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Thread: Cement board under shower and tub?

  1. #1
    DIY Junior Member ehlinn's Avatar
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    Default Cement board under shower and tub?

    I am getting ready to start rebuilding my gutted bathroom. Is it a good idea to cover the entire floor with cement backer board, and install the tub and shower on top of that rather than just installing them on the plywood subfloor? I noticed my old shower and tub were placed right on the plywood subfloor and the tile underlayment and tile were butted up to the sides of the shower and tub.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The only thing you might want to put on the floor under the tub is some deck mud to support the bottom (assuming it is not cast iron). My unprofessional opinion.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member ehlinn's Avatar
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    Thanks. It is a fiberglass tub, and I am definitely going to put some deck mud under it.

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    DIY Member Squints2See's Avatar
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    is deck mud slang for concrete?...........or is this something else

    I'd like to know because I gotta do the same thing, install a new one piece fiberglass tub and the bottom of the tub sits about 4 inches off the floor......Do you have to have support under the fiberglass tub, or is this an "optional" thing?

    Thanks,
    Todd

  5. #5
    DIY Junior Member ehlinn's Avatar
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    A lot of guys use sand mix, which you can buy already mixed or mix yourself. It is portland cement and sand.

    http://www.quikrete.com/catalog/SandMix.html

    It is highly recommended to set a fiberglass or acrylic tub or shower in a bed of topping mix to prevent sqeaks and future cracking of the tub or shower floor. It also makes the tub or shower feel much more solid.

    I still want to know if anyone covers their entire floor with cement board before installing fixtures. Seems like it would be good to have under the tub and shower. Of course I will still be using the topping mix.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default board

    There is no reason not to do it, but also no overpowering reason to do it, since the water is not going to leak through the tub or shower base.

  7. #7
    Plumber RioHyde's Avatar
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    Thumbs up

    I dont put cement backerboard under tubs. I use sandmix, set it in place, anchor it, pipe it up. For asthetic reasons I like to have the backerboard as well as the tile come up against the skirt of the tub. I guess its really a personal preference. As was said by HJ, no real reason to do it nor any overpowering reason to do it. A few dollars more involved to put it under the tub I guess.

  8. #8
    DIY Junior Member ehlinn's Avatar
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    Default thanks

    Thanks for the tips guys.

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