Replies

1. 
   fingers | Mar 25, 2010 05:29pm | #1

   I'd follow the manufacturer's directions.  I haven't installed Kerdi but was under the impression that the thinset under the kerdi was one way and under the tile the other.   IOW if unmodified under the kerdi then modified above it. or is it the other way around. . . anyway read the directions again.

2. 
   sapwood | Mar 25, 2010 09:51pm | #2

   Following advice given on the John Bridge forum, I used Versabond which is a semi-modified thinset. There have been numerous tales of woe by people who have used heavily modified thinsets, so think twice before using it. For more advice read up on johnbridge.com.

1. 
   weemalky | Mar 29, 2010 12:37pm | #3

   Thanks for directing me to that site, isn't the internet great? I received 8 replies in a couple of days from guys who strictly do tile.The problem is voiding the Schluter warranty, but if people who do these systems all the time have success with that method, I trust real world advice.

3. 
   plumbitup51 | Mar 29, 2010 12:51pm | #4

   Most of the thinset manufacturers have rapid set modifieds, which my Schluter rep tells me are fine with their product when you are setting stuff where modifieds are recommended, like glass and porcelean. The rapid set I use behaves just like their other thinsets, has similar open time, but when it goes it goes fast.

4. 
   andybuildz | Mar 29, 2010 03:07pm | #5

   You CAN use Versabond which is "lightly" modified. It's not to spec by the company but even John Bridge'll tell you, you can use it. I've done it countless times with success.

   You use unmodified when the surface behind Kerdi or Ditra isn't breathable...like between tile and Kerdi.

   If it's between Kerdi and a CBU or plywood or concrete then you can use modified. The moisture needs a place to evaporate/dry next/in/to but between unbreathable spaces you need unmodified.

1. 
   weemalky | Mar 30, 2010 10:46am | #6

   From what I gather in the specs, unmodified dries without needing air(breathing), whereas modified needs up to 28 days depending on conditions. Since Kerdi  is watertight and doesn't breathe, if you put modified on the substrate/wall  then cover it with Kerdi, how does it evaporate? Also aren't certain tiles and grout breathable,porous? Is it all engineering gobbledygook? I just wanna get 'er done.

1. 
   plumbitup51 | Mar 30, 2010 01:04pm | #7

   Once you set tile over Kerdi or Ditra, It can only dry through the grout joints, which is why it can take so long, depending on the amount of joints, their size , permeability of the tile, etc. Setting Kerdi over a substrate like plywood or CBU,  the moisture can migrate into the substrate and eventually dry out the back side.

   The rapid sets have the benefits of being modified, but have some sort of chemical trigger to accelerate the whole drying thing.