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Unread 07-09-2022, 03:42 PM   #1
fish2222
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Kerdi Advice

So, I thought I did all of my research, but after installing 3 sections of this (2 more to go) I feel like it's not as strong as it should be. It's been a week since I installed it, and I have attached a picture of what the amount of coverage is after it has dried for the most part. So, my first mistake apparently was using Customblend unmodified thinset, since all the instructions I found (even from Schluter) say any unmodified thinset meeting a certain Ansi number, which it does. I am however installing over sheetrock, and I did wet the area down with a sponge before hand so it wouldn't dry it out prematurely. The Kerdi is one uniform color everywhere, so I think it's all excatly like what I pulled up for the picture. Maybe I am being worried for nothing, but I thought I should ask before I finish this project and start putting up tile. Thanks everyone!
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Unread 07-09-2022, 05:31 PM   #2
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Welcome, Mike.
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Originally Posted by Mike
...since all the instructions I found (even from Schluter) say any unmodified thinset meeting a certain Ansi number, which it does.
Yeah, that's a problem. Schluter usta say in their literature that it should be a "good quality" (or words to that effect) un-modified thinset mortar meeting ANSI A118.1, but even that was no real help. The mortars are advertised as either meeting that standard or not. In truth, some meet it quite handily and are fairly easy to use; others, not so much. In my opinion, Custom Blend is clearly on the side of not so much. I have no doubt that batches of it have been made and tested that do, in fact, meet that standard. Other batches, again, not so much. Can I prove that? No, I cannot. See my warranty information below.

I see nothing wrong with what I see in your photo except for a guy trying to peel the Kerdi off his wall. Don't do that! The required bond strength of the thinset mortar to the Kerdi membrane is a shear bond of at least 50 pounds per square inch. No pull bond requirement is specified and except for a ceiling application, your tiles will not try to pull the Kerdi off your wall. And neither should you.

I would recommend you switch to a modified thinset mortar meeting ANSI A118.4 in lieu of that Custom Blend. Wherever you purchased the Custom Blend will also sell Custom's VersaBond, which I assure you will work just fine with your Kerdi and be much, much easier to use. I know that; many other users of Kerdi Know that; Schluter knows that. Now you know that.

The only tile related application for which I'd recommend you use the Custom Blend, if you must, would be as a bedding mortar if you install CBU on a floor application.

My opinion; worth price charged.
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Unread 07-09-2022, 06:00 PM   #3
fish2222
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Thank you. And now I do know. I will definiltey look into the other brands and go that route. I was just more worried about the tile pulling it off the wall at some point.
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Unread 07-09-2022, 07:22 PM   #4
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Mike, I think your mortar was too dry at the time you put the sheet up. I can see an impression of the back of the sheet on the dried mortar, but no fleece.

When I spread mortar for Kerdi, I have the wall pretty damp to begin with. No water running on it, but wet enough. The mortar will hold a notch, but just barely. And I'm usually pretty fast at getting the wall covered with mortar before I start combing it out. That keeps the moisture from escaping the surface of the board.

Then I quickly notch out the mortar and cover it immediately. I have all my sheets pre-cut and ready to go up. There's no stopping for anything once the mortar is on the wall until it's covered with Kerdi, and the Kerdi is embedded.

And the time to check for coverage is immediately after you get the sheet embedded. That's really the only time you'll be able to fix a problem, if there is one.

If you're trying to cover an entire wall at once, you might consider cutting that in half. Even an experienced installer has to work very quickly to cover an entire wall. I hope to goodness you're not using the Kerdi Wide Roll.
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Unread 07-09-2022, 07:41 PM   #5
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CustomBlend has a lot of sand in it, and the particle size is fairly large. Some of the premium unmodified brands have significantly more cement in the blend and finely sifted, small sand grains which make it much creamier, and easier to fully embed the membrane. THat mortar will work, but it's lots harder to do than with a better product.

As they show on their videos, your best test is when you think you've got it done right, is to pull back a corner immediately, and IF you have full coverage on the back of the membrane and the wall, you're doing it right. If not, you need to change your materials or technique. If it was right, the sheet will quickly and easily go right back up. If not, you might want to remove it, and try again.

