Kerdi Question

Discussion in 'Shower & Bathtub Forum & Blog' started by Kestrel, Jul 22, 2012.

  1. Kestrel

    Kestrel New Member

    Aug 16, 2011
    Hello gang - many thanks again for the help on this blog.
    Basement remodel almost done - nearing drywall. I'm planning on using a Kerdi drain, floor, curb and membrane for the walk-in shower stall, with tile over. Now their literature says to use regular drywall, but all of the installers (2 so far) I've had look at the job think it's nuts and propose some other, I think fiberglass stuff, with ceramic screws.

    I guess I'm looking for some experience-based opinions that the Kerdi membrane and system are reliable, and that going with their (Schleurter's) specs - despite the drywallers' reticence - is OK.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Drywall is an industry approved method according the the TCNA when used underneath Kerdi. You do NOT want them to mud and tape the seams where you are going to install the membrane, the thinset and membrane will provide that function. When installed properly no moisture from the shower will get beneath the membrane and the drywall stays dry, retaining all strength it had upon installation.

    Installing cbu, and I'd guess (haven't looked recently) the fiberglass faced backer boards, are also one of the listed approved backers. Because you have a lot of area to spread the thinset on, if you use cbu, you want to wipe it down with a wet sponge to do two things: remove any dust and to provide some moisture so it doesn't suck it all from the thinset, making it dry, which is then hard to embed the membrane's fleece into properly.

    Schluter has decades of history of using drywall, but they also list other materials that are suitable...go with the one you prefer, but in any case, done right, none of them are a problem waiting to happen. If you had a leak from say the plumbing, cbu would likely survive, drywall might not depending on how bad and how long it went before discovery, so that is a strike against that...but, consider all other areas in the house where there's drywall and plumbing behind you worry about that, too? Plumbing does not magically leak. Obviously, it can, but it's not all that common unless disturbed or incorrectly installed (same with the membrane). Do it right, do it once, and don't worry.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The TCNA NEVER calls out brand names, but 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 the year of the manual you have, it discusses membranes and specifically DOES list drywall as one of the allowable materials. Now, SOME places prohibit it, and if you live there, you abide by your local requirements or your preferences. It is not REQUIRED to use drywall, and they list other materials that will also work. They list in the requirements section that the solid backer must comply with either: W201, W241, W243, W244C, or W244F where W243 (INterior walls, wood or metal studs, gypsum board with dry-set or latex-portland cement mortar) is the gypsum board as a solid backer material. These requirements have not changed for ages, and saying it is not an approved method (for most places, anyway), is just plain wrong! If the document wasn't copyrighted, I'd post a scan of the appropriate pages, but you can look it up yourself. There's another provision in there: installed per the manufacturer's instructions, and yes, in the Schluter documentation, drywall IS listed as one of the approved backer materials.

    That method is called Solid Backing Bonded Waterproof Membrane Integrated Bonding Flange Ceramic Tile.

    Whether you like it or not, it is an approved method for most people's use. Should you prefer to use something more robust in case of potential system failures (don't happen if you follow instructions and have good workmanship), then I don't have any problem with that - go for it.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Okay, it took me awhile to get this, but by permission, I'm quoting an e-mail I got from the TCNA rep. Basically, they no longer recommend gypsum board underneath a waterproof membrane in a shower construction because not ALL membrane or board manufactuers do. But, they DO allow it IF both the backer board and membrane manufacturers are in agreement. In the case of Schluter and Kerdi, Schluter has paid an independent testing agency to verify their claims that it works, and done properly, they will warranty the work. Since the TCNA has no general agreement from manufactuers of other products with the same general characteristics, they do not make a generic recommendation, but reference it to basically, follow the manufacturers instructions. Now, anything can fail if you don't have good workmanship. Here's the quoted e-mail info:

    "Hello Jim,

    While some membrane manufacturers promote use of their
    membrane in a B422 installation with gypsum board (W243) as the solid backing,
    other membrane manufacturers do not. At the same time, not all manufacturers of
    gypsum board recommend their product be used in a B422 installation. Thus, the
    Handbook leaves the determination of a suitable membrane/backer board
    combination up to the membrane and backer board manufacturers to declare and up
    to the design professional to select/specify. Ideally, one should find
    recommendations from both materials manufacturers that agree. However, as you
    are aware, manufacturers do put forth proprietary recommendations, which the
    Handbook does not address, such as in the case you are asking about.

    Many in the industry feel that building code prohibits the use of gypsum
    board in wet areas, even when a waterproof membrane is applied to the surface.
    However this is a matter of code interpretation, with the actual code language
    somewhat ambiguous, and with some feeling the code only prohibits direct bond of
    tile to gypsum board in wet areas.

    At the Handbook meeting last month,
    the Handbook Committee voted not to include a shower method where a mortar bed
    (cleavage membrane, metal lath, and wall mud) is installed over gypsum board.
    This is different from the method/materials you are asking about, but I am
    mentioning it to provide some perspective on recent consensus out of the
    Handbook Committee regarding use of gypsum board in wet areas. Again, the
    methods are different; in no way do I mean to imply the Committee feels the same
    about gypsum in a shower if a waterproof membrane were applied. What I can say
    is, the amount of water/steam exposure generally is raised as a factor for
    determining suitability in discussions of this topic.

    For further
    information, you may also want to contact the Gypsum Association.

    Stephanie Samulski"
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