - they are suggested for very good reasons and your job is to take the advice given - and apply.
 

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hope you meant your underlayment is 1/2" not the subfloor
Tpolk raises a very good point. Is 1/2" plywood all you have on top of the floor joists? I don't think that's enough to support marble without flexing a lot and cracking up your grout joints but you should probably have 3/4" plywood as an absolute minimum.

I'm definitely not a pro though, so maybe someone with more knowledge could chime in on this...

Also, after reading tons of forums on this, the choice of modified or unmodified thinset depends on the two layers of material you are putting it between. Unmodified thinset will cure completely in a couple of days between two layers of material that barely breathe (i.e. concrete, ditra, and tile). Modified thinset will take a month or so to completely cure in between two layers of material that barely breathe. So if you're using modified thinset you should be applying it over something like plywood that will allow the required evaporation to occur. Unless, of course, you are ok with staying off the tile for a month.

And don't use cheap unmodified thinset, it should cost somewhere between $10-$20 per 50 lb. bag. HD sells CRAP for $5, don't get that (just paraphrasing what I've been told).
 

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Discussion Starter · #14 · (Edited)
Thanks for all the replies.
I misspoke in my most recent message, prior to this one (I said it right in my first message): Under Ditra, use Latex p.c. thinset (or modified); on top of Ditra & under tiles, use unmodified. I'm clear on that aspect, Ccarlise; p. 25 of the Ditra Handbook really lays that out nicely.

As for questions about floor under tile: underlayment is 1/2 plywood, per Ditra's specification (those specifications are "underlayment – minimum 11/32", 3/8" nom.(10 mm)-thick Exposure 1, plugged-face plywood or OSB with 1/8" (3 mm) gap between sheets")

The SUBFLOOR (under the underlayment) is 3/4" plywood.

Thanks for the concern - I don't understand all the ins & outs (that you learn with years of experience) but I am paying close attention to the Ditra Handbook. Where it spells things out clearly, I am fine.

But, I appreciate the feedback regarding taping joints. But, what do I tape them with?
Can I use the same tape on plywood underlayment that is used on Cement Backer Board? (Home Depot thinks I can ...)
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?isShowFreeShipping=true&jspStoreDir=hdus&amtLeftForFreeShip=249.00&catalogId=10053&productId=100645273&navFlow=3&keyword=underlayment+seam+tape&isOrderQualifiesForFreeShip=false&langId=-1&searchRedirect=underlayment+seam+tape&storeId=10051&endecaDataBean=com.homedepot.sa.el.wc.integration.endeca.EndecaDataBean%40153d32ff&ddkey=Search

What's better - tape or caulk for the joints? Or is it not a big deal?

I bought the Mapei Ultraflex 2 Tile Morter with Polymer (latex portland cement mortar), which meets ANSI A118.11 specifications, per Ditra specifications (over $20 per bag).
 

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Discussion Starter · #16 ·
ALSO, did you know that granite tile is thicker than most ceramic tile? Granite tile will be 3/8" thick. Personally I don't think you will have as much variance as you say you will.:)

Use a cheap caulk to fill the plywood gaps.
I'm not sure about what you're saying in the first point. My granite tile is going on both the old underlayment/subfloor and the new underlayment/subfloor - perhaps you thought it was going on only the new subfloor/underlayment?

I am clear on the issues about mortar (see my message which immediately precedes this one).
 

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There will be added thickness from the thinset used to install the DITRA, hard to say exactly how thick.

There will be added thickness from the thinset used to install the tile, hard to say exactly how much.

The tile will be thicker than standard ceramic tile by about 1/16". I'm sure you'll right were you need to be when all is said and done. If not...you can always fudge the tile a little if need be.

Plywood does not get taped, only use caulk to close the joints and keep thinset out. DITRA does not get taped, unless you are waterproofing it. To waterproof DITRA seams, KERDI Band is applied using thinset.:)
 

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Discussion Starter · #19 · (Edited)
An update on regarding using slightly more thinset under Ditra to make up a small difference in height of 2 subfloors (I asked about this earlier in the thread): I installed the Ditra before the reply about small humps resulting if you do this. In fact, that's what happened. I think if anyone tries use extra thinset under Ditra as I did: (1) think hard about using self-leveling compound (I guess I could have done that although there were reasons why it could have created its own problems); (2) if you do it, really press down with the float to try to get the thinset spread out well under the Ditra; (3) do NOT walk or place anything on the Ditra after step (2) until the thinset is set up. I DID do the skimcoat of thinset & let it cure before the installing the Ditra - but I must have gotten my thinset too thick when installing the Ditra.

The humps haven't been a particular serious issue, although I did decide to go with the trowel with the deepest teeth recommended by Schluter. (Disclaimer: this is based on my one experience with using Ditra & installing granite tile). I also finally tracked down a dealer that could get Kerabond for between the tile & Ditra.
 
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