One of the bigger mistakes people make is trying to spread too much thinset at once prior to covering it with the Kerdi. You can use a bigger sheet, but spread your thinset for part of the wall, embed the membrane, add more thinset and continue. You don't need to cover the entire section before you start to embed the material. Most find it easier to just use smaller sheets, but either method will work.
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Unread 07-10-2022, 03:00 PM   #6
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So, I guess from what I am reading, it sounds like there should be fleece that pulled loose when I pulled it away from the thinset after a week, correct? So this probably means this installation will fail with the weight of the tiles? I am using the 33" wide x 7ft tall pieces, so it is possible it was too dry. I tried to keep it wet enough it would hold a notch, but there is definitely a fine line where it goes one way or the other. If I pull it all back off and the fleece stays in tact, can I reuse it?
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Unread 07-10-2022, 04:05 PM   #7
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Keep in mind that cement-based products are rated for strength after 28-days of curing. Even after a week, you can probably peel the membrane back off. If it was properly embedded, that would tear the fleece off of the body of the membrane. After a month, things are MUCH firmer.

When you peel the membrane, you're putting all of the stress along a very narrow line. A tile is trying to pull the membrane away from the wall. The Kerdi membrane, and similar products, must meet a minimum shear strength of 50-psi. Kerdi typically is closer to 75-psi in shear, and, with that thinset, it might be closer to the standard minimum, but should still work.

But, if you did pull it back, and none of the fleece tore off, yes, you could reuse the sheet. That's unlikely, though. If you took say a 10x10" tile, that's 100-sqin * even 50psi, that's 2.5 tons to pull it straight off the wall...you'd tear the wall down before you broke the bond. IOW, there's a lot of margin.

The bigger thing is if your seams are not well bonded...those can leak.

It is very important to wipe the walls down with a wet sponge prior to spreading the thinset. They're not too wet unless there's liquid water pooling. That allows the water you mixed into the thinset to remain in the thinset so it remains pliable. Thinset achieves its strength by curing, not drying. During the curing process, the water is chemically incorporated into the cement. The only way to break those bonds is to heat it up. Well, you can fracture them, but that's a different story. When it's made, things get heated up to about 2700F, so you can see, cement is fairly tough stuff! At that temperature, it breaks the chemical bonds.

Custom Building Products still makes a decent unmodified thinset, but they don't sell it in the USA any more. The one they do sell is fine for use under cbu, and can be used for other things, but it's a price leader product, not a premium one. You get what you pay for.
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Unread 07-10-2022, 08:00 PM   #8
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Thanks. As I did say previously, I did wipe the walls down with a wet sponge to the point of the paper discoloring on the sheetrock, which with my experience is to the point of the paper wanting to fall off. Haha. Anyway, I think I will try the versabond on the rest of it, just for piece of mind. I did the 2" overlap so I think the seams should be good, I hope. Thanks everyone!
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Unread 07-11-2022, 09:46 AM   #9
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Hi Mike,

I've done a bunch of Kerdi showers over sheetrock, and I've never wet the drywall before starting. I do mix the thinset very "loose," meaning just short of runny. I've used Versa-Bond extensively. Also, I never spread more area than I can cover before the thinset starts fo "skin over" or dry on its surface.
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Unread 07-11-2022, 03:50 PM   #10
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So, the versabond fortified is what you all recommend?
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Unread 07-11-2022, 04:24 PM   #11
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There are premium dryset mortars that work well. Versabond is a modified mortar that people have found does work and is fairly easily available. Not all modified mortars will work, and unless you choose the Schluter one, would void your warranty. Cement cures over time, some modifiers also require drying to become stable. Waterproof membranes can slow drying for months, which is why it's important to use one that becomes stable while still being damp.
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Unread 07-12-2022, 04:57 AM   #12
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To answer simply Mike, yes Versabond works great with kerdi and you shouldn't have any doubts about using it.
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Unread 07-31-2022, 05:42 PM   #13
fish2222
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Well, I have to say, I put the last two sheets of Kerdi up today with the VersaBond and man, that is a lot easier to use for sure. I wish I would have known about that sooner!
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Unread 07-31-2022, 08:10 PM   #14
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We wish Schluter would tell everybody about that, Mike.
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