Can I overlap Durock over a 1/8" tub lip without notching or shimming?

Discussion in 'Shower & Bathtub Forum & Blog' started by Don Williams, Nov 27, 2018.

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Can I overlap Durock over a 1/8" tub lip without notching or shimming?

  1. yes

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  2. no

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  1. Don Williams

    Don Williams New Member

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    Can I overlap Durock over a 1/8" tub lip without notching or shimming?
     
  2. cacher_chick

    cacher_chick Test, Don't Guess!

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    What are you using to waterproof the backerboard?

    You could bring the durock down to within an 1/8th of the lip and then seal the 2 together with kerdi-fix & kerdi-band.
     
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The big question is, do you want the bottom row of tile to bow out? If not, you have to either shim things so the cbu can clear the tiling flange or stop it above the lip.

    I'll second the moisture containment question. There are two general ways to deal with moisture:
    - put a moisture barrier behind the cbu and lap it over the tiling flange OR (not both)
    - apply a surface waterproofing material to the ENTIRE surface.

    FWIW, using a waterproof layer of banding (like KerdiBand) does not meet either the manufacturer's or industry standards. Since it is waterproof, any moisture that gets into the board above it will be trapped behind the wall. Neither tile, grout, nor cbu are waterproof, so you need to do something to keep moisture from getting into the walls. They won't be damaged by becoming wetted. The walls behind can and probably will long-term. This is why it needs to be CONTINUOUS at least up to the height of the shower head to abide by industry standards, either continuous behind it, or on top of it. Kerdi is good stuff, but like anything, needs to be used properly.
     
  5. cacher_chick

    cacher_chick Test, Don't Guess!

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    There was no mention of it being a shower, so my greatest concern is sealing the gap at the tub lip.

    Jim, with your method of installing the vapor barrier behind the backer board, the water runs down and is trapped at the bottom because everyone caulks the gap below the bottom row of tile. This is where the failure point is in many installations.
     
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    FWIW, it's not my method...it's one of those listed in the TCNA handbook, which is the industry guidelines. Some suggest leaving small gaps for any moisture to escape. But, so many people think tile is waterproof, it gets messy. With today's porcelain tile, they nearly are, though. Some studies have shown 70-80% of tiled showers are not done to industry standards, things get a bad rap. It's not particularly hard technically, but all it takes is one omission or error, and things can fail...you must have good workmanship and understanding...something sorely missing in the industry except for much too small of a minority.

    Basically, if you use waterproofing on top of the backer, it must be complete up to at least the top of the shower head. If you opt to do a moisture barrier behind it, it must lap into the tub over the tiling flange. Anything else and you risk moisture getting into the structure and rot. So many people don't level the tub, and that can allow stuff to pool and create problems, too. It all works if you do it 'right'.

    Personally, I like a profile at that junction...no caulk to fail, and it can breath a bit. Same in the corners. Per industry standards, all changes of plane or materials needs an expansion joint. The most common is caulk which can fail, followed by an actual gap, which in a shower isn't a good idea!
     
  7. Don Williams

    Don Williams New Member

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    What I men is the lip of the tub is 1/8" THICK (it is a Bootz sheet metal tub. If I overlap the thickness of the tub (1/8") will the Durock make the transition or crack due to the 1/8" difference between the studs and the tub lip?
     
  8. cacher_chick

    cacher_chick Test, Don't Guess!

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    You should not overlap the tub flange with the backer board without shimming the studs out so that everything is flat when completed.
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The general rule depends somewhat on the size of the tile. If at least 1/2 of the bottom row of tile will be supported by the cbu, you can cantilever the tile over the tiling flange. You still need to have either a moisture barrier behind the wall or a waterproofing done on top of the cbu, and that needs to be sealed to the flange, or if behind, lapped over it.
     
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