Problem with Ditra [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-27-2011, 04:55 PM
I'm in the process of laying 12x12 granite tile in by 80 sf bathroom.

Base is post tensioned concrete slab with some cracks. I poured SLC to level it (up to 5/8" thick on one side), placed ditra on top because of cracking and was about 3/4 done tiling yesterday before stopping.

When laying the ditra, I had to wet the SLC base quite a bit to keep it from sucking all the water out of the thinset before it embedded into the ditra.

Today, I noticed that in intersecting corners of 4 tiles and about 4 inches back, it sounds hollow when tapped. The rest of the tile sounds solid. I also noticed another small area where I haven't layed tile yet where the ditra sounded hollow and it wasn't completely embedded in the thinset below.

I'm assuming the hollow tile is another non-attached area and I'm wondering what would be the best way to correct it without tearing out too much?

The tile has been down for about 24 hours now.



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03-27-2011, 05:42 PM
Welcome back, Bob. :)

Couple issues I see there.placed ditra on top because of cracking andDitra is not advertised as a crack isolation membrane. I has some properties as such, but that's not what it's designed or tested for.

Did you use the SLC as prescribed by the manufacturer, including cleaning the concrete and using the appropriate primer? I'd be as concerned about the bond of the SLC to the slab as the bond of the Ditra to the SLC.I had to wet the SLC base quite a bit to keep it from sucking all the water out of the thinset before it embedded into the ditra.Just how wet are we talking here? And which SLC and which thinset mortar are we dealing with?

Give more hints and we'll see can someone help you figger it out.

My opinion; worth price charged.

03-27-2011, 06:24 PM
Hi CX, Thanks for the quick reply.

I used a hand grinder to rough up the slab and get rid of any paint from the original construction, cleaned and primed as recommended before pouring the SLC. The SLC pour went fine and other than priming and building up one small 9sf area a little more, I haven't had a problem with the SLC.

The SLC was the LevelQuik ES but I needed another 2 bags of LevelQuik RS to finish, all poured at the same time.

I used Mapei Keraset to put down the ditra and granite and used a very wet sponge to wet the SLC before putting it down. No standing water, but I had to go over it a couple of times with the sponge.

Since I found one area of ditra that didn't get embedded completely and sounded hollow, I assumed that this is the problem. I just don't know what will happen if I try to pull out the four tiles to replace that small section of ditra.

03-28-2011, 07:18 AM
You could certainly be correct about the source of your problem, Bob. Only way to verify it is, of course, to remove the hollow sounding tiles.

One suggestion I would make is for you to get a better grade of thinset mortar. The KeraSet you're using is the absolute bottom of Mapei's line and not the best choice. Yes, it technically meets the requirements, but it's not a "good quality" dryset mortar as described by Schluter for installing the Ditra.

And while it may finally cure to sufficient characteristics, there's little question that it'll be more difficult for you to use properly than would a better mortar from that line or another brand. Place selling KeraSet should also have access to KeraBond, an excellent dryset mortar from Mapei. DitraSet, by Bostik, is another really good choice if you can find it.

Worst case, get some VersaBond from Homer. It doesn't technically meet Schluter's requirements, but it's known to work quite well in your application. You'll have no Schluter warranty, but, as you're seeing, any problems you have are not gonna be warrantied problems, anyway.

My opinion; worth price charged.

03-28-2011, 07:03 PM
So much for calling Schluter's help line....they're the ones that told me to use KeraSet. I had just picked up another bag too. Oh well, I'll take it back and pickup the KeraBond and use it for the rest of the install.

Any advice on the best way to bust out a 12x12x3/8" granite? I planned on just cleaning it up around the edge, covering it with a towel (so a chip doesn't destroy the glass shower) and busting it out with a hammer and chisel. Would it be better to score it with a grinder and diamond blade or would that be a waste of time?

Thanks for all your help.


03-28-2011, 07:36 PM
Bob, the KeraSet meets the ANSI standards for an unmodified thinset mortar, A118.1, and as such meets the requirements for installing Schluter membranes. Schluter is unlikely to tell an unknown caller not to use a setting products manufacturer's mortar if it meets the published requirements, even if the rep you talked with had an opinion on the subject. Very bad for bidness, that.

We here have a little more leeway and feel an obligation to help our visitors the best we can even if it means ruffling some feathers at times. And we don't discriminate at all amongst the companies discussed here.

Keep in mind I'm not saying KeraSet won't satisfy the requirements of your installation, I'm saying you're much more likely to have a problem making a good installation with it than you would with KeraBond.

Mapei makes a superior product in KeraBond and that's pretty well known. I recommend you use it.Would it be better to score it with a grinder and diamond blade or would that be a waste of time?Not a waste of time at all. If that tile is set over your Kerdi, you're gonna wanna be as careful as you possibly can in removing it, and scoring it deeply before you try to break it up is good thinking.

My opinion; worth price charged.