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Unread 03-02-2021, 02:41 PM   #1
ZoeJR
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Zoe's DIY Bathroom - Dilemmas abound!

Hello,

I am a new homeowner in the process of finishing a bathroom. When we bought the house, the bathroom plumbing and electrical was already roughed in, so we are finishing it. I have decided to go with a Kerdi Schluter shower system, and have everything ready to go (I think). There is so much great info out there from the company themselves and this forum that I thought it was an easy decision.

The one question I can't find a definite answer to (even after speaking to customer service themselves!) is what to do about the cement backerboard seams in the shower. I know you can use drywall, but I didn't want to, and went with CBU instead (particularly because when we did the drywall install, I wasn't sure which shower system we would proceed with). Rather than RedGard the walls and mix systems, I decided to use the Kerdi membrane on the walls.

However, this is meant to be applied with unmodified thin-set, and everything I read says to tape the CBU seams with fiberglass tape and modified thin-set. Schluter says drywall seams don't need to be taped, but did not have any info for CBU seams. The rep told me that the tape will repel thin-set, which seems incorrect since the tape is made to be covered with thin-set, and since it is a different tape than the drywall tape referenced in the handbook section they referred me to.

So, do I:

1) Tape and mud the seams with fiberglass mesh tape and modified thin-set, wait for it to dry and risk creating speedbumps when I apply the unmodified and the Kerdi?

2) Tape and mud the seams with fiberglass mesh tape and unmodified thin-set at the same time as I am applying the Kerdi to avoid speedbumps at the seams?

3) Leave the seams open as advised by the Schluter rep and apply Kerdi over top, risking the crack lines across all the open seams?

Thanks in advance for any input and I apologize profusely if this question has been addressed elsewhere - I searched high and low but fully admit I may well have missed it. That said, I wanted to go ahead and create an account here either way as I do have some questions about the flooring we will be installing after the shower is installed, too.

-Zoe
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Last edited by ZoeJR; 03-02-2021 at 03:54 PM. Reason: Edited to clarify what I mean by "Tape and mud the seams", as I do NOT intend to use drywall mud or drywall tape.
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Unread 03-02-2021, 04:07 PM   #2
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Welcome, Zoe.

You wanna do #3. You don't wanna fill or tape any seams that will be covered with Kerdi or similar sheet membrane. The membrane, properly applied, will result in the CBU seams being better treated than with the alkali resistant mesh tape usually recommended.

The CBU manufacturer, any CBU manufacturer, requires the use of a modified thinset mortar (ANSI A118.4) over his product. All other manufacturers of sheet-type direct bonded waterproofing membranes (ANSI A118.10) require the use of a modified thinset mortar with their product.

Only Schluter wants you to use un-modified thinset mortar (ANSI A118.1) with their product.

You gotta violate somebody's recommendations when installing Kerdi over CBU. I recommend you use a modified mortar for your entire project. See my warranty information below.

My opinion; worth price charged.
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Unread 03-02-2021, 04:27 PM   #3
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Thanks so much for your response - and greetings from a lifelong Texan now transplanted to the PNW! (Portland, Oregon). I hope Boerne is thawing out without too much damage.

As for the mortar, I'm so glad I asked, I have several bags of unmodified mortar upstairs that I was just about to start using. Looks like I'll be making some exchanges! Just to clarify - this project also includes: a Schluter Kerdi shower pan, drain, bench, and curb with Kerdi band seams, as well as Ditra for the broader bathroom floor - I should use modified thin-set for the whole kit and kaboodle?

I had previously only purchased one bag of modified (Versabond) to go between the Ditra and the plywood subfloor, but planned to switch back to unmodified between the Ditra and the marble floor tiles, per the recommendations of the Ditra handbook.
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Unread 03-02-2021, 05:09 PM   #4
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Zoe....While Schluter might not smile on this, we've pretty much found out that Versabond can be used on most of your project....
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Unread 03-02-2021, 05:19 PM   #5
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Zoe, I'm not telling you not to use your unmodified thinset mortar for your Schluter installations, just telling you in my opinion you'll be happier using a modified mortar for the entire project.

Schluter, you'll note, always wants you should be using a good quality unmodified mortar. You cannot purchase such a mortar at Home Depot where I'm guessing you purchased your Versabond. They don't carry such a mortar last time I checked.

What specific unmodified mortar do you have?

