So, here’s my dilemma. Have a 59” wide acrylic shower base going into a 59 ¼ opening (drywall was already on all 3 walls). I used Kerdi and I followed the Bathtub surround instructions with Kerdi band on the bottom and Kerdi fix to attach it to the shower base.

In order to ensure the tiles would install over the lip on the shower base, the manufacturer recommends building out the walls above the lip of the shower base such that the wall is flush with the lip prior to installing tiles. Then when the tiles are installed they come down over the lip (with silicone on the bottom to seal the bottom edge of the tiles to the shower base.)

Now my issue… I’m concerned with having used plywood (exterior grade) as the substrate to build out my walls. I think I made a mistake when I read some of the instructions on the Internet (I know, my own fault). I’m a bit of a DIY guy and never for a second thought that using plywood on the walls would be an issue for the tiles. In fact I thought plywood would be a better choice than gyproc. The plywood is installed tight to the wall (gyproc underneath) and was nailed very well. I know some threads talk about “movement” but I’ve install tiles on plywood floors 5-6 times and never once had an issue.

I did some research and decided to use quality products. I used Laticrete 255 modified mortar between the Kerdi membrane and the plywood ( MY key problem is I mistook the DITRA instructions that indicated to use modified). Then I sat back this afternoon and was reading the Schluter Shower system installation handbook and got a “sinking feeling”.

I’ve got a lot of hours into this already but I’m very concerned now about putting the tiles on over the Kerdi/plywood.

I have a few questions:

1. Am I likely to have problems with the plywood given it is now fully covered in Kerdi.
2. I know I was supposed to use unmodified mortar between the Kerdi and the substrate but I mistook the DITRA instructions and used Laticrete 255 modified. Am I likely to have problems ? Will an extended drying time help ?
3. The Kerdi seems very well adhered to the plywood. Is there a likelihood this would change over time (i.e. would tiles start falling off the wall !!)
4. The Laticrete 255 instructions state it is suitable for use over exterior grade plywood (that is what I used to build out the walls). Am I safe ?
5. I realize my mistake and would use unmodified between the Kerdi and tiles if I’m safe to go with what I have there now.

Any other suggestions about where to go next ? I cannot stomach the thought of having to tear the plywood off the walls and start over again. But I also don’t want my tiles cracking or the grout cracking in a few months if I forge ahead.

And yes, I know, read the manual next time ….

Randy

I think that you don't need to worry. The modified thinset will cure under a membrane, it just takes longer to do so. The main problem tends to be that when you start setting the next day and go to pull a tile off the wall the kerdi comes with it. The other issue is that any warranty from schluter is void.

Yes, you made a mistake, but if you wait a few days to start setting everything will be fine. As for the plywood, it is less dimensionally stable than drywall and cement boards, but unless you have large humidity swings in your wall cavity it will not move.

I agree with Pete, you should be Ok as is.

However, in # 2 you said;



Good thing you used modified to bond to the plywood. Had you used unmodified, (as you think you should have), I would be telling you to start over now. For both Ditra and Kerdi you have to use a "substrate appropriate" thinset, and unmodified will definitely not work over wood.

Jaz

Kerdi is not approved over plywood at all. I would worry about tiling over an unstable backer board like plywood.

Did you use 1/4" plywood?

Better check with Schluter since I sure they will have issues with this.

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