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Thread: Can I Use Any Thin Set With Kerdi? The ICC-ES Evaluation Report On Kerdi Says You Can

  1. #16
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Retired Defense Industry Engineer jadnashua's Avatar
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    Remember, concrete products are at their rated strength after 28-days. The required shear strength of the bond to Kerdi is 50psi, but it tests out at 75psi. Real uses don't have huge cantelevered loads, only shear. On a tiled wall, any local load is shared by the adjacent tile since they are tied together with grout on a vertical load (shear), but a much less tenacious situation in tension.

    I haven't received any official response from anyone yet, but it appears that the primer should provide all implied requirements - it's laytex based to prevent moisture migration and it is compatible to concrete based bonding to anchor the thinset. We'll see what the makers really think.

    Best practice is to not use drywall mud in an area that is going to be tiled, regardless of what is going over it, Kerdi, or just tile (say in a backsplash).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    HandyWOMAN pitterpat's Avatar
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    1st, Schluter says that drywall is ok, look here, http://www.schluter.com/media/ShowerHandbook.pdf Schluter Shower Handbook, pg 7.

    2nd, no primer and no drywall mud. If it bubbles, flakes or peels off then your Kerdi loses it's bond and no waterproofing. Also if the drywallers have been sanding in the area then you need to brush down then wipe down the drywall in the area that the Kerdi will be installed with a damp cloth to get rid of all of the drywall dust. Dust means there is a chance of poor Kerdi adhesion.
    Pat Harris
    A HandyWOMAN Service, LLC
    "Why call a handyman when you can call A HandyWOMAN?

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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  6. #21
    Retired Defense Industry Engineer jadnashua's Avatar
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    The point is, don't use drywall compound on the nails/screws or seams when using Kerdi. If you do, then you should treat it as specified. The way you write it, you imply that it is required all the time, and this is patently false. this is all part of being professional...following the manufacturer's instructions and the approved method, whatever system you choose.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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  8. #23
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you're doing this yourself, don't use drywall compound. If it ends up there, then search out the necessary fixes to your mistakes. If it was only over screws/nails, I wouldn't worry about it, as if it did get soft, it would reharden and the areas around it would bond well with the thinset. I'd only worry about it if it was a large area like say a seam where it could be as much as 12" across with the mud and likely thicker than over just a screw hole dimple. Primer over drywall is for a mistake in the installation, it should not be the primary issue during an install.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

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  10. #25
    Retired Defense Industry Engineer jadnashua's Avatar
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    I have to ask, as you indicated you've installed Kerdi for years, what did you do in these circumstances when drywall mud was there? My training said not to use it, I haven't, and therefore haven't run into that situation. I never claim to know all the answers...I do know how to do it per the manufacturer's instructions, and that's the way I've done it. That's also the way I guide people...the way the manufacturer says they've tested it and it works.

    The TCNA handbook never calls out specific products, only classes of products, but I see under Shower Receptors method B422-11, which appears to be the same as B422-09, which appears to be the same as B422-07, depending on which year's book you're looking at. You don't have to be a rocket scientist to infer they're talking about Kerdi in that method of approved, tested install.
    Last edited by jadnashua; 12-26-2011 at 05:11 PM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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  12. #27
    Retired Defense Industry Engineer jadnashua's Avatar
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    They specify at LEAST a 2" overlap of the seams to account for minor errors to protect the SYSTEM. The narrowest Kerdi-band is 5" (you can get wider, if you wish), so you've got to be a little careful with your seams, but if you can put up wallpaper, with a little practice, you can put up Kerdi. Use whichever approved backer that makes you feel comfortable. With any product, if you don't maintain at least some level of craftsmanship, it could fail. Personally, the thinner material is a plus: it's easier to see if it is embedded properly and minimizes buildup. Plus, it folds easier to get squarer corners. Any surface membrane is a step up from a conventional shower and provides more flexibility in design and options. Kerdi is only one of those available. It happens to be one I've used and been trained on, and found to be functional.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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  15. #30
    Retired Defense Industry Engineer jadnashua's Avatar
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    Commercial verses DIY'er often run into different things. A DIY'er, if he's smart, reads the instructions. If so, and this is a remodel, there's no drywall mud in his shower because he put it up. If he installs things right, the shower doesn't leak, the drywall stays dry, everything works. Over drywall, unprimed, no mud, assuming you wait the 24-hours they recommend when doing a flood test, the sheet is adhered pretty well and probably better than with a modified. Yes, a modified has better initial tack and ultimate strength, but if it isn't embedded into the fleece well, it could easily end up being weaker than a proper dryset installed properly. And, where you're talking about TONS of pull required to break the bond with either dryset or modified, why not use a dryset? When installing the Kerdi, they recommend you peel back some to verify that it is properly embedded into the thinset. A dryset flows better than the sticker modified thinsets and gets into the fleece better rather than just sticking to its surface. If you verify this as you go, the stuff is quite tenacious. Just like you can peel tape off if you pull it at a steep angle, you have lots more trouble pulling it off in tension. you can tell if you have a good bond by looking at the membrane.

    When the membrane is sandwiched between two rigid surfaces, the backer material and the tile, it is protected and thickness is somewhat irrelevant. Goretex (TM) is very thin, but works, and you never see it as a surface treatment. It really isn't very hard to protect it during construction - you'd want to protect any membrane, either painted on or sheet, and what you do for one will work for the other. And, it's quite easy to do most of the walls and tile before you even start to lay the pan. Leave the last row or two out at the bottom, and you can install the pan, install the membrane with the required overlaps, and there's little opportunity to drop tools on it or mess it up while working on the walls since they're already done!

    The TCNA does list gypsum board as an acceptable backer material in the TCNA handbook in the section where they describe sheet membrane shower construction.

    If it took you that long to do your first shower with Kerdi, I feel sorry for you. It really isn't that hard. Some research beforehand should have prepared you to do it right the first time in much less time. The video in the box with the drain also available on-line is pretty good if you can't get a class. It took me about the first sheet to get the hang of it, and the rest was easy, and for a type A personality, that's saying something! I've mellowed some as I have aged.

    ANY system takes a little practice. I think it's tougher to paint on nice, even layers of waterproofing, achieving the required min/max thickensses without getting some pooling or drips that can come back to haunt you when you get ready to set your tile; or, get more buildup in a corner. Thicker sheet membranes also have buildup issues.

    Drywall is stong enough, stable enough, and easily workable for the majority of walls in homes installed in the USA and probably Canada. They hold tile quite well in dry areas, and, if kept dry by a properly installed membrane, do an equally good job in wet areas (therefore, not really wet). If using a different backer makes you feel safer, use it, but it works, has worked, and will continue to work, when installed properly.

    All of those people have a liability problem you've talked to, and I fully agree - drywall should not be in a wet area. The main issue is, if you install a Kerdi shower properly, the drywall is NOT in a wet area. There will always be a yahoo that never reads instructions, slaps something together, then blames it on the product. If your shower leaks, regardless of the material it is made out of, you'll likely have problems. It's just a function of time and severity.

    With non-setting drywall mud, it doesn't magically fall off the wall if it gets wetted - well, maybe if you used a high volume or pressure hose, that's not the same as brushing or rolling some onto it. Even if the sealer contains some water, once it dries, everything drys out, and then protects the stuff underlying it from getting wet again. You should not cover the primer until it has properly cured - again, read the instructions on how long to wait after installing. Fail that, just like anything, the results are unpredictable.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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