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Unread 06-20-2021, 07:15 PM   #1
zinctoo
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Bath Remodel with a Tub

Hi everyone!

This forum was really helpful for me last year when I remodeled my shower, which is documented in this thread: https://www.johnbridge.com/vbulletin...d.php?t=129275

I'm now working on my guest/hall bath, which is a tub/shower combo. The room is currently gutted and I'm waiting for my cast iron Kohler Bellwether tub to arrive (actually, I'm waiting on my second Bellwether to arrive. The enamel finish on the first one was badly damaged in a few spots on the skirt, a problem which I didn't notice until after I installed it. Kohler agreed that it was damaged and Home Depot agreed to an exchange).

Having spent hours on this forum for my shower project, and now having spent several more hours on the forum reading about tub surrounds, I have most of my questions answered. However, I have one major question remaining, which is not a unique question at all. But I thought I'd ask about it too, as it relates to the specifics of my project. Plus, I figure I'll have other questions too and so I should go ahead and get a thread started for the whole project.

My current plan for this surround is to use Permabase CBU (readily available where I live) and use Aquadefense liquid-applied direct-bonded waterproofing. I'm going to tile to the ceiling and I'm going to install the CBU up to the ceiling as well.

The current alcove is 60 3/4" long which is slightly larger than the Bellwether tub, which is 60" long. I did not measure the original tub before I busted it out, but I would imagine that it was a standard 60". The original 1968 install used a mortar bed, which I guess explains the extra space. You'll note in the attached photo that some of the studs on one of the alcove walls are new: the studs that were there were chopped up from a replumb in the 90s so I replaced them, but I kept the wall in its original location since I only replaced half of it.

At any rate, when I installed the new tub for the first time, I centered the tub in the alcove, leaving an extra 3/8" gap on each short side of the tub. A few hours later I realized the problem that this would present for waterproofing and tile install, which I can now correct.

3/4" is a lot to make up, but I believe that I can do this on the "plumbing" wall easier than I can on the wall opposite of this, which I'm referring to as the "door" wall. This is because the pluming wall is only about 5 inches wider than the tub itself so there are no consequences to other things by furring this wall out. Furring out 3' for the tub on the door wall would also require me to fur the entire rest of the wall so that the drywall and CBU will meet. Consequently, the door jamb would need to be made wider. I'm hoping to use a prehung door here and it may not accommodate the extra jam depth out of the box. I would like to avoid furring this entire wall if possible, which means that the 3/4" needs to be made up on the plumbing wall (moving the valve and any other plumbing as necessary).

Furring 3/4" gets me a tight tub surround but it still leaves open the question of how to bring the CBU into the tub. From reading through every post about this topic that I could find on the forum, I've identified eight different ways of handling this:

1. Furr the walls an additional 1/8", which is the thickness of the tub flange, and bring the CBU down into the tub, keeping it up from the tub itself (the "shelf") 1/4". This is the method described in TCNA (2020, page 230). However, this is the option that I prefer least because it requires me to furr out the entire door wall which also requires making the door jamb wider. The Bellwether's flange is only 1/2" tall, so I question the value of having only 1/4" of the flange behind the CBU anyway.

If I did go this route, since I'd already be committed to furring out the door wall, I'd probably divide the total inch that I need to make up (60 3/4 - 60 + 1/8 + 1/8) and furr each short side of the tub 1/2". I'd just figure out a solution to making the door jamb wider when I got to it.

2. Keep the CBU on the same plane as the flange (no furring), leaving a 1/4" gap between the CBU and the tub shelf, and fill this gap with 100% silicone. This is the method described in Kohler's instructions. However, on my shower project, I was advised to not paint the bottom of my CBU with Aquadefense because if water managed to get into CBU, with this bottom edge painted, it would have nowhere to evacuate. Caulking the bottom of the CBU where it meets the tub with silicone would present this same problem: if water got into the CBU, where would it go? (Out the back into the stud bay, I guess.)

3. Keep the CBU on the same plane as the flange, give it a 1/4" gap above the tub shelf, and drape 6 mil plastic down over the flange to bridge this gap and take the plastic about 6" up the backside of the CBU. Paint the entirety of the CBU with Aquadefense. This is similar to how a shower pan liner comes up the backside of the CBU in a traditional shower. There's a guy on YouTube who does this but using stick-on window flashing instead of plastic: https://www.youtube.com/watch?v=FYSZQp-Y_kI.

4. Keep the CBU on the same plane as the flange, give it a 1/4" gap above the tub shelf, and bridge this gap with alkaline-resistant mesh and fill it with thinnest (just like you'd handle corners and joints). Paint the entirety of the CBU, including the mesh/thinset bridge with Aquadefense. This seems like a good plan, but by bridging this gap in this way, haven't I effectively removed the 1/4" gap?

5. Keep the CBU on the same plane as the flange, give it a 1/4" gap above the tub shelf, and bridge this gap with Kerdi-Band. Use Kerdi-Fix to glue the Kerdi-Band to the flange and thinset to affix the Kerdi-Band to the CBU. Paint the CBU with Aquadefense, bringing the Aquadefense layer down atop the Kerdi-Band few inches. This is the method used on this forum post and it's even John Bridge-approved: "That will certainly work" - https://www.johnbridge.com/vbulletin...ad.php?t=98181. On first impression, I'm averse to mixing products but if it works, who am I to question it?

Also a note about the thinset type: in this scenario I would expect that I could use modified thinset, since the reason for requiring unmodified thinset – sandwiched between two Kerdi products, the water cannot evaporate so the thinset doesn't gain strength –*doesn't apply in this situation because the Kerdi-Band is being affixed to CBU, which is far from impervious (source: https://www.schluter.com/schluter-us...mic-stone-tile).

