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Old 08-14-2019, 03:49 PM   #1
makemenuconfig
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Help me fix my tub-to-backer transition

Hi, I've been reading for quite some time but finally have a question to talk through with you all.

I am working on a bathroom remodel in my own house. The acrylic tub has been set in mortar and is quite sturdy, It is screwed into the studs through the flange. I know there's a mixed opinion on whether to overlap the substrate over the tiling flange, or to end it above with a gap. I chose the latter to better tie into the adjacent drywall. I plan to waterproof with Aquadefense/RedGard but have not done so yet.

I got the 1/2" cement board installed and used thinset and mesh tape on the joints. I had read to thinset and mesh-tape across the transition between the tiling flange and the bottom of the cement board.

Now yesterday I was hanging some drywall on the ceiling above the tub and had to step on the front apron / back ledge to reach some screws. I have done this a few times without issue. When I was standing there this time, I heard a small crack sound. I went to investigate and see a tiny crack in the thinset at the tiling flange. If I push hard on the front edge of the tub, I see the tub moving slightly behind the thinset like the thinset separated from the flange. Not sure if I am describing that well enough.

The front edge of the tub flexes a little since the bottom of the apron isn't quite touching the subfloor. It needed to be this way to level the tub. Now this is a separate issue, but I was thinking of either shimming the bottom of the apron or getting a little thinset under there when I tile the floor. The feet of the tub rest on the floor and the tub is level.

My main question is what to do about the backer-tiling flange transition. I am worried about leaving it as-is considering it seems the thinset came loose from the flange in this one spot. I am worried about it not holding up over time as the tub undoubtedly flexes as people get in and out, fill it with water, etc. I was considering re-doing the flange transition with kerdi-band. I assume kerdi-fix to bond the tiling flange to the kerdi-band and modified thinset to bond the kerdi-band to the cement board.

Are there any tips for removing the thinset in this area? Do you have other ideas for how to transition from cement board to tiling flange?

Thanks!
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Old 08-14-2019, 04:30 PM   #2
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Welcome, Evan.

The only good method I've seen for dealing with that junction when using a direct bonded waterproofing membrane on your walls is to use a strip of a sheet-type membrane that you can bond to the tub's tiling flange and to your wallboard. Preferably with even a tiny bit of a wedgie in the gap for movement accommodation. If you'll go to the Schluter Kerdi website and download the Installation Handbook you'll see (I think on page 34 these days) a very good depiction of the procedure for that.

While you can probably do that with a liquid-applied membrane and a reinforcing fabric, I'd really recommend you get a strip of either the Kerdi membrane or the USG Durock Shower System membrane to use for the purpose. The USG product is easily found on Amazon, even. Kerdi Fix is very good for the adhesive to the tub flange, or you could use something similar, like SikaFlex, from your local home center. 100 percent silicon might also suffice, but would not be my first choice. You'll bond to the CBU with thinset mortar.

Your thinset mortar is not gonna bond with the tub flange and even if it did the mortar would not allow for any movement at all. And there will be movement there as you've seen. Cutting out the mortar and mesh tape in that area should not be a difficult task.

My opinion; worth price charged.
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Old 08-14-2019, 10:56 PM   #3
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The way I handled it in my hydroban waterproofed tub area is the following.

Hung cement board with a 3/16" ish gap above tub rim to allow for movement. The acrylic tub (MTI) had a 1/16 factory bonded tub flange that is designed to go behind the wall board. I shimmed the studs above tile flange 1/16 to make up for tile flange thickness.

I carefully cleaned the tub deck with denatured alcohol. I then blue taped the tub deck leaving only the area directly under the wall board exposed. I then carefully pumped this gap full of plenty of hydroban sealant. I immediately tooled the sealant with a straight edge insuring the gap was well filled with no holes and flush. Immediately pull away the tape. You should now have a pefectly filled gap between wallboard and tub deck that allows for tub movement. Allow sealant to fully cure.
I then taped the tub deck again putting the edge of the tape right up the sealant bead. I then waterproofed the cement board with hydroban liquid membrane and used a brush to fully cut the hydroban in all the way to the tub deck. Hydroban is painted over the hydroban sealant. Immediately pull up the tape before the hydroban cures.
Repeat for 2nd coat.

When you tile leave a similar sized movement gap with tile and tub deck. Fill this gap with 100% silicone in your desired color. Do not grout this gap! Grout will transfer movement to your tile installation.


What you did there is a no no as per CX. Thinset will not stick to that tub and that fibertape is not doing anything. Further you need to have a movement joint there and the thinset does not provide that.
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Old 08-16-2019, 09:22 AM   #4
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Thanks all for the advice, this sounds like a plan. I'll have a go at removing the thinset and mesh tape and will use kerdi-band with kerdi-fix on the tub and unmodified thinset on the CBU. Schluter has a great video demonstrating this technique.

Everywhere I look people say you shouldn't mix systems because there's no reason to and you loose any warranty. But I'm not particularly interested in using kerdi in the whole surround and still want to use aquadefense. Can I just go over the kerdi-band transition down to the tub with aquadefense? I know neither company tests this scenario. I don't want to set myself up for failure, but I'm not seeing why this would be an issue unless one of the materials won't bond to the other.
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Old 08-16-2019, 09:42 AM   #5
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Should work fine, but I'd recommend you apply your HydroBan to the walls first, then apply your KerdiBand to the wall/tub flange junction.

We know the thinset mortar (I'd use a modified mortar) will bond the KerdiBand to the HydroBan, but I'd be less confident of the bond between HydroBan and the KerdiBand. Up to you.

My opinion; worth price charged.
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Old 08-19-2019, 02:58 PM   #6
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Got the thinset removed; took less than an hour to do. It chipped off the flange fairly easily, but was pretty tight between the flange and backer board.

Doing the liquid membrane first makes sense in my head but kind of feels backwards to me. I ended up making a mockup test this weekend. I took a piece of kerdi and coated it with aquadefense, leaving an area masked with blue tape to give me a corner to start peeling. I did 2 coats and left it for about 24 hours.

Peeling off the aquadefense was not easy. Once started, took moderate pressure to pull apart. The fleece completely separated from the orange kerdi leaving a smooth orange sheet behind. That would imply the bond between aquadefense and kerdi was higher than the bond between the two parts of kerdi. This test doesn't speak for the longevity of such a solution, but it does show the aquadefense can certainly grab onto the fleece fibers.

I like this solution because it gives me a complete, uninterrupted waterproofing membrane from top of the surround to the bottom of the flange like I originally wanted. I think this is sufficient, but still weighing my options.
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Old 08-19-2019, 03:17 PM   #7
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It's your shower, Evan. If you like it, use it.
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