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Unread 10-28-2021, 09:47 AM   #1
jeffnc
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Kerdi/thinset waterproofing

Have a couple questions about this. Called Schluter technical support but the person I got ahold of didn't have the technical expertise to understand my question, let alone answer it. Called my local rep, but have not heard back yet, so thought I'd get the ball rolling here if possible.

In general, I kinda sorta have an idea of how Kerdi waterproofs (I'm talking about seams - it's obvious how it waterproofs as a full membrane). Since thinset isn't waterproof, it supposedly relies on something having to do with the fibers in the Kerdi fleece interfering with the ability of water to flow through the thinset.

Assuming that is true, it leads to a couple questions I've run into.

1) How does waterproofing work when attaching Kerdi to Ditra? Ditra has no fleece, so does the fleece/fibers on only one side of the assembly work to do the waterproofing?

2) Let's say you're doing your Kerdi shower split up over 2 days. The first day you install the walls, pan and curb. Not wanting to disturb that assembly until it's set, you wait til the next day to install the seams and corners. But let's say you got some spillover thinset onto Kerdi near the pan/curb/wall seams. (Not big globs, but stuff you scraped off with a trowel, leaving it flat). Once that fleece is "locked in" by dried thinset, does waterproofing still work when you apply Kerdi over it? Or does the Kerdi fiber on both sides of a seam have to "interlock" with wet thinset?

I've also heard this explanation which doesn't mention anything at all about the fleece/fibers. This guy says it's simply the thickness (i.e. lack of thickness) in the thinset layer that allows very little water in (explained at the end.)

https://www.youtube.com/watch?v=vpCC2mTyMUk
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Unread 10-28-2021, 10:07 AM   #2
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1. I dunno. Schluter says it does. I've never actually tried it. I think it might be OK in a floor/wall application where there might be an occasional large spill that was of short duration, but other than that I'm a bit skeptical.

2. You would always have cleaned your exposed Kerdi with a wet sponge and never need to be concerned with your theoretical problem. It is my understanding that it is really necessary to have a clean fleece to clean fleece interface in order to take full advantage of the Kerdi waterproof overlaps.
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Unread 10-29-2021, 08:30 AM   #3
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I don't know either. I have generally wiped over the connections with a damp/wet sponge before the thinset fully dries. I know one thing, though. I've done maybe a hundred Kerdi showers and have had no leaks that I'm aware of.
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Unread 10-29-2021, 04:08 PM   #4
ceramictec
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I did tons of Kerdi showers and a bunch of decks outside over mud with Ditra and Kerdiband on the seams and never a problem. Rain soaking in the grout doesn’t really get under the tile and enough pressure to push the water into the thin layer of thinset between the Kerdiband and the Ditra. I don’t know if you went to the Schluter school but they talked about “head pressure”.

https://en.wikipedia.org/wiki/Pressure_head

So if you do a Kerdi shower pan and flood test it that little bit of water weight is putting some pressure on the joints and could push water into the thinset. Then they explained if you were to take that shower and seal it up high and fill it up 7’-8’ with water, that pressure would probably push the water through the joints. But that doesn’t happen when tile is installed on the top.

Also the unmodified thinset between Kerdiband and membrane gets harder/dense overtime and less absorbent.

Also a good quality Unmodified thinset is pasty which creates a thin layer. That’s why they don’t want you to use some cheap sandy thinset and large notch trowel which would create a thick porous joint.

* I watched the video and do agree with him at the end and what I said above with the thickness of the thinset between the Kerdiband/Kerdi or Kerdiband/Ditra
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Last edited by ceramictec; 10-29-2021 at 04:14 PM.
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Unread 10-29-2021, 06:09 PM   #5
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Water pressure, or head, amounts to 0.43#/foot...that's enough, given a decent water tower, to supply water across a large area for your domestic water needs, but it also can stress any seal/joint if it gets high enough.

As was noted, a cheap dryset mortar may tend to have larger grains of sand, and even raw cement. A quality dryset mortar suitable for use with a membrane like Kerdi needs to start out with a small grained, well sifted aggregate, and cement.

When cement cures, it literally incorporates water into its structure and grows into interlocking crystals. If you get the membrane well bonded because you've used a quality thinset, it was clean when you started, and embed the seams well, it works. This requires that you use the proper gauge trowel so that you don't have excess thinset there, and you need to realize, you will get some squeeze out when you embed the two sheets together. You need to run your embedding tool out past the edge of the joint so you don't leave a lip, scrape up the excess, and it doesn't hurt to wipe the seam down with a wet sponge to clean up the excess so you don't have a buildup when you're later going to apply more to set your tile after things cure and you can do your flood test.

