Kerdi Drain Flange Installed Directly on Wooden Sub-floor Without Mortar Float?

Discussion in 'Shower & Bathtub Forum & Blog' started by Tony31, Jul 9, 2019.

  1. Tony31

    Tony31 New Member

    Jul 9, 2019
    Houston TX
    Searched a lot for any discussion on this topic and found the proper mortar thickness but not much discussion on what happens if you don't follow the manufacturers instructions. I know that the manufacturer required mortar thickness under the Kerdi drain on wooden sub-floor is 1" and on concrete its less because it provides more support.

    However, In my shower I am able to achieve a curbless shower if I slope my mortar bed to 0" under the Kerdi flange and screw the flange to the wooden sub-floor. My wooden sub-floor is 3/4" plywood with a 12" span and the drain is very close to a double-wide 2x10 floor joist. It seems like this would provide more than adequate support for the drain to avoid any future structural issues.

    My question is... am I taking too much risk on the structural support for the reward of a fully curbless shower? Has anyone installed these Kerdi flanges direct to sub-floor? I am not worried about the manufacturers warranty at all but I am worried about whether this will work.

    P.S. I am not going to lower the sub-floor any further because I would require an engineer to review the floor joist modifications and I don't want to deal with that. I would rather live with a 1" lip into the shower than lower the sub-floor.

    Attached Files:

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The lip of the drain is maybe 1/4"'s been awhile since I had one in my hand. Deck mud is just not structural at anything close that that minimum level which is why the recommendation is 1". In reality, the deck mud manufacturers call for 1.5" in most cases, but some get by with as little as 3/4" over a wooden subfloor. Only when you can bond it to a slab is it viable thinner. Over a wooden subfloor, the mud is floating - usually it is installed over a slip sheet and lath. Throw that in around the edge of the flange, and the mud thickness decreases even more. It doesn't take much of any flex for it to crack apart when that thin.

    Personally, I think you're asking for problems. A preformed, foam pan MIGHT work, if the size is closer to what you want. If it's too small, you can fill in around the edge(s) with deck mud, as that is where it would be the thickest. When using the foam pans, that does get bonded to the subflooring, so it adds some strength for the pan and tile on top.

    You might get some suggestions over at where they build showers daily.

    Sometimes, a linear drain can help, but those drains are lots more expensive as the long grate that must not only look pretty, but not have any twist to it is much more difficult to make.
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