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Thread: Installing Kohler cast iron tub (alcove) on plywood floor

  1. #16
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Quote Originally Posted by lemmy View Post
    Are you referring to midspan blocking? I rarely see that in houses and the middle of the wall isn't even in the pictures.
    Mid Span blocking and missing wall studs for curtain rod on the right side of your tub.

    Sorry for voicing my concerns over your choice in backer board. You seem to know all the answers and Jim here has filled in maybe 40% of the missing info.

    You really need a strong plan for your build and understand the entire job.

    If using drywall and Kerdi I have been told (By Schluter) that you should involve your local Schluter rep if you want any kind of warranty. Jim refers to movement. I called you on missing blocking. But hey - if you did not have it before why do it now.... I'm kidding of course.


    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.

  2. #17
    DIY Member lemmy's Avatar
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    Quote Originally Posted by johnfrwhipple View Post


    As a bathroom builder this is what we see most times when removing drywall from a shower's wall. Nasty Black Mold.
    You point is made. I am not trying to disagree with you because I know that your experience is far far greater than mine. Have you ever seen mold like that when kerdi was used over drywall? If so, where is the most common failure point? Is it cracks in the grout and then the Kerdi doesn't do a good job protecting the drywall, or is it almost always at the tub drywall interface?

    I am not so far along (as you can see from the pictures) that I can't easily change to concrete board. Would I just run the concrete board out near to the point where I would stop the tile then but up drywall to that?. Then bring the kerdi out over this joint between concrete board and drywall then tile out just over the kerdi? I am not concerned with increased price, I just don't have much experience working with concrete board and I had read from so many people that kerdi over drywall worked well. I know you may disagree, but if you have faith that the Kerdi is 100% water proof, you can see why someone would think that the material behind the water proof membrane wouldn't need to be water proof.

    BTW, I had also considered using the DensArmor but I didn't know how the Kerdi would stick to it. I was going to just get it for the entire bathroom but then I read it has a rough finish and requires a skim coat to make it look smooth. But if it was just going to be under the tile around the tub I guess it wouldn't matter.

  3. #18
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Concrete board can handle some mositure and not fail. Drywall when it gets a little moisture turns to mush and the paper is a food source for mold.

    I have not ever repaired a Kerdi Shower that had failed. Same is true for any other shower that has had red guard, HPG, Hydro Ban, Ardex 8+9 and on and on. Ecvery shower I have repaired with mild and discoloured grout has had drywall behind it. Every single one.

    The local inspectors so no to drywall with kerdi. I know because I asked.

    Denshield can work fine. Kerdi band over the tub's flange and set the Kerdi band to the denshield with Kerdi Fix. Then use 8+9 over all screws and seams.

    When we come across Denshield I often skim coat the entire wall with a self curing product from Mapei called Grani Rapid.

    How much Kerdi do you plan to install? If you are not going up the walls 6' then you best do a traditional cement backer board design with the Kerdi going up the wall studs, poly or builders felt lapping over the Kerdi and then the cement backer board - tape the seams....

    Any split design has holes in the planning. If you plan to go topical - go all the way.

    Did you see my blog post on Kerdi Band? Simple design. You could go this route with ease.

    It's horrible the advice Schluter gives out about drywall. I hate this about this company. I do however like the waterproofing and Kerdi Fix more than I can't stand the advice they give out. For years I have blasted Schluter over this point.

    I think Kerdi Board is crap as well. I don't use any foam products.

    The TCNA and TTMAC list one backer board for steam showers - Cement Board. That's it. Ask yourself why...

    I have never fixed a shower with cement board in my life.

    We find showers all the time built with drywall that are perfect. Makes no sense - but it CAN be done. But. every failed shower we fix is also done with drywall. Why for the sake of a few dollars take the chance.

    Deflection (movement) cause lots of failures.

    Bond Breakers + number one reason for tile failure.

    If you can find Ardex setting materials first install your concrete board and tape the seams. Skin out the entire wall with a flat trowel of thin-set. Then the next day install your Kerdi. Ardex will allow you to use their modified setting material. This way you will have more working time and not have to fuss with spray bottles of water over drywall.

    One thing the yaw whos on John Bridge don't tell you about water application is that water can by itself be a bond breaker.

    A shower is so easy to screw up. Kerdi Fix. Ardex 8+9 together cost about $100.00. together with Kerdi you can build anything. While you have these items why not build one of my LED shower niches out of your scrap building materials....


    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.

  4. #19
    DIY Member lemmy's Avatar
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    I was planning on doing Kerdi band around the bottom and the Kerdi membrane up to within 1.5 foot of the 9' ceiling (all the way to the point where the tile will end).