At CTEF, they (used to, don't know if they still do) made a drink cooler box they'd fill with water bottles each morning along with ice. The box after a long time would start to soften the cardboard...not because it got wet, but because of the constant flexing over months...then, they'd make a new one.

If you have a leak, you did something wrong, or maybe you dropped a sharp object onto it, and it poked a hole in it. If you don't tile fairly soon afterwards, and it might get foot traffic, you need to protect it so you don't damage the fleece, or poke a hole. Otherwise, it's pretty robust. Once you get the hang of it, it goes up fast, too.
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Unread 10-29-2021, 06:17 PM   #6
ceramictec
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Quote:
Originally Posted by jadnashua View Post
At CTEF, they (used to, don't know if they still do) made a drink cooler box they'd fill with water bottles each morning along with ice.


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Unread 10-30-2021, 01:16 PM   #7
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There are systems out there that encourage you to use sealant on the seams instead of thinset. Noble ValueSeal is one. Hydroban is another.

I'm not saying thinset doesn't work but sealant works better, IMO
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Unread 10-30-2021, 05:42 PM   #8
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The only advantage to the sealants I can see is that you can flood test after about 15-minutes...the stuff is not cheap, and it's messy to deal with.

You could probably use KerdiFix if you really wanted to...one of the instructors said he uses it to patch holes in his swimming pool liner without draining the pool.
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Unread 10-30-2021, 07:18 PM   #9
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Actually sealant such as KerdiFix does not work that well on Kerdi membrane. The reason is the fleece. Supposedly the fleece helps waterproof with respect to thinset. With a sealant it's the opposite. If you stripped the fleece from the Kerdi membrane, then yes it would work well because the sealant can bond completely to the plastic. With the fleece there, the fleece actually gets in the way of the sealant bonding to the plastic, and therefore water is allowed to sneak in under the sealant. It's not a major leak because that is a very small space, but still.

Of course I'm comparing apples to oranges in a way. Schluter requires a 2" overlap when sealing with thinset. When people use a sealant they are just using a standard caulk bead. Maybe if people sealed that seam with a full 2" of sealant it would work better. But generally if you want to seal with KerdiFix, you should remove the fleece in that spot.
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Unread 10-31-2021, 09:14 AM   #10
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I can't speak for every system out there but personally, I use Noble's guidelines for seams and they still want a 2-inch overlap on the seams with two beads of sealant 1-inch apart. Doing it that way will fill the majority of the 2-inch seam.

I've used Kerdi fix for this purpose before and found that it works very well. However, it is against Schluter's guidelines to use it to seal seams.
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Unread 10-31-2021, 09:31 AM   #11
jeffnc
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I've used Noble Seal once (instead of Ditra for a floor tile job, since the tiles were only 1"). I noticed the edges of the Noble Seal (about 2-3") did not have fleece on them. I wonder if that was to promote a better seal at the seams, if one were to do that.

Have not done a shower with Noble, but of course I would follow their directions as well. I would trust a Kerdi seam with sealant if it were not in a place where water could accumulate, but not for example at the shower pan bottom corners. If I removed the fleece along that seam to apply sealant, then I would probably trust it.
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Unread 10-31-2021, 04:30 PM   #12
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I've not used the Noble products, but did read through the instructions, and yes, they do not have any fleece on a strip where they want you to use their sealant. Probably similar to seaming a vinyl liner...it wants it to be smooth. Noble has two ways to set their material...a roll-on adhesive for both the main material and seams, or thinset plus the adhesive on the seams. To me, their original stuff is too thick, and makes seams more likely to give problems when setting tile.
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Unread 10-31-2021, 08:49 PM   #13
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Quote:
Originally Posted by jadnashua
...too thick, and makes seams more likely to give problems when setting tile.
The non-fleeced edges are half thickness to combat this very probem.
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Unread 11-01-2021, 12:03 PM   #14
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Jim-TS is about the same thickness as Kerdi DS and Valueseal about same as Kerdi.
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Unread 11-08-2021, 07:03 PM   #15
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I like the videos that Isaac Ostrom (“tilecoach” on youtube) has posted. I like his skepticism. I believe that the thinset application is sufficient to guarantee no leakage during intermittent use. However, there is a reason that Schlueter makes Kerdi Coll. I do not know why they wouldn’t make this product available in the U.S. For peace of mind one could use it to install one’s membrane and would have a truly waterproof installation.
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