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Thread: waterproofing under tub

  1. #1
    DIY Junior Member tbb2's Avatar
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    Feb 2009
    Arlington, Virginia

    Default waterproofing under tub

    I have seen enough water damaged wood and cracked tile in old bathrooms to want to waterproof under the tub during my remodel.

    After a lot of exercise, Schluter's Ditra looks like it will handle some movement from heating cables and waterproof the floor.

    Schluter requires setting the tub apron onto the tile to distribute the weight over the Ditra. The only testing with Ditra was done with tile in place over it.

    I have never seen a cast iron tub apron set on top of the tile.
    Other than the challenges of installing the tub on a finished tile floor are there other reasons to bury the tub in the floor finish?

    I am not planning heating cables under the tub so I have a 1/4" difference in levels between under the tub and outside the tub.
    (If anyone has run cable under a cast iron tub to keep it warm I'm all ears.)

    Can I get votes on setting the tub on tile 1/4" lower than the floor outside the tub and then set tile scribed to the tub over that tile?
    Will you share what special prep and mortar are needed?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Sep 2004
    New England


    A properly completed tiled bathroom does not leak and cracking a tile only happens if you drop something heavy onto it! As a result, waterproofing underneath a tub is not generally done unless it's something like a clawfoot tub. If the tub is installed level, the walls are properly waterproofed to the tiling flange, and you install the glass door properly, or use a shower curtain, you'll not get any water out of the tub except what you might drip when you exit it, which wouldn't rot out anything! In new construction, the tub is one of the first things installed, so having the apron below the finished floor is normal. It also can hide some minor imperfections of either the tub or the floor. You could install Ditra underneath it, but you'd want to fill it with thinset prior to installing the tub. In fact, some people do this on a regular basis as it somewhat protects the towers and makes layout lines easier to mark since the thinset will hold a marking pen better than the plastic Ditra material. The key to a long-term, care-free install, is proper prep, planning, and execution per industry standards. Unfortunately, getting all of those accomplished is getting harder, as any one oversight, intentional or accidental, can compromise the final result.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Test, Don't Guess! cacher_chick's Avatar
    Join Date
    Jan 2008
    Land of Cheese


    I'm not sure if I completely agree with Jim, as most places I've been in the people leave a puddle outside the tub when they stand there drying off. Imagine having a house full of kids doing that. Waterproofing the entire floor for that reason might be a fine idea.

    For the slim chance of water under a tub being a problem, you could simply roll a couple of coats of Redguard or another similar product on the sub-floor where your are setting the tub. If you want to go to the extreme, you could build the entire bathroom floor like a big shower pan. A good cast-iron tub might be there for 40 years or more without anyone ever knowing what it looks like underneath it.
    Last edited by cacher_chick; 01-11-2013 at 04:12 PM.

  4. #4
    Forum Admin, Expert Plumber Terry's Avatar
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    Aug 2004
    Bothell, Washington
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    I have never seen water proofing done under a tub, much less a heavy cast iron tub.

    Normally you lay a bead of caulk between the tub and the finish floor. That is what prevents water from seeping under the tub.

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