Can I Use Any Thin Set

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

  1. johnfrwhipple

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

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  2. BobL43

    BobL43 DIY Senior Member

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    does this qualify as a conundrum or a dilemma?
  3. johnfrwhipple

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

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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. johnfrwhipple

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

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  7. 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
  8. johnfrwhipple

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

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  9. 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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  10. 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.
  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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    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.
  13. 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?
  14. johnfrwhipple

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

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  15. 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
  16. johnfrwhipple

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

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  17. 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).
  18. johnfrwhipple

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

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  19. 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.
  20. johnfrwhipple

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

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