Can I Use Any Thin Set

Discussion in 'Shower & bathtub Forum & Blog' started by johnfrwhipple, Dec 19, 2011.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  2. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  4. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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: Dec 26, 2011
  6. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  8. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  9. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  11. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    If drywall mud getting wetted by a laytex paint/primer/thinset/whatever really caused a bond-breaker, the paint on millions of rooms would be falling off when someone rubbed against it or bumped it. Trying to minimize the possibility is a best practice issue, how important in the real world, not sure, but I'm sure the legal gurus want it. Except when you're actually trying to remove drywall mud, how often have you had it fail when paintin, or adding thinset to it? It takes a lot of water and a lot of rubbing to get it fluid enough to do anything. Thinset has little 'excess' water in it...the water that is there is chemically incorporated into the cement as it cures. If you were using a medium bed mortar you might have an issue, but it still probably wouldn't. To preclude the small possibility, a prudent practice is to mitigate problems, and Schluter recommends a primer to seal away the moisture. Best practice is to not put it there in the first place. With the cost of labor to scrub the stuff off, it might be just as cheap to just replace the stuff with virgin drywall, then you've not got the problem at all. Tear it off, screw some new stuff up, and go for it.

    Nobody should be trying to do anything to dusty drywall, neither paint, nor wallpaper, nor thinset. So, trying that is just an indication of poor practices, and anything you try is open to subjective results.

    If you want to determine 'ultimate' stength, then wait the industry standard for cement products - 28-days. ALL products are spec'ed at 'full' strength after curing for that timeframe. You get a fair portion after 24-hours, and more at 7-days, but full strength is 28-days. In fact, cement products continue to get stronger the longer they cure. And, the bond between a waterproof membrane and a nearly impervious tile will achieve the full-rated strength more often with a dryset than with a modified that has trouble drying. Given enough time without stress loads, it might reach full strength. If you're talking about bond breakers, a modified over a waterproof membrane and a good porcelain large format tile, now you've got one if it is distrubed, as it needs to dry, and that can literally take more than the spec'ed 28-days.
  13. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  14. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  15. jch

    jch New Member

    I may be kicking the hornet's nest here, but here I go anyway....

    I'm building a tiled shower in an area with a sloped ceiling with rafters 24" OC. Because of this span, I was going to use 5/8" drywall + Kerdi, rather than 1/2" Cement Board + Kerdi.

    The rest of the ceiling would be 5/8" drywall (again, because of the 24" span).

    Questions:
    - Is 1/2" Cement Board stiff enough to span 24" on a ceiling?
    - Does Cement Board come in 5/8" sheets?
    - How do you handle the transition from 1/2" Cement Board (to be Kerdi'd and tiled) to 5/8" drywall in the rest of the room? The transition occurs between rafters.

    *This* is the reason I was planning on using 5/8" Drywall behind the Kerdi in shower area...

    Any better approaches??
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You'd be better served with your own new thread...

    I've not seen 5/8" cbu, doesn't mean it doesn't exist. I'd not want to tile a ceiling with either with that span. Could you add some intermediate supports up there? Will there be insulation stacked on top of this as well? I'd worry about the weight of that, the board, and the tile trying to end up bowing the material which would lead to failure...not nice to have that fall! Some cbu specify the maximum spacing - you'd want to read the installation instructions very carefully to see if they'd allow it on 24" spacing on a ceiling. Then, you have to deal with your local code people.

    As an aside, the title to this thread is misleading...the referenced report says to follow the manufacturer's installation instructions, and the report covers both Kerdi, Ditra, DitraXL, and Kerdiboard. Depending on what you are installing, would call for different materials suitable to what you are installing it on, and the manufacturer's instructions do not call for just 'any' thinset, only one meeting specific specifications, brand independent, for the specific material and location of that installation.
  17. Since kerdi requires thinset on both sides, why not use thinner substrate instead of the exact same thickness? I don't get it. The way you are planning it, your tile + thinset + keerdi + thinest will be thick, on a 5/8" thickness.... And this is because you want it to line up?? Btw, what are you planning to do at the end, use a Schluter profile, or a special tile, or nothing?

    In any case, I would make it all 12" between ceiling support members. Easy as pie. HAve you called the gypsum manufacturer to see if even 5/8" drywall will be warranteed hanging from studs 24" apart ??
  18. chefwong

    chefwong New Member

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    Hi John -

    Love to see some updates on the testing progression.
    I stumbled across this thread in research....or whether I'm going to use Kerdi or Liquid for my next project.
    Regardless, it would have been CBU for the wetwalls. It's just a lil more dust to hang the boards and more heft lugging it up a flight of stairs, but the labors all the same to me.

    Any reason not to post this same Topic of Discussion over at the JB Pro Forum....
  19. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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  20. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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