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Thread: Kerdi Shower Tray ST Subfloor Requirements

  1. #1
    DIY Junior Member lespaulboy's Avatar
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    Default Kerdi Shower Tray ST Subfloor Requirements

    I'm in the process of installing a Kerdi 48" x 48" shower tray that will be cut down to 35" x 35". My subfloor is level and is 5/8" OSB. My joists are on 16" centers. I've been looking through the Schluter Installation Guide and this forum, but can't seem to find any definitive information on subfloor requirements other than having to be level.

    Will I be fine installing my tray over the 5/8" OSB, or should I be looking at putting down a layer of cement board (don't want to raise height too much)? The rest of my shower walls are Durock cement board.

    Thanks.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A couple of things: cbu does not increase the deflection rating of a subfloor - IOW, adding some will not make the floor stiffer; 5/8" material, is the absolute minimum strength required to lay any tile. If that first layer is not T&G, I'd want a second layer of ply before I did anything. Then, the 'system' requirement would be that the floor deflection ratings should be adequate for setting tile and that would depend on the type of tile, ceramic or natural stone. It's been awhile since I read the installation instructions on the Kerdi pan, but I seem to remember they want the floor structure to be at least L/360 deflection rating which is the minimum for ceramic tile. This is determined by the thickness, depth, spacing, species, and length of the unsupported span of the joists. If your deflection ratings are at least L/360, you should be able to just thinset down the pan and go. If you were going to tile the rest of the room, I'd consider adding another sheet of ply, at least 3/8" thick (the thinnest suitable to a second layer with tile) under the whole floor. Hassle with 5/8" first layer, especially with OSB, is that it's very easy to strip the hole with the screw. It may take predrilling the holes and careful torqueing the screws to get things together well.
    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 Junior Member lespaulboy's Avatar
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    Thanks for the input Jim... I ended up calling Schluter Canada directly (probably should have done that before posting) to ask the question. The response was that I would be fine to install the system over 5/8" OSB. To address you concern above above, the OSB is T&G. Being an engineer as well I tend to question things and wanted to throw it out to the masses to see what others have done and their experience and recommendations on this matter.

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