Gap Bottom Shower [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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08-04-2019, 08:13 AM
I built a niche shower using a Kohler shower base that has a 1 inch flange. I furred out the 2 x 4s, covered them with heavy tar paper (never again) and covered that with concrete backer board. The tar paper was sealed to the top of the flange and the backer board goes down over the flange to a quarter of an inch above the shower base. I used Versabond and standard 6 inch square tiles. If you add up the thickness of the backer board, the Versabond and the tile is it least an inch deep from the surface of the tile back to the flange. The first time I sealed it, are used caulking backing strip and silicone caulk. Please try not to laugh but the second time I used multiple layers of silicone. However every couple years it’s disgustingly moldy and I end up ripping the caulk apart again. It always seems like the bottom of the caulk is not stuck to the shower base and there is definitely moisture underneath. Short of ripping the whole thing out, does anyone have a decent suggestion as to what should be done with this gap. I’ve wasted countless google hours and really can’t find a right answer. Please note, the shower base does not have an indent for a drain on the surface next to the flange.

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08-04-2019, 09:33 AM
Here is a drawing of the problem.


08-04-2019, 01:33 PM
I'm thinking a picture or a sketch would be helpful. So you're saying you tried to put caulking on this one inch bottom of your tile cement backer unit assembly for why?

08-04-2019, 03:25 PM

08-04-2019, 03:27 PM
Gap at bottom...depth is the question.

08-04-2019, 03:31 PM
Better photo.

08-04-2019, 03:37 PM
The gap between the tile and the shower base is an inch deep...goes all the way back to the flange. Trying to understand how that gap should be caulked or what else I should have done.

08-04-2019, 04:00 PM
The base is on a double subfloor and is sitting in concrete, so it doesn’t move around. I sistered the floor joists to reduce deflection so the floor tile in the rest of the room wouldn’t crack.

08-04-2019, 08:08 PM
That joint will probably be a maintenance issue from now on. Just like the joint where the tile meets a bath tub, the caulk or silicone has to be periodically replaced.

You might find some foam backer rod to fill in with and just silicone the outside 1/4. It might be best to leave some small gaps/weepholes in the silicone so the water can escape.

08-05-2019, 07:54 AM
Just for my edumacation can you install the backer board over the flange to 1/4" above a solid surface pan with a bead of caulk or Kerdifix between the flange and bottom back edge of the board and then have the tile overhang the backer by 1/8" leaving the ensuing 1/8" gap un-caulked to allow for drainage?

Case of beer for the first person who can de-code what I just asked. :yeah:

08-05-2019, 08:06 AM
You can, of course, but it's not what you want to do. If you stop your backer board above the top of the tiling flange, you want to use a strip of direct bonded waterproofing sheet membrane to attach to the tiling flange and to the wallboard above. Good to have a minor wedgie in the membrane at the junction for movement accommodation.

If you'll go to the Schluter Kerdi website and download the Installation Handbook and look at (I think) page 33 you'll see good photos of what I'm describing.

My opinion; worth price charged.

08-05-2019, 08:59 AM
My pan is different than the OP's but just for the sake of clarification as long as the backerboard waterproofing is properly tied into the pan is it necessary to caulk the gap at the bottom or can you just leave it open to allow for better drainage?

08-05-2019, 10:42 AM
Really wish these questions were on your project thread, Lynne.

You can leave that gap open if you want.

08-05-2019, 10:50 AM
I would leave it open also.. That allows any water to escape!

08-05-2019, 11:13 AM
Yeah, CX, I figured you would say that which is why I deliberately kept my project specifics out of it. I really think my question is pertinent to the OP's. Clearly caulking it is trapping water and causing mold. So does the gap need caulked at all? If it doesn't then perhaps that is what the OP needs to do to prevent it but I didn't want to suggest it because I'm just a DIYer not an expert.

08-05-2019, 11:31 AM
Thanks to all of you for the input. I will probably caulk and leave gaps for weeping.

08-05-2019, 11:50 AM
Lynne- I am the definition of a DIYer. The shower in question was the first thing I ever tiled. (Well, that whole bathroom - I did the floor too.) Since then, I took the half bath that was in our master and turned into a full bath. I’m attaching a snap of that shower. My husband thinks I’m crazy, because what 50 something-year-old woman takes on something like this? It makes me happy though, so he doesn’t complain too much.

08-05-2019, 11:57 AM
Sterlinghick, please check your Private Messages.