Kerdi membrane questions

8 years ago

Hi all,

I have immersed myself in forum reading on this topic and it has been so helpful. I have a few questions that seem to get different responses and one I can't find answered.

My husband and I bought a home that hasn't been updated since it was built in 1950. We are remodeling while we live here with our three children, so...that's fun. Ha! We are concentrating on the bathroom at the moment. We had a shower line put in as there was only a tub spout in the one working bathroom we have as of now. The walls are plaster over a layer of drywall (not lathe...weird, I know). Now we are faced with waterproofing the walls which have for all their existence been painted with semi-gloss and left alone. They aren't in bad shape, and we didn't want to rip out the walls to the studs. Instead, we discovered Shluter and bought Kerdi membrane to install over the painted plaster walls. However, I used joint compound over a layer of plaster of Paris when trying to even out the walls that had been cut into during the plumbing work described above. (There was also work done around the tub spout, so that is plaster and joint compound as well.) Additionally, the space between the tub and the wall is about 1/4 - 1/2 inch wide in some places and is definitely not flush despite putting in a layer of silicone. I have attached a picture to help. I did learn that I need to prime the joint compound before using the thinset, but I have several other questions.

1) Can I use a lightly modified thinset (versabond) instead of unmodified? (I know this will void the warranty, but will it hold better?

2) The plaster wall substrate is not even, particularly where the wall meets the tub, as mentioned. Will this cause a problem when tiling or will the Kerdi membrane help create a flush surface for better tiling?

3) Should I score the painted wall before applying the thinset to help it adhere better?

Thanks in advance!

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Comments (4)

MongoCT
8 years ago
If you have a 1/4" to 1/2" gap at the tub that needs to be filled, consider installing a veneer of cement board on the walls. That would cover the gap and give you a pristine surface to work off of.

The negative to that will be twofold; you'll need to consider the detail at the edge of the tile on the two end walls with regards to hiding the edge of the cement board, and you'll want to make sure your valve trim kit can accommodate the extra thickness of the cement board.

The cement board edge can be hidden with a tile trim piece, a return, or with a wood trim boarder that melds with the other trim in the bathroom.

The semi-gloss paint on the walls is a big negative for me. Thinset is less likely to develop a bond with it, and because of it's permeability, semi-gloss can create difficulty with modified thinsets drying versus an unmodified curing.

With that info given, I'll try to answer your questions:

"1) Can I use a lightly modified thinset (versabond) instead of
unmodified? (I know this will void the warranty, but will it hold
better?"

Modified will develop a stronger bond then unmodified in a pristine installation. But you could encounter drying difficulties by sandwiching modified thinset between the semigloss paint on the wall and the kerdi membrane.

2) The plaster wall substrate is not even, particularly where the
wall meets the tub, as mentioned. Will this cause a problem when tiling
or will the Kerdi membrane help create a flush surface for better
tiling?

The kerdi will follow the lines of whatever it is installed upon. If the wall is wonky, the kerdi will be wonky. if there are just a few recesses on the wall, they could be built out to the proper plane with applications of thinset or mortar feathered into those areas. Let the thinset or mortar cure, then apply the kerdi. If the recesses are deep and numerous, it might be easier to skin the entire wall with cement board, shimming the board out as needed to get the walls in plane.

3) Should I score the painted wall before applying the thinset to help it adhere better?

Yes. Especially with a semi-gloss paint. I'd recommend you go further than that and attack the paint with high-grit sandpaper to really scuff it up, or to even remove it. If you end up with a variety of materials on the wall when you are done (some semi-gloss paint residue, some thinset, some drywall mud, etc) it might make sense to prime the wall in its entirety. Let the primer fully cure, then thinset and Kerdi over the primer.

You recognize yours is a messy installation based on your existing substrate. Some plaster. Some semi-gloss. Some drywall mud. Some thinset. Working off a Frankenstein base like that offers no guarantees. That's why my advice, especially considering you have that gap at the tub that needs to be bridged, is to veneer the wall with cement board.

Since your walls seem to be a thin plaster veneer over blueboard and not a thick plaster over lathe or a thick multi-coat plaster, you could even strip the walls to the studs. It sounds like a lot of work, but you may very well spend more time trying to make your existing walls right without them ever really being right, versus going to the studs and making them right. Get to the studs. Shim the studs out if needed so the three alcove walls are all in plane. Cement board. Kerdi. Tile. Grout. Done.

Shortcuts sometimes take longer than what seems to be the more arduous path.

I HIGHLY recommend you contact Schluter tech services. I can somewhat assure you that because of the hybrid nature of your walls, they'll not guarantee you anything. But they can offer you another perspective.

Good luck!
mrs_laurayankee thanked MongoCT
mrs_laurayankee
Original Author
8 years ago
Thank you so much for the advice! You are right in that we have already spent a lot of time trying to get the substrate ready and I'm still highly doubtful it will be a success. I will contact Shluter today.
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MongoCT
8 years ago
If photos will enhance the discussion, have a file of photos ready when you call schluter. You can email them to the tech as you speak to him, or you can refer him to a hosting site so he can see your photos as you have your discussion.
mrs_laurayankee thanked MongoCT
bjb817
8 years ago
I agree that a coat of primer throughout would be the way to go and then just use unmodified thinset per Schluter's recommendations. Whatever you do, don't go over raw plaster or joint compound!
mrs_laurayankee thanked bjb817

Kerdi membrane questions

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Kerdi membrane questions

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MongoCT

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