Can I Use Any Thin Set

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

  1. johnfrwhipple

    johnfrwhipple BATHROOM DESIGN & BUILD for both Canada & the US

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    Can I use any thin set?
     
    Last edited by a moderator: Nov 26, 2016
  2. BobL43

    BobL43 DIY Senior Member

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    does this qualify as a conundrum or a dilemma?
     
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  4. BobL43

    BobL43 DIY Senior Member

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    I'm still trying to decide which matting to use; Ditra, Spider Web or the Noble Seal TS. I thought that modified, like Versabond was fine to use to bond the Ditra to plywood. According to what you just wrote, that is up in the air now too? One you did not mention (for tiling over plywood) is Tavy thin skin and his glue. Would you rather me start another thread on that?
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The installation instructions are referenced and cover this.

    Bonding Ditra or DitraXL is dependent on the substrate it is going over: they call out modified to bond it to a wooden subfloor and (I think, I'd have to reread the manual, either modified or) unmodified over masonary materials...it's pretty clear. Since Ditra is only spec'ed for floors, and the deflection issues and movement issues with wood are the biggest unknown, the use of a modified is called for, espcially since there's a moisture release path, both through the substrate and by the open channels of the membrane itself, out to the sides.

    They call for unmodified when installing Kerdi, both under and over, and unmodified on Ditra over as there's no good moisture migration path between the tile and the waterproof membrane.

    If you want to deviate from this, they say you need to call both them and the material (likely tile) manufacturer to come up with a suitable solution, if there is one. I have heard, indirectly, that they've allowed a rapid set modified when asked. Rapid set mortars are not likely a good solution for a DIY'er because of the limited pot life.

    IF you want Ditra or Dita XL to be waterproof, then you need to band the seams, just like you do with Kerdi and that calls for an unmodified, since it is on top of the membrane. This may call for two different thinsets, or the same one and mixing it with either water or their addative to make it into a modified (probably the simplest since you don't have extra thinset in the wrong configuration - you make it up to need as required).

    This is all called out in the manual. The key is: suitable and per the manufacturer's instructions.
     
    Last edited: Dec 19, 2011
  6. BobL43

    BobL43 DIY Senior Member

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    Thanks Jim, I did download and read the manual a few weeks ago, but can't seem to remember anything about using their own additive. going to re-read it again. This sounds like more art than the science it should be to me. And maybe that art is in the form of BS
     
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    John, you're not interpreting it correctly. They state: 'The membranes are separated at expansion joints to avoid restriction of the joint movement, and the seams are sealed with flexible KERDI-FLEX.'

    Got a crack in the floor? Need to keep it waterproof? Butt the DITRA seams and seal it with KERDI-FLEX. Build a shower: read section 3.4.1 again...it calls out Kerdi-Band for waterproofing the seams.

    Determine acceptable backer boards? Read section 2.0, it lists gypsum board. This is from August 2011, not old, although they also did a test on it a long time ago. The test reports are not valid forever.[TABLE="class: std"]


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  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Most thinset manufacturers offer an addative that can be used with their dryset (unmodified) thinsets. WHen you use this addative, it essentially becomes a modified thinset. You use that instead of water when mixing the thinset up. It is not a Schluter thing...it is entirely a thinset manufactuer thing. If your job might require both types, you can stock up on the unmodified and the addative, then, by choosing how you mix it (add water for unmodified or the addative/admix) and you get a modified). This can simplify things for you. There's nothing in the Schluter installation manuals about this, they only state what type to use, not how you obtain it.
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The report specifies following the instruction manual...the manual specifically identifies which thinset is required. Unmodified for Kerdi, and surface dependent for Ditra. One thing to keep in mind with Ditra is that on the top of it, the thinset only has to hold the tile...it does not bond to the membrane, and, because it can't outgas to dry, they specify an unmodified on top. On bottom, depending on the substrate, you may have up to two paths for drying: one through the channels created by the waffles and another through the surface. The 'pillars' that form when you fill the dove-tailed waffles lock it into the membrane, and hold the tile up. The sizing of the waffles is such that to get a minimum of three pillars, you need at least a 2" tile (thus the restriction in the manual when using Ditra as to minimum tile size - the pillars form the vertical support structure).

    I'll go back to my example from another time: is 1/8" glass any less waterproof than 1/2" stuff? Then, consider that it is OVERLAPPING the underlying material, so it is essentially adding to the thickness of the SYSTEM and doesn't stand alone. Kerdi installs must be taken as a SYSTEM, made up of correctly applied parts.
     
  10. mtcummins

    mtcummins In the Trades

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    Did I just hear John say that drywall in a shower is proper given a proper installation? Did hell just freeze over?
     
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The best thing is to not mud the walls before applying Kerdi. The thinset and the membrane works great to fill minor (nail/screw) holes and fill the tapers of the boards or joints. I'll try to get a read on what works, and if I do, I'll report back. My personal feeling is, if it is a small area, you have virgin drywall around it, and the tile is fairly large, it's not a big thing since even if the moisture from the thinset does soften the stuff, once it dries, it's good, and the whole area around it is tight in the interim. Now, on a large area like a seam, it could be a problem best averted by not mudding in the first place.

    Being a holiday weekend, responses will be delayed, but I would think that this, for example would work fine: http://www.custombuildingproducts.com/docs/TDS234_LevelQuik_Primer_3_11.pdf?user=arc&lang=en

    Every thinset manufactuer sells a laytex primer for its SLC, and similar products that improves the bond and seals the underlying materials so the cement products will stick better.
     
    Last edited: Dec 24, 2011
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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).
     
  13. pitterpat

    pitterpat HandyWOMAN

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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.
     
  14. 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.
     
  15. 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.
     
  16. 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
  17. 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.
     
  18. 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.
     
  19. 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.
     
  20. jch

    jch Member

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    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??
     
  21. 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.
     
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