6. Keep the CBU on the same plane as the flange, give it a 1/4" gap, and bridge this gap with the Mapei-approved waterproofing fabric which gets painted with Aquadefense along with the rest of the CBU. This is the same fabric that I will use in the corners and around the niche. By my understanding, the fabric would not be affixed to the flange in any permanent way. This approach isn't very appealing to me because Aquadefense can be messy to work with and painting the fabric on the wall without something solid behind it will be very difficult. Should I paint the fabric with a few coats of Aquadefense first, and allow it to dry, in order to to stiffen it up before "gluing" it to the CBU with more Aquadefense? This approach has the benefit at least of not mixing products.

7. Use Kerdi for everything. Drywall and Kerdi Waterproofing Membrane (the stick-on kind) or Kerdi boards. Bring the Kerdi product down over the flange, but leaving a 1/4" gap. Frankly, this seems like the simplest method, but it is the most expensive.

8. Forego using Aquadefense entirely and use 6 mil plastic behind the CBU the whole way. Bring the plastic down below the CBU, over the flange. This is a well-known approach, but I've always wondered, what about the water which gets into the studs by way of the CBU's screw holes?

Of all these approaches, I prefer numbers 3, 5, and 7, with number 7 coming in last only because of cost. What does everyone think? Which approach would you take? Are there additional methods that I've not accounted for?

I imagine that there are some folks who would be against any approach except number 1, furring to allow the CBU to come down in front of the tub flange. If you're in this camp, do you have a backup preference? Or is this a hard and fast, wont-do-anything-but requirement?

Thanks to everyone in advance!
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Unread 06-23-2021, 07:34 AM   #2
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Skyler,

Fur out the plumping wall to close the gap, move plumbing as required, though that copper and brass drain assembly might pose a challenge.

Then, either 7 or 8. If 7, I'd use Kerdi boards and simply notch the bottom edge so that it fits over the tub flange, then Kerdi Fix the edge to the flange. If 8, don't worry about the screw holes through the poly. I recently gutted my 40 year old guest bath and the tub alcove was 2X4's, drywall, mastic, and tile. No vapor barrier at all. Some drywall damage, a little bit of mold in the insulation of one wall, but zero wood damage.
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Unread 06-23-2021, 03:15 PM   #3
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Might want to fur out the other end wall and leave the plumbing alone.

But if you set the tub dead in the middle of the space you'd have 3/8 on each end. That could be handled by the thickness of the cbu.
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Unread 06-24-2021, 09:28 AM   #4
zinctoo
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Thanks for your replies, guys.

John, your suggestion to center the tub without furring (if this is what you're suggesting) is certainly the easiest. Then I suppose I'd use one of the waterproofing methods endorsed by Dan.
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Unread 06-24-2021, 09:31 PM   #5
zinctoo
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John, a follow-up question. Are you saying that it's OK to have the CBU on the same plane, or real close to the same plane, as the tub flange? If I center the tub, the CBU will hang over the flange by 1/8" at most.

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Unread 06-26-2021, 08:20 AM   #6
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I believe that's what John is suggesting Skyler.

The challenge there is that your tub's "flange" has very little height so your bottom row of tile will not hang much below the top edge of the flange.

But it's done all the time I suspect. If you use poly behind a CBU you'll have very little flange for the poly to drape over. If you use something like Kerdi board and bring it down to within 1/16th to 1/8th of the flange I imagine you could seal the gap between the two with something like Kerdi Fix.
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Unread 07-02-2021, 07:01 AM   #7
zinctoo
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I was all set to use Kerdi but I called my local Floor & Decor, the only place that sells it around here, and they say the Kerdi boards are on backorder and have been since May! Apparently none of the stores anywhere near me have any in stock, and they don't know when they'll be getting more.

Lowes sells Kerdi boards, but they're out of stock too, and Home Depot doesn't carry the 4x8' boards, at least not according to their website.

Guess I'll be using one of these other methods. I'm leaning towards #8, draping the poly behind the CBU and over the flange, but I've got so little flange there, I worry that the poly will get pushed behind the flange while I'm working on it, or it otherwise wont form a complete waterproofing surface from behind the walls to the tub.

I'm sort of leaning towards option #2 as well, at least so far as filling the gap between the tub shelf and CBU with 100% silicone. This is what Kohler says to do for this tub, and what TCNA says to do in general. Though, more accurately, TCNA says to use "flexible sealant." Does 100% silicone count as flexible sealant?
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Unread 07-02-2021, 10:00 AM   #8
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I didn't read your whole thread but many times over the years I've beveled the back bottom edge of the tile slightly to keep it from hitting the tub flange. This allows the tiles to slide down closer to the tub.
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Unread 09-03-2021, 12:20 PM   #9
zinctoo
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Hi all, long time since my last update. I managed to find enough Kerdi in town to do this tub surround. I shoved the tub against the door wall and furred the plumbing wall (and the back wall as necessary to get everything plumb).

The Schluter instructions say to secure the boards with screws horizontally every 16" and vertically every 12" (maximum). But what about on the ends of the board where it meets surrounding drywall?

Do I screw the Kerdi boards on the panel ends where they abut the drywall? And if I am supposed to screw here, do I need to extend the Kerdi band (which covers the screws) onto the drywall?

Also, forgive my in-progress tape and mud job.

As always, thank you in advance!
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Unread 09-05-2021, 07:31 AM   #10
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Hi Skyler,

Yes, you'll need to install the Kerdi screws and special washers on those edges where the Kerdi abuts the drywall. You'll then want to use thinset mortar and Kerdi band to seal those seams.
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