    I did see your blog post. The only thing I didn't quite grasp is where you have a gap between the backer board and the tub, then you say not to fill the gap between the tub and the back board. Are you saying you don't want to push down on the Kerdi Band too hard (in the area of the flange) so that the Kerdi Fix gets up in the area between the tub flange and backer board? Why do you not want the sealant to get in to that area? So does the 8+9 provide a water proof seal for the Green EBoard or is the Eboard water proof already?

    In all honesty I don't think it is cost (at least for me) that makes people want to go with the drywall and Kerdi membrane (not just the bands, along the seams and tub, but all the way up the wall with the Kerdi membrane). I think it is the simplicity. Attach drywall as usual, no need to finish the drywall (except for tapered edges), apply mortar and kerdi, then apply your tile. Maybe the other methods (i.e. concrete board) aren't any more complicated, but they seem to be.

  5. #20
    Retired Defense Industry Engineer jadnashua's Avatar
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    This is a TUB only install, NO shower, at least that's what was said, and is the basis of my thoughts on what is required. So, no shower head to get things wet on a regular basis, and the only splashing is unlikely to go very high , or be very consistent. If this were a shower, THEN, it would need to cover the drywall up to at least the height of the showerhead (per industry standards). To add a showerhead after completion would likely take a remodel, and then you could address the issue, although you could change the spout and put in a handheld. IF that is a possibity, reconsider your waterproofing and go higher. BIG difference in being a shower and a tub only in how things MUST be prepared. It is crucial to get the tub/wall joint waterproofed, and not allow any moisture to get in from there, and if there's waterproofing up a foot (probably higher than required) or so above the tub rim, it should be pretty bulletproof. A little splash first, won't penetrate far, and would then have an opportunity to dry in between tub uses.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #21
    DIY Member lemmy's Avatar
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    So would this work?

    1) Screw Durock directly to the studs (no vapor barrier behind it) from about 1/4" above the top tub flange (which is very small on these cast iron tubs, not nearly as big as the one in your blog) to about 1.5' from the ceiling (the tile will stop just a few inches above this).
    2) Spread mortar on the Durock and Install Kerdi membrane over the Durock and Kerdi-band in the corners and along the tub flange and using Kerdi-Fix to attach the Kerd-band to the Durock
    3) Seal around shower outlet, valve etc with the Kerdi
    4) Install tile down to within about 1/8-3/16 of the tub and seal the tub/tile interface with caulk



    Questions:

    1) I assume I would use the special screws used Hardi makes for their backerboard?
    2) Do I just bring the Kerdi membrane up over the drywall/Durock joint?
    3) Do I need to do anything for the screws in the durock or joints between different pieces of Durock if I am sealing with the Kerdi membrane? Or sould I use thinset and tile (alkali resistant) mesh tape on the joints? Should I also do this for the corners?
    4) I know you can score/snap the Durock, but what is the best way to cut holes in it?
    5) would hardie backer or some other board be a better choice?

  7. #22
    DIY Member lemmy's Avatar
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    Yes, no shower. I was still planning on Kerdi on the drywall all the way up the wall to near where the tile ends.

  8. #23
    DIY Member lemmy's Avatar
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    Quote Originally Posted by johnfrwhipple View Post
    Mid Span blocking and missing wall studs for curtain rod on the right side of your tub.
    There is blocking up high for a curtain rod, but this is just a tub so I wasn't even sure if I would install a curtain rod. I have been thinking that I might prefer a sliding door which would require some blocking lower down as well so I am glad you mentioned that. i need to add some blocking for that.

  9. #24
    Retired Defense Industry Engineer jadnashua's Avatar
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    It depends on whether this would ever become a shower how high you need to bring your waterproofing. IF the cbu joints will be covered with Kerdi, the Kerdi membrane will act as your reinforcement, and you do not need to band them separately. If the joints are not going to be covered with Kerdi, then yes, use the tape at all joints. Bond the Kerdi to the wall with thinset...use Kerdifix everywhere else you need to bond it to something (as in your tub).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #25
    DIY Member lemmy's Avatar
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    But for the most part, the installation is the same for cbu and drywall if everything is covered with kerdi, right? The main difference being the screws that are used to attach it to the studs...right? With both you attach the Kerdi to the backer board with thinset and then tile.


    Another issue I have been wondering about is how do you accomplish this to a tub that does not have a flange? My flange is small and runs almost to the front edge of the tub, but I know some tubs do not have a flange. How do do you attach the Kerdi-band to the tub then?

  11. #26
    Retired Defense Industry Engineer jadnashua's Avatar
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    There are two reliable ways to waterproof that that I'm aware of (and probably others): most tub manufacturers offer an add-on tiling flange...basically, you'd adhere it to the tub deck with something like Kerdifix, then proceed as before; wrap Kerdiband onto the top of the tub deck, secure with Kerdifix (depending on the thickness of the tile would determine about how far you could wrap the membrane without it being seen). There are other waterproofing membranes, and they probably have similar options.
    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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