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Thread: Backerboard questions for cultured marble shower

  1. #1

    Default Backerboard questions for cultured marble shower

    I ordered my cultured marble shower today and want to make sure all my ducks are in a row before it arrives. I have a few questions about backerboard.

    1.) Is durock sufficient backing material. I had read somewhere that it may not be strong enough by itself and should be used in conjunction with greenboard for strength. Is that so?

    2.) Each wall of my shower will be covered by one solid cultured marble panel. Is a vapor barrier needed?

    3.) I'm still not clear on what material I should use to imbed the fiberglass tape when I tape the seams of the durock. I will need to make a smooth surface. I had read to use thin set mortar, but it seems to me that if I used thin set I wouldn't have a flat surface. What am I missing?

    Thanks in advance for your advice. This forum is a lifesaver.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Can you go to the manufacturer's website and download the installation instructions? That would be best; otherwise, you'll get generalities.

    1/2" durrock is strong enough on a wall to support a tiled wall, I don't see why it wouldn't work under the panel. Now, if you really need it is another story all by itself.

    The durrock (or any other brand of cbu) is NOT waterproof, it is just unaffected by moisture. My guess is that the manufacturer will probably request a vapor barrier behind the panels - simple plastic sheeting will work. Now, since the panels are water and moisture proof (except for any failed seams), they may not even require anything.

    When using cbu as a tile substrate (which is what it is designed for), you use alkali resistant fiberglass mesh tape and embed it into thinset. It is easiest if you do this while tiling, otherwise you usually end up with speedbumps that make tiling (or putting up a panel) difficult, since the surface ends up with a hump at the seam. The mesh tape and the thinset at the joint is there, not like doing a drywall seam, to create a monolithic surface, not a pretty paintable surface like with drywall. Since you won't have tile and grout to crack, my guess is that if they want cbu up on the walls, you won't have to do anything with the seams. I'd still put up a vapor barrier behind it and bring it down to the lip of the pan. My unprofessional opinion.

    The manufacturer's instructions always trump opinions if you want a warranty.

    Also note, it may offer setting the pan without a mortar bed as an option...don't take it - use it and you will increase the life and your satisfaction with the install. Make absolutely sure that it is sitting level in both directions, or you will have problems with the walls and draining, too. That is another reason to use mortar under it - you can smush it down into the stuff, it is fully supported, and you can get it level.

    Good luck.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    Thanks for your help, Jim. I am bothering the forum with these questions because the manufacturer's instructions are very vague. Nothing is said about vapor barriers or what type of backer is recommended. I have even called the manufacturer with questions and have not been given satisfactory answers. So I really appreciate the help that you and others on this forum have to offer.

    To follow up on your reply, the shower pan that I have ordered does not have a tiling flange. If I put up a vapor barrier, where would I direct the bottom of the vapor barrier? I know normally the bottom of the vapor barrier would go over the flange and end there. Without a flange, should I just cut the vapor barrier about half an inch above the shower base and leave it hanging? I was actually hoping that I wouldn't need to mess with a vapor barrier at all since a cultured marble panel shouldn't be porous.

    As far as the seams, I guess I will leave them unfinished. Makes things easier for me, as long as that is a safe practice to follow.

  4. #4
    Plumber jimbo's Avatar
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    They make a waterproof joint compound to use with the fiberglas tape. This tape joint ( seams and corners) doesn't have to be glass-smooth like wallboard. You are adding a thickness of mortar or adhesive which more than covers any uneveness at a seam.

    I don't see how you have a shower pan without a flange. Doesn't sound like a pan. This is a recipe for a leak.

  5. #5
    DIY Senior Member sulconst2's Avatar
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    i would mesh tape with thinset then after it dries skim coat the entire surface with thinset. plus were the cbu hits the pan use a high quality caulk. then again for the panel.

    jimbo 1000th post congrats!!
    Last edited by sulconst2; 12-08-2005 at 05:14 AM. Reason: hail to jimbo!

  6. #6
    DIY Member khayes's Avatar
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    I just had a cultured marble shower surround installed. I was told by the manufacturer, no need for backer board. We just used greenboard fastened directly to the studs. No vapor barrier. The shower pan itself resembles a box with no top, that is the sides extended up the wall about 9 inches. The cultured marble panels where 3/8 inch thick and were siliconed to the green board and extended over the pan sides and rested on the bottom of the pan. All joints were siliconed. It looks great!

  7. #7
    Forum Admin, Expert Plumber Terry's Avatar
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    The installers around here use greenboard too.

    The cultured marble is a good product.

    Designers Marble in Woodinville
    Last edited by Terry; 06-03-2010 at 03:29 PM.

  8. #8

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    Do you think greenboard is superior to Durock for this application?

  9. #9

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    Cheaper and easier to work with.

    Being you are covering it with a product which is impervious to water, greenboard is more than enough. If you were looking for additional insulation you could consider Wedi board, but it is rather expensive.

