DIY Home Improvement Forum banner

Modified Adhering to Unmodified Thinset

8711 Views 34 Replies 4 Participants Last post by  nltkocur26
I'm currently redoing my master bathroom. I purchased a Schluter shower kit to use for the shower pan. Per directions, I used an unmodified thinset to adhere their waterproof membrane to the backer board walls. I was not comfortable with how I adhered them, so after a few weeks, I pulled the membrane off the wall. I've decided to go with Redgard to waterproof the walls and use the Schluter shower pan.

I was told that I should fill in any gaps with modified thinset over the unmodified thinset to smooth out the wall before using Redgard.

My question is can I used a pre-mixed modified thinset over the unmodified thinset to smooth it out? Will it adhere properly?
1 - 20 of 35 Posts
Why on earth would you scrap the Kerdi system? What was it exactly that you weren't comfortable with?

What do you mean by "gaps" exactly? And what do you mean by "pre mixed"? Just that the latex additive is already in the powdered thinset?

There is no problem with adherence of modified or unmodified thinset to each other in general.
I don't want to scrap the entire system. Only the membrane on the walls. My concern was that when I originally mixed the unmodified thinset, I mixed it too thick, and covered too much area, so as I put the Kerdi up, it was already beginning to dry. I found some air pockets, so after going back and forth, I pulled it off the wall. While there was a lot of areas that were adhered properly, there was an equal amount of Kerdi that wasn't adhered well, so it pulled right off very easy.

I will still use the foam shower pan, but I'll use Redgard as the seal on the walls. I just have to fill in a few gaps including the trowel marks to make the wall smooth again.

By pre-mixed, yes, I meant where it's already to use, not the powder. Just wasn't sure how a pre-mixed would adhere to unmodified vs a modified that I would first have to mix. I figured it was fine, but just wanted to be sure.
See less See more
Pre-mixed thin set is NOT thinset, it's mastic. Manufacturers have a play on words but, thinset by definition must contain Portland Cement. You can NOT store Portland once it's mixed. Thinset mortar only comes in a bag, it's powder.

You should not use that pre-mixed stuff in wet applications and definitely not under/over Redgard or any other liquid membrane.

Too bad you had problems mixing and installing the Kerdi on the walls. How far up the walls from the floor will you be flashing the Kerdi now?

I don't plan on using the Kerdi membrane on the walls moving forward. I'm going to Regard the walls from the floor to the ceiling, and then put the Schluter shower pan in, and use the Kerdi strips to seal the edges around the pan to the Redgard covered walls.

I will use modified thinset to fill in the unevenness from the unmodified thinset that is already on the wall. Then Redgard over that.
Yes I realized what you're going to do now. I mentioned "flashing" for that reason. May not matter much, but I'd install the Kerdi on the shower floor then Kerdi Band the corners and finish with the Redgard. That way you can use the correct thinset for the Kerdi Band.

This might not matter in practice, but I called Schluter (and Mapei) about a similar product that Mapei makes (similar to Regard). They said they don't recommend using Kerdi over it because Kerdi required unmodified thinset and the product required modified. Also, I don't know if the Kerdi will be waterproof over Redgard as opposed to drywall or cement board.

Like I said, might work fine in practice, but I doubt you'll have any warranty coverage on it.
Right Jeff, and that's why I said to install the Kerdi first, (unmodified thinset), then apply the liquid membrane. This way the only non compliant part is bonding the tiles to the last few inches of the wall over Kerdi Band using modified thinset.

So just for my clarification as a a novice, what you're saying is to first put in the shower pan and drain, then put Kerdi on top of the pan, and seal the seams with Kerdi band using unmodified thinset, And then smooth up the walls of the current dried unmodified using a modified as a filler, letting that dry, and then going over the walls with Redgard up to the pan?

Thank you all for your help, as I've been learning a lot through this process, and your knowledge really helps.
My only point is to install the Kerdi Band flashing using unmod. before applying any liquid membrane. You can do the shower floor at the same time or later. So, yes, you've got the idea.

What exactly do you mean when you say flashing?
Flashing is what you're doing when you apply Kerdi Band on the inside corners to make them watertight.

Gotcha. So once I do that, can I redgard right up to the shower pan and go over the wall part with the Kerdi Band?
No. You'll have the Kerdi installed already, and flashed up the wall a bit. Then you'll apply Redgard on the walls to meet up to the Kerdi Band flashing.

Is it not safe to apply the Redgard over the Kerdi membrane, even if it's only a few inches just above the shower floor?
It shouldn't hurt, but if you can just butt to it you'll avoid the dreaded Kerdi thickness build up. Regular Kerdi is 8mm, Kerdi Band is 4mm, plus the thinset. So thinset + Kerdi + Redgard can create an extra little hump.

I understand. So that being said, since about half of a wall already contains a layer of unmodified thinset, should I then thinset the remainder of the wall with a modified, just so it will even out, so I'm not applying the Redgard on half backer board and half thinset?
Why is half the backer coated with a layer of thinset?

That is where I originally had the Kerdi membrane up before pulling it off the wall. I never finished the full wall.
Oh yes, the aborted Kerdi instal........ Have you tried to scrape with a flex putty knife yet? I'll bet most of the thinset will just come off.

1 - 20 of 35 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.