kerdi and unmodified thinset [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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11-09-2012, 01:18 PM

when installing the kerdi membrane, what would happen if i did NOT use an unmodified thinset mortar as recommended by schluter...

my tile setter wants to use his tried and true thinset when installing my 12x24 marble tile over the kerdi membrane...

is this a bad idea?


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Tool Guy - Kg
11-09-2012, 01:25 PM
You are in danger of the modified thinset (which cures differently than unmodified) not curing properly in a sufficient amount of time.

If this contractor isn't comfortable using unmodified thinset, I would inquire to his experience level with Kerdi and with tiling in general. Perhaps it would be helpful if you told us a little about your project. While I'm not saying that the following is certain, but this type of question you've asked is often followed by other prep steps done incorrectly and we would be happy to review what has been done so far. It's apparent that you need some confirmation and you're on one of the best tiling knowledge sources anywhere. Tell us what you want to build and the materials you are using.


11-09-2012, 01:57 PM
i'm tiling a 32x60 stand up shower that has a shower base already installed... the shower base a flange and there's permabase on the walls already...

actually, i'm not going to do the tiling but my contractor is... he has 20yrs experience but none with kerdi... hence these questions...

i'll be using marble tile 12x24 in a subway pattern... there will be a marble corner shelf...

he's actually installing the kerdi as i type with the unmodified thinset... how long should he wait before these heavy tiles go up?

thanks very much for the warm response.

11-09-2012, 03:00 PM
Welcome, Dan. :)

You might suggest that your tile man familiarize himself with the Kerdi System before he continues, since he's the one installing the product, especially doing a full shower with it. If this is part of your "how much should it cost" thread, I'd say you're paying far too much if you've hired a contractor who is not even willing to read and follow the product manufacturer's installation instructions. :shades:

There is no need to wait at all between installing Kerdi and setting tiles.

My opinion; worth price charged.

11-09-2012, 03:24 PM
There are a few key points on installing Kerdi, and they're all covered in the installation manual available free on the Schluter website. But, you do need unmodified (good quality, not the cheap stuff - it does make a difference!) that is mixed a little looser than 'normal'. It must still retain the ridges from the trowel, but you don't want it dry and tearing when spreading the thinset out. The fleece must be well embedded in the thinset and you need at least a 2" overlap to get a waterproof seam. Once you get the hang of it, it's pretty easy and the learning curve isn't bad.

Kerdi is essentially waterproof. A modified thinset has two different things that must happen for it to reach its full strength - it must first dry the modifiers out, and the cement component must cure. Until both happen, the thinset does not have adequate strength and the bond can be broken fairly easily because it will still be 'soft'. An unmodified thinset only needs to cure, and it will do that without any air or drying (in fact, it will create a stronger bond if it stays wetter while it cures, which is a chemical change, not a physical drying thing like drying your clothes after washing). So, you risk the modified thinset taking literally weeks to properly dry, if then, to achieve its full strength when used on a waterproof membrane as any moisture must evaporate through the small grout lines or literally through the tile, and that can take a very long time.

The difference in ultimate strength between modified and a (good) unmodified is there, but not huge. The wall would fall down before you broke the bond with either a modified or unmodified, if it is installed properly.

Tool Guy - Kg
11-09-2012, 09:51 PM
Hi Dan,

And just to add a couple thoughts to Jim's:
Please make sure nobody does the "peel" test in the next few days. It seems to be the default test everyone loves to do to "make sure the Kerdi is sticking". But a peel test is quite inappropriate for two big reasons. First, thinset takes awhile to develop a strong bond. While 28 days is its full cure, it gets pretty strong in a week. And most folks that come onto the forum asking about why their Kerdi peels off so easily tried peeling after only a day or two. And secondly, peeling doesn't mimmic the force the membrane will go through when a stiff tile is hanging on the wall. If you fall into the trap of'll find that it indeed DOES peel off. Don't do it. It will only make you nervous.

But Kerdi will stick and perform very well if the directions are followed. A high-quality unmodified thinset needs to be used like: Hydroment's DitraSet, Mapei's Kerabond, Tec's FullSet Plus, or Laticrete's 317 (all of which are mixed with water). We highly suggest you avoid cheap $6 bags of unmodified thinset. Custom's CustomBlend would fit this description and hopefully he isn't using this. The thinset needs to be mixed properly and allowed to slake and be remixed. The cement board or drywall should be wiped with a damp sponge before the thinset is spread to help keep it from prematurely drying out. Make sure the thinset doesn't skin over before embedding the Kerdi membrane into it with a flat trowel. Make sure the overlapping seams are embedded.

Good luck. :)

Oh, and can I ask what kind of marble tiles you're using?


11-10-2012, 12:30 AM

i'm using 12x24 white carrara turkish tile... it's pretty nice...

he installed the kerdi and it looks ok but there's an outside corner that seems "unattached" to anything underneath... i'll assume it's because the drywaller didn't have a proper edge there... hope it's not going to cause any issues...

now can he start tiling tomorrow? i know schluter says you can tile right away but with large, heavy tile don't i know want the kerdi to adhere somewhat to the cement board?


11-10-2012, 08:49 AM
Dan, "large, heavy tile" has the same or greater bonding surface area per pound as does small, heavy tile. Don't worry about it.

Do worry about the seemingly unattached area of Kerdi. While it may not make a real difference when installing your large format tiles, it's not a good idea to have any of the membrane unbonded. Perhaps a photo of that area would help.

My opinion; worth price charged.