    Paul

  10. #10
    Engineer chassis's Avatar
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    omo, if you would like additional advice on this question, try posting your question in the Tile Forum/Advice Board at www.johnbridge.com. Good site focusing mainly on tile and stone issues, but showers are a large proportion of the discussion topics.

  11. #11
    veronashowers
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  12. #12
    DIY Member TWEAK's Avatar
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    The cultured marble is waterproof. But the joints are not.

    It's similar to tile. The water that gets behind tile gets through the grout lines, not the tile. Yes there is far more linear feet of grout than you will have with your cultured marlble, but don't ignore the joints. The silicone or polyurethane sealer the installer will use in the corner is good when new. It doesn't last forever.

    In my experience, I do not like greenboard in showers or tub surrounds. It is fine for the rest of the bathroom where humidity is high but in "wet areas" I would never use greenboard. If your caulk starts to leak it will get very wet and turn to mush.

    Cement board could be used, but it's not the best method. Cement board is also not waterproof. Put a garden hose on a piece of Wonderboard and you will see this very clearly. If you want to use it, put down drywall first, then a vapor barrier of 6 mil polyethylene. Some people use 15# felt paper, but I strongly believe that 6 mil or heavier poly will last forever, the paper will not. Then hang your cement board. You can use alkali resistant mesh tape and thinset at the joints. If you REALLY want a watertight job, put some P&L polyurethane door and window sealer (not silicone) one each cement board screw. Then hang your cultured marble.

    The problem with the above method is that water will eventually get through the seams unless you re-caulk with 100% silicone or polyurethane every year or two. If your seams start to leak, the cement board behind will get saturated and you can definitely grow mold in cementboard. That's why the above is not the best method.

    The most bulletproof, long-term thing thing you could do IMO would be regular sheetrock. Do not tape or mud the seams. Then install one layer of Schluter Kerdi over the sheetrock, exactly like a tile job, according to Schluter's instructions. You apply the Kerdi with unmodified thinset, mixed very loose. The Kerdi is pricey, it will cost you about $250-400 depending on the size of your shower. But that is cheap compared to what your spending on the cultured marble. The Kerdi IS totally waterproof and vapor proof. Use Kerdifix sealant around all the penetrations and where the Kerdi goes up over the lip of the pan. Kerdifix is over $20 for a caulking gun size tube, but you only need one. Then put up your cultured marble walls with thinset directly to the Kerdi... maybe two screws through to the studs located up high (the cultured marble can be filled so that you will never see the screws).

    Most tile places sell Kerdi in full rolls, which is very expensive. You can order just what you need from "tile experts" on line.

    The thing about the Kerdi method is that the if any water gets through the caulk joints, the first thing it sees is the Kerdi membrane -- not cement board or greenboard which it will saturate. You don't want anything that holds water behind the cultured marble.

    Some may think this is overkill but if it's your house and if you've ever had water damage you will probably want the best solution. Good luck!

  13. #13
    Forum Admin, Expert Plumber Terry's Avatar
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    All cultured mable in the Seattle area is installed over greenboard.

    The marble panels are water tight.
    The joints are made with Silicone.
    The stuff looks great 30 years later.

    TWEAK is guessing on his answer.
    Don't make it harder then it needs to be.
    The reason for greenboard, the glue sticks to it.
    You don't want the marble panels falling down.
    The water never gets past the marble and silicone.

  14. #14
    DIY Member TWEAK's Avatar
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    Terry, I agree that your solution may be fine IF the silicone holds. Have you ever seen caulk on a tile install that needs to be re-done? This is no different.

    If there is a bit of dust, skin oil or other surface contamination the material the sealer won't hold as it should. Or if the installer doesn't get it down perfectly. The homeowner will not know about the problem.

    If the house settling , and normal expansion and contraction, the shower can move a bit and the joints start to seep water. The homeowner will not know about the problem.

    If the escutcheon around the valve lets a bit of water past, the greenboard will turn to mush. The homeowner will not know about the problem.

    Until it is too late.

    I am not guessing, I have done this. Before I did it I was in touch with Schluter Systems, asking about how to insure water couldn't get through the corner seams. This was their recommendation for an absolutely bulletproof installation.

    The panels will not fall down. I suggested screws at the top into the studs -- better than glue. You can fill the screw holes easily on cultured marble and never see it.

    People said the the same thing about tile installed directly over greenboard. Now it's no longer approved because years later many people had water damage.

    For 1/2 a day of effort and a few hundred bucks of Kerdi - which is easy to hang - you can have an installation that you will never have to worry about.

    Judgement call.

  15. #15
    DIY Senior Member chefwong's Avatar
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    I agree with Tweak with one exception. CBU in any wet application. If you want to take it to another level, CBU with Kerdin.
    I would not do regular sheetrock with Kerdi . If a PRO did it, I would consider it.

    Sheetrock with Kerdi - the variable I see for a DIY is making sure you are sealing the areas where the valve stems, stubs, etc don't penetrate and make the wallboard underneath soft..

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