confusion about ditra and unmodified mortar

[DonH]

Got a quick question about installing Ditra membrane. I've used modified thinset to stick the Ditra to my ply floor. I'm now to use unmodified thinset to bond the ceramic tiles to the Ditra. Schluter says that the unmodified mortar is needed because:

Latex-modified mortars must air dry for the polymers to coalesce and form a hard film in order to gain strength. When sandwiched between two impervious materials such as Schluter -DITRA and ceramic tile, including porcelain tile, drying takes place very slowly through the open joints in the tile covering. [Might take from 14 to 60 days to cure.]

[But] Portland cement-based unmodified thin-set mortars are dependent on the presence of moisture for hydration in order to gain strength. Since Schluter -DITRA is impervious, it does not deprive the mortar of its moisture. This allows the cement to properly hydrate, resulting in a strong, dense bond coat. In fact, after the mortar has reached final set (usually within 24 hours), unmodified thin-set mortars achieve higher strengths when cured in continually moist conditions.

My question concerns the unmodified thinset. When I look at the mortar specs, none of them say anything about bonding to polyethylene, the stuff that Ditra's made of. And there are substantial warnings about making sure you use latex additives if you want to bond to exotic things like vinyl, linoleum, etc. Maybe there's no bonding at all -- maybe the mortar is instead grabbing those little square cutouts in the Ditra.

Should I worry? Or should I just plow ahead with straight unmodified mortar?

Many thanks ... the expert advice from this forum is terrific!

Don

[jdm]

Don --

All of those square recesses in the Ditra are shaped like dovetails -- wider at the bottom than at the top. The thinset mechanically locks into those spaces and holds firm.

And although Schluter insists on unmodified thinset over Ditra some of the pros here have found that lightly modified thinsets (like Custom's VersaBond) work over Ditra, too.

[Mike2]

Don, here is a cross-section drawing depicting the Ditra cavities that lock the thinset mortar into place. Nothing is actually going to stick to Ditra, mechanical bond only.

[DonH]

Thanks, Jeff and Mike. I had no idea that there was no chemical bonding going on at all. I'll get on with it as soon as the wind-chill is above the single digits.
Don

[sleepy hollow]

I have to say, this is the first I have heard of the spec to use unmod'd thinset on the DITRA. I have done three floors with DITRA, one using the older ribbed version about 4 years ago, and 2 using the newer grid version. I used CBP premium Flexbond for all three and they are rock solid after all this time.

Since I am a DIYer, I rarely get to grout until a week or two after setting the tile, so that may have inadvertently helped in the curing, but I honestly read everything I could find about DITRA and never saw this mentioned two years ago when I did my last floor. In fact the video on their site used to show the fellow using apparently the same bucket of thinset for laying the DITRA and then tiling.

Is this relatively new?

[bbcamp]

Un-modified thinset has been in the Ditra installation handbook since 2004.

[sleepy hollow]

Ok, well that explains it. My installations were clearly before that. Schluter must have learned something to lead them to change the spec.

I have to add, though, that the bond strength of unmod'd thinset would be considerably less, especially for porcelain tiles, at least accoring to the data sheets for those products. Interesting tradeoff it would seem.

So, now if I use a waterproofing membrane on top of CBU, will the same issues be present? Should those installations also use unmodified thinset?

Alternatively, if I wait, say 30 days, between setting and grouting, does that change the equation and make mod'd thinset more acceptable?

On the other hand, since all cementitious products, including grout, are permeable, then why wouldn't the whole installation (thinset, tile, and grout)eventually cure properly anyway, even with modified thinset? Cetainly the water will evaporate eventually, yes (assuming I do not seal it for 30 days or so)?

Just some thoughts on my part. I am sure I am missing something. I am no chemist, that's for sure.

[Mike2]

Sleepy, those are all good questions that cannot be addressed adequately in a single Reply. They have incidently come up before and we've hashed this out dozens of times over the past few years.

Here are just a few of many threads on this very subject. Makes for some interesting and informative reading. You can find dozens more by doing a search on "Ditra and Modified"
.
http://johnbridge.com/vbulletin/show...ditra+modified

http://johnbridge.com/vbulletin/show...ditra+modified

http://johnbridge.com/vbulletin/show...ditra+modified


Happy reading.

[madmax]

Originally Posted by Mike2

Don, here is a cross-section drawing depicting the Ditra cavities that lock the thinset mortar into place. Nothing is actually going to stick to Ditra, mechanical bond only.

If the top layer is impervious and the bottom layer is impervious and the dovetails mechaincally bond the thinset to the bottom layer then what bonds the thinset to the flat top layer?

[Mike2]

Good question max. Ditra has an anchoring fleece laminated to its underside. This fleece becomes fully engaged in the thinset mortar forming another mechanical bond.

[sleepy hollow]

Thanks, Mike2. I had no idea. Should have thought to do a search. Very good discussions.

[DonH]

Mike, thanks for the pointers to those other threads. I have searched this forum for hours and felt that I had seen every pertinent post. But I had not seen any of those. Very useful. And it's also useful to see that there can be valid differences of opinions between the experts.
Don

[madmax]

Originally Posted by Mike2

Good question max. Ditra has an anchoring fleece laminated to its underside. This fleece becomes fully engaged in the thinset mortar forming another mechanical bond.

I am referring to the bond between the tile and the thinset. The bottom is Ditra which is impervious but the dovetails provide a mechanical bond. On the top their might be glass or porcelain which is also impervious but their is no dovetail. What is providing the bond to the tile? The manufacturers of those products recommend using latex to insure a good bond.

[jdm]

In post #22 of the first thread Mike posted links to Dave Gobis addresses the porcelain and dryset issue.

Glass would be another story.

[AnnieKay]

Was most interested in the discussion in this thread as farther down in the posts on today's forum threads I posted a directly related question - please see my post on the 2nd page of the thread under title "masonite subfloor concern" by AnnieKay. Your points about pros having their different ways of proceeding may apply? Would appreciate very much your wisdom on this, all who weighed in, Thanks, Annie (I have e-mailed Schluter but haven't yet received a reply)