And let me be the first to congratulate you on actually reading and trying to follow the product manufacturer's recommendations. We harp on that alla time hereabouts. But the backerboard manufacturer and the stone tile producer also have recommendations and both recommend you use a modified thinset mortar with their products. I'm suggesting that since you absolutely must violate somebody's recommendation, you violate the odd man out, which is Herr Schluter. Except when it comes to bonding to plywood when even he recognizes you must use a thinset mortar meeting ANSI A118.11.

And you know that's good advice on accounta you got it from some guy you've never met nor heard of who you found on the Internet!

My opinion; worth price charged.
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Last edited by cx; 03-02-2021 at 05:26 PM.
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Unread 03-02-2021, 05:38 PM   #6
ZoeJR
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Ha, thank you, I'm trying my darndest to follow all the rules! I made sure to get ones that meet the recommended ANSI specs for both, with Versabond for my modified (ANSI A118.11) and CustomBlend (ANSI A118.1) for my unmodified.

I have read all the Schluter literature and really tried to follow it but you are right - I have not read the CBU recommendations nor the stone tile company's, yet. If you guys think that I can use Versabond for all of the above, I welcome the opportunity to simplify! I will have to do some brand specific research on the mortar recommended by the brand that makes the marble floor tiles, as I haven't gotten that far yet - though I did make sure our subfloor was up to snuff and fortified it accordingly.
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Unread 03-02-2021, 07:43 PM   #7
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Please don't use the CustomBlend for any tile application other than bedding CBU on a floor installation, regardless any other mortar choices you might decide upon.

My opinion; worth price charged.
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Unread 03-02-2021, 11:02 PM   #8
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Welcome, Zoe! May I suggest that you look into using Schluter All-Set modified mortar for your entire project. A bit more expensive than Versabond, but it's approve by Schluter for setting all of your materials. As you stated, I also think it be a good idea to find out the recommended setting material for your marble, if you can find that info.
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Unread 03-03-2021, 12:27 PM   #9
ZoeJR
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Thanks so much for the advice y'all - I will return the CustomBlend. As much as I would like to use AllSet, I can't find it within 150 miles of here, and shipping it is pretty expensive and slow. VersaBond it is, I'm afraid. Unless the Mapei "Uncoupling Membrane Mortar" is better for the floor?

I do have another matter I would like input on, if you would be so kind:

We will be installing 4" hexagon mosaic marble tile, already purchased and en route. This is NOT going in the shower or tub areas, just the flooring for the broader bathroom. For this I will probably go ahead and use a professional installer, so that we don't damage the marble (feels a bit out of our depth DIY wise!). I followed as much internet guidance as I could find on the subflooring, but I am not sure if it is sufficient and don't want to ruin the marble by putting it on an insufficient subfloor.

What we had was this: 1" tongue and groove on 16 inch joist spacing. The joists are 2x8's, which gave me a deflection of L/480 on the handy deflection calculator here. Obviously, this is insufficient for stone tile.

I went through and screwed down anywhere there was a squeak or movement, and then installed 1/2" exterior grade plywood running perpendicular to the joists. So, with the T&G and the plywood, I am at 1 1/2" total, which exceeds the 1 1/8" minimum for stone, to my mind. On top of that, I am planning to use Ditra instead of CBU as the underlayment for the decoupling benefit, which I have seen can be particularly useful for stone. I read CBU won't technically improve deflection so I figured Ditra was the way to go.

Is this sufficient? I am seeing mixed messages on whether the required 2 layers for stone can be one of T&G and one of plywood. Do I need another layer of plywood (one installer recommended it - the others haven't balked at the current setup)? Should I use CBU for additional rigidity or stick with Ditra for decoupling?

Thanks in advance for any guidance.

-Zoe
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Last edited by ZoeJR; 03-03-2021 at 12:46 PM. Reason: Edited to clarify total thickness
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Unread 03-03-2021, 02:23 PM   #10
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Quote:
Originally Posted by Zoe
What we had was this: 1" tongue and groove on 16 inch joist spacing.
Tongue and grove what, Zoe? Plywood, OSB, sawn boards, other?

There is no "exterior grade plywood." What you want is an exterior glue plywood with no face of grade lower than C.

There is no 1 1/8th" subfloor requirement for natural stone tile installation. There are specific combinations or the mandatory two layer subflooring.

If you have an adequate subfloor, you can certainly use the Ditra. You could also use a CBU if you want.

Do you plan to improve your joist deflection or just tile what you've got?

My opinion; worth price charged.
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Unread 03-03-2021, 03:07 PM   #11
ZoeJR
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The tongue and groove is not OSB or plywood, they're 1" thick wood planks - so I guess sawn boