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Thread: Why does drywall work in a Kerdi Shower?

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  1. #1
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default Why does drywall work in a Kerdi Shower?

    First, neither tile nor grout is totally waterproof, so whatever you use in a shower must not be damaged by moisture. Typically, people would use cement board on the walls with a vapor barrier behind it since none of those components are waterproof, BUT, none of them are damaged by being wet. The vapor barrier prevents any moisture trapped in those materials from being in contact with the wood studs and structure. The same thing is true on the floor of your shower...there's a waterproof layer preventing water from getting to it.

    So, it's not a long stretch to realize that if you do put drywall in a shower, if must be kept from ever getting wet or you'll have problems. This is exactly what the Kerdi membrane does...it is a very effective waterproofing layer. Just like the liner in your shower keeps the wooden subfloor from rotting out or ever getting wet, the membrane does it for not only the walls but the floor. So, if you built it properly - the drywall stays dry, the floor stays dry, and everything works out well.

    As with anything, if you make a mistake or breech the waterproof layer, you'll have problems. But, after installation and a flood test, the entire surface is covered with tile, so that layer remains well protected, and is easily able to do its job.

    Some related concepts: why can you use wood underneath your roofing shingles? Because when properly installed, the roof doesn't leak. Why can you use drywall on the inside walls next to your windows? Because a properly installed window doesn't leak. Sure, you can break the window or have wind blow a shingle off, but that's hardly likely to happen inside of a properly built shower covered in tile!

    Schluter has ben using Kerdi over drywall for decades, and since the certification doesn't last forever, has been getting it retested to reverify it still works to stay in compliance bi-annually. The ICC (International Code Council) test report ESR-2467 covers the certification for both Ditra and Kerdi and is only valid if you follow the manufacturer's installation instructions. While other methods might work over drywall, to my knowledge, none have the history or the certification, or test data to show that they work properly in that environment. So, do not take Schluter's experience with Kerdi make you think that another system is certified, tested, and warranted in other than their tested, approved substrates.

    FWIW, that report is valid over the following surfaces in a shower (from section 2.0):
    "Kerdi is intended for use on floors over concrete, mortar and tile backerboard substrates. Kerdi is also used as shower lining, as required in IRC Section P2709.2 and IPC Section 417.5.2, over concrete, mortar, tile backerboard, expanded polystyrene foam board, prefabricated polystyrene shower trays, curb and ramp, gypsum board and masonry substrates."

    So, you can choose what board you want in your shower when using Kerdi. Schluter's recommendation is drywall since it is available in larger sheets so you have fewer seams, accepts thinset well, and allows excess moisture from the thinset to be purged while maintaining a suitable environment for the thinset to cure and attain its best strength.

    Kerdi will work equally well with any of the approved substrates - choose the one you want, install it per the manufacturer's instructions and you'll have a long-lasting, problem-free shower. Don't let any nay-sayers force you into any other extra steps by making you feel guilty...IF you do it right, it will not leak. IF it does, fix it before you put up your tile. Based on the certification, doing it otherwise could void the warranty and the certification of the Building Codes.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #2
    builder:anti-builder dhagin's Avatar
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    Yep, pretty much sums it up.

    For even more info, see the "Schluter - Shower System Installation Handbook".

    http://www.schluter.com/media/shower...v=201401250602
    dana
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  3. #3

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    Quote Originally Posted by jadnashua View Post
    First, neither tile nor grout is totally waterproof, so whatever you use in a shower must not be damaged by moisture. Typically, people would use cement board on the walls with a vapor barrier behind it since none of those components are waterproof, BUT, none of them are damaged by being wet. The vapor barrier prevents any moisture trapped in those materials from being in contact with the wood studs and structure. The same thing is true on the floor of your shower...there's a waterproof layer preventing water from getting to it.

    So, it's not a long stretch to realize that if you do put drywall in a shower, if must be kept from ever getting wet or you'll have problems. This is exactly what the Kerdi membrane does...it is a very effective waterproofing layer. Just like the liner in your shower keeps the wooden subfloor from rotting out or ever getting wet, the membrane does it for not only the walls but the floor. So, if you built it properly - the drywall stays dry, the floor stays dry, and everything works out well.

    As with anything, if you make a mistake or breech the waterproof layer, you'll have problems. But, after installation and a flood test, the entire surface is covered with tile, so that layer remains well protected, and is easily able to do its job.

    Some related concepts: why can you use wood underneath your roofing shingles? Because when properly installed, the roof doesn't leak. Why can you use drywall on the inside walls next to your windows? Because a properly installed window doesn't leak. Sure, you can break the window or have wind blow a shingle off, but that's hardly likely to happen inside of a properly built shower covered in tile!

    Schluter has ben using Kerdi over drywall for decades, and since the certification doesn't last forever, has been getting it retested to reverify it still works to stay in compliance bi-annually. The ICC (International Code Council) test report ESR-2467 covers the certification for both Ditra and Kerdi and is only valid if you follow the manufacturer's installation instructions. While other methods might work over drywall, to my knowledge, none have the history or the certification, or test data to show that they work properly in that environment. So, do not take Schluter's experience with Kerdi make you think that another system is certified, tested, and warranted in other than their tested, approved substrates.
    If we have to look at an engineering POV , using the drywall in showers or wet areas is a no-no . The codes are a minimum -- repeat a minimum -- of building procedures to respect . Kinda strange to see you coming so strong with it as a recommendation .

    Drywall compared with plywood is by far an inferior product . And talking about roof leaks -- most of the showers are build under roofs -- , if the tiled ceiling gets wet , the drywall will collapse . If the walls are getting wet ...

    Quote :
    " Moisture Movement (Wicking) within Gypsum Wallboard
    Paper # 580
    Dale J Greenwell and Marc Y Menetrez
    US Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC
    27711
    ABSTRACT
    Gypsum wallboard with repeated or prolonged exposure to water or excess moisture can lose
    its structural integrity and provide a growth medium for biological contaminants. Poorly sealed
    buildings, leaking or failed plumbing systems, or improperly constructed HVAC systems can all
    contribute to water and moisture problems. Gypsum wallboard readily absorbs moisture through
    direct contact with standing water and differences in water vapor pressure. Regular gypsum
    wallboard sample sections were hung vertically and exposed to a continuous source of standing
    water 1.3 cm in depth. Both water absorption (measured in percent moisture content) and
    vertical movement (wicking height) were monitored within the wallboard over several days. The
    moisture content measurements revealed a water movement pattern similar to paper
    chromatography. A leading edge of relative high percent moisture content (~ 20%MC) moved
    upward along the width of the wallboard. The immersed portion of the wallboard maintained a
    moisture content level ≥ 20%. Between the leading edge and immersed portion of the wallboard
    was an area of moderate moisture content (~ 12%MC). Water wicked to a height of 15 cm
    within the first three hours of testing after which the rate continued asymptotically appearing to
    reach a maximum height by day 16.
    IMPLICATIONS
    Most wallboard remediation techniques involve visual inspection and moisture content
    measurements to determine the extent of the water damage and the presence of or potential for
    mold growth. Remediation not performed within a reasonable time following discovery, may
    result in ineffective remediation as moisture may continue to move beyond the original detected
    region. Because of the wide spread used of gypsum wallboard in commercial and residential
    construction, understanding the moisture movement (wicking) of water in gypsum wallboard
    products is essential. ''

    End quote .

    If you want to rely only on the membrane -- Kerdi or similar -- for waterproofing and not consider the safety of the assembly , I think you are doing a good job .
    Roberto

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    No argument that when drywall gets wet, it loses integrity. But, it is no more likely to get wet in a properly built Kerdi shower than it is on the ceiling under a solid roof or next to a window or a door. The point being - Kerdi is proven waterproof when installed properly. Therefore, any tested and approved material used underneath it can produce a reliable, long-lasting shower. Use any one of the approved materials and it still works fine. When installed properly, the only way drywall (gypsum board) will get wet is from some other way, and that's not a problem with the shower build.

    Once the shower is flood tested, then covered with tile, it is going to take some other fault in the house for it to cause a problem in the shower. I don't have any issue about which substrate you end up deciding to use, I have an objection to the years of test data that says it works, passes building codes, and is warranted by the manufacturer that has a vested interest in it working being touted as wrong.

    Just in case the roof leaked, do you put CBU on the ceiling? What about next to a window or a door? No, you accept that whoever put the roof up or the door or window in did it properly, and if you have a fault, you fix it.

    If you're really paranoid, you could make your house entirely out of cement, stone, and tile...then, if it got wet because of some misstep, you'd just mop up.
    Last edited by jadnashua; 01-26-2014 at 10:33 PM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5

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    All of this coming from an engineer .........surprising.

    I guess you are working only on the membrane level and the rest is not important . Drywall , like other products, are only accepted by Schluter for a shower installation . None of the drywall mfgs recommends the use of drywall in wet areas.

    I guess long lasting is less important than your coating of inferior products. Reminds me of the arguments that drywall contribute to increase the structural #s when calculate them . Probably they will also accept an inferior product saying that if everything else is done right , the walls will not buckle. Such a mediocrecy .
    Last edited by eurob; 01-27-2014 at 03:12 AM.
    Roberto

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Post(s) deleted by John Whipple
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    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Default Dry Wall and Kerdi - Jim's first discussion start of 2014

    Post(s) deleted by John Whipple
    Last edited by johnfrwhipple; 03-15-2014 at 02:48 PM.


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

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Default Dry Wall is crap

    Post(s) deleted by John Whipple
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    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by johnfrwhipple View Post
    I know it's wrong and the only reference you can hang on to is that Schluter says it's OK.
    Do you not put any credibility to the ICC? If when they tested if it failed, would they risk their standing and reputation? What about the hundreds of thousands of Kerdi showers that have been built that don't leak? ANY shower can fail if there are workmanship problems, and if it does, it really doesn't matter what's behind the waterproofing...it has to come out to be fixed.

    Massachusetts requires any plumbing work to only use approved materials and methods - they list Kerdi, the Kerdi drain in their database WHEN INSTALLED PER THE MANUFACTURER'S INSTRUCTIONS. They do not, the last time I looked, exclude or make exceptions for the use of drywall in that install. http://license.reg.state.ma.us/pubLi...tion=&psize=50 This is just one example...
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by eurob View Post
    None of the drywall mfgs recommends the use of drywall in wet areas.
    Explain to me why if Kerdi works as a pan liner, and one does not consider the subfloor under a shower a wet area, why is drywall behind a waterproof membrane that has been tested and approved for use in a shower, that drywall is in a wet area?

    The whole point of this is that when installed properly, NOTHING on the other side of Kerdi is in a wet area.

    If you do not believe Kerdi is waterproof, why would you ever use it as a pan liner?
    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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    DIY Senior Member DougB's Avatar
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    As an engineer (I don't think that matters), and as a pratical person, I can't for the life of me understand why someone would usee drywall in a shower! If you had just a teeny tiny leak - the drywall is going to go to shit. Cement board could tolerate it, and maybe with no ill effects.

    What do you save by using drywall? $50? Why risk it? What's the point? Because it can be done?

    If a fixture would leak behind the drywall - you would be sunk.

    I would say using drywall in a wet area: safety factor = 0.9
    If a hammer won't fix it, it's an electrical problem.

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    DIY Senior Member DougB's Avatar
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    Quote Originally Posted by jadnashua View Post
    As with anything, if you make a mistake or breech the waterproof layer, you'll have problems. off, but that's hardly likely to happen inside of a properly built
    There is a concept called risk. That's why there's insurance, hedge funds, etc.

    Quote Originally Posted by jadnashua View Post
    Don't let any nay-sayers force you into any other extra steps by making you feel guilty...
    Well it seems to me (just a DIY'er - gutted three bathrooms) that cement board and hydroban or redguard is less work than the extra steps of mixing thinset and applying the Kerdi

    Quote Originally Posted by jadnashua View Post
    IF you do it right, it will not leak. IF it does, fix it before you put up your tile.
    You're not going to know if it leaks at a seam.

    Quote Originally Posted by jadnashua View Post
    Based on the certification, doing it otherwise could void the warranty
    Warranty? Really? I would suppose nobody has ever collected on it, cause if was installed 'properly' it would not leak. Therefore if it leaks, it wasn't installed properly.
    If a hammer won't fix it, it's an electrical problem.

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    I used Hardi on all the wet side walls of my shower. Green board is on the dry side of the same walls. I covered the Hardi with Regard, so mines still shit anyways.

    Can someone pass me the popcorn please?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by Vegas_sparky View Post
    I used Hardi on all the wet side walls of my shower. Green board is on the dry side of the same walls. I covered the Hardi with Regard, so mines still shit anyways.

    Can someone pass me the popcorn please?
    Custom Building Products has specific instructions on how to use RedGard as a waterproofing layer in a shower. If you follow their instructions carefully and have good workmanship, it can work. Personally, I prefer a sheet membrane. RedGard applied too thin or too thick or with a pinhole is less than ideal.
    Jim DeBruycker
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    DIY Senior Member Vegas_sparky's Avatar
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    Quote Originally Posted by DonL View Post
    I am just checking in, I have no traffic.

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