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Unread 01-20-2022, 04:08 PM   #1
max_power
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Gap Between Tub and Floor

Hello. I am working on a bathroom renovation and have run into an issue. We set the tub so that it is level but now there is a ~3/4in gap under the tub apron at one end of the tub (the other end sits right on the sub floor). The subfloor is 3/4in plywood and the floor is about 4.5'x8'. Would it be best to add an additional layer of plywood or should I try pouring SLC? I plan to use ditra and a 2X2 porcelain mosaic to finish the floor.
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Unread 01-20-2022, 04:36 PM   #2
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Welcome, Alex.

Without knowing more about your floor structure, I can't say whether you actually need a second layer of subflooring, but it never hurts. If you don't think the visual difference in height will be a problem for you, the plywood would seem to solve your problem.

You could also use the SLC if you want, presuming your floor structure is suitable for a ceramic tile installation as it is. Keep in mind that your tiles don't care a whit about level, they care only that the floor is flat.

My opinion; worth price charged.
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Unread 01-20-2022, 04:51 PM   #3
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Thank you for the reply CX. I am mostly concerned about the potential of water damage that would be caused by having a gap between the tub and the surface I am putting the tile on.

When choosing between SLC and plywood I was thinking going with plywood would give me a lower maximum thickness and be easier to install since I have not used SLC before. For plywood I did not know if I could get away with using 3/8"; thinking that the 3/8" of plywood plus the 1/8" for the ditra would leave me with a 1/4" gap that I could fill with Schluter fix. Would this be a bad idea?
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Unread 01-20-2022, 05:22 PM   #4
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I would use nothing thinner than nominal 1/2" plywood in your application. While the nominal 3/8ths" might be technically suitable, most of the plywood that thin you'll find is not sufficiently flat and if it is flat it's still difficult to install flat on the existing subfloor.

If you actually want to make a waterproof bond between your Ditra and the face of the tub, you'll really need to use some KerdiBand, fold it up onto the tub face a quarter-inch or so and bond that with some KerdiFix. Then bond it to the Ditra with thinset mortar. Still won't give you a waterproof floor, but at least it would take care of that particular joint if that's your goal.

Entirely up to you.

My opinion; worth price charged.
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Unread 01-20-2022, 05:37 PM   #5
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Would it be acceptable to do the following:

1. Install 1/2" plywood
2. Install ditra
3. Install a strip of kerdi band on top of the ditra, leaving a 1/4" between the tub and the kerdi band and applying Kerdi fix to this 1/4" gap.
4. Installing my tile

This would still leave a gap between the new 1/2" plywood and tub but after the tile is installed there would be no visible gap.
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Unread 01-20-2022, 05:46 PM   #6
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Not for me. The Ditra is already waterproof, Alex. What you're trying to waterproof is the gap between the Ditra and the tub face. Putting KerdiBand over the Ditra won't help. You need to fold the KerdiBand up the face of the tub and bond it to the tub with the Kerdi Fix.

Just using a caulking or flexible sealant such as KerdiFix alone in that gap would not be sufficient in my opinion. But, again, entirely up to you.

You would also want to apply a bead of sealant, most likely one color matched to your grout, between the tile and the tub face. That is not for waterproofing, that's just to fill the gap you have there and provide movement accommodation.

My opinion; worth price charged.
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Unread 01-20-2022, 06:06 PM   #7
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Thank you for explaining. I think I am getting confused since the Schluter videos show them putting down a strip of Kerdi band over the Ditra along the length of the tub apron (but not on the tub). So, for my scenario where there is a gap, to get the same level of waterproofing as a tub that sits flush with the floor I would need to attach the Kerdi band to the tub?
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Unread 01-21-2022, 09:59 AM   #8
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Probably needless to say, even if the length of the apron sits flush on the plywood floor it won't be water proof on its own. At a minimum that joint would still need a sealant applied, but that would only prevent water from getting under the apron, and does nothing to protect the plywood.

I've not seen the Schluter video you're referencing, Alex, but my guess is it shows the Kerdi band being installed on top of the Ditra, with the edge of the band butted against the apron, then the band is sealed to the apron with Kerdi Fix. Which would be fine, if everything is flat and level.

In your case, given the 3/4" gap at one end, after the 1/2" ply and Ditra is installed you're still going to have about a 1/8" gap, maybe a bit less with the Kerdi on top. You might be able to seal the remaining gap with KF, but you can definitely seal it by lapping the band onto the apron.

The challenge you'll face is keeping the band to only 1/4" high on the apron so that your floor tile hides it. You'd also want to tape the apron to prevent the KF from ending up places you don't want it to.
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Unread 01-21-2022, 11:25 AM   #9
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If I went with 3/4" plywood it would be fine to then butt the Kerdi band up to the tub apron and not worry about lapping it over the tub apron a 1/4"? If so, it sounds like that might be the easiest solution from a waterproofing perspective. It may cause issues with the toilet flange and transition to the hall so I will just have to decide which option will cause fewer headaches.
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Unread 01-21-2022, 03:46 PM   #10
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In my mind it would be ideal to fold the KB onto the apron. Whether you can achieve that while hiding the edge of the kerdi with the floor tile, and getting a full end-to-end seal of the band to the apron with kerdi fix - while not getting kerdi fix all over the apron (no idea how easily it wipes off), I dunno. I would be pretty comfortable with butting the kerdi edge against the apron, then sealing it with kerdi fix. I believe between that seal, and the silicone caulk you'll use between the floor tile and the apron (which - as cx pointed out above, isn't intended as water proofing but will, at least, prevent most water from getting in there), you'll be in pretty good shape.

Other considerations: The two inside corners created by the left and right ends of the apron where they abut the side walls, where those side walls meet the floor, the top front radius (left and right) of the tub, and the tub deck/wall junction. What are you using for the tub walls, a cement backer? Hardie? A water proof foam board? Does the tub have a flange? What water proofing method do you intend to use?
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Unread 01-21-2022, 06:51 PM   #11
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For what its worth this is what I did. I used a bead of kerdi fix right at the edge of the tub and laid the band flat.
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Unread 01-22-2022, 09:27 AM   #12
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Thanks for the reply Dan. Some of the additional info:

1. Tub surround walls are drywall.
2. Kerdi will be run over the drywall.
3. Kerdi band will be run on the tub flange-wall joints.
4. Kerdi band will be butted up against the tub, leaving a 1/4" gap to apply kerdi fix, from the floor to the tub deck, there is no flange here.
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Unread 01-22-2022, 09:28 AM   #13
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Phil - Was there a gap between the floor and the tub apron there? If so, about how big was it?
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Unread 01-22-2022, 09:54 AM   #14
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If I remember correctly probably about 1/4in and I filled it with kerdi fix.
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Unread 01-22-2022, 10:05 AM   #15
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Has it held up so far or are you still finishing up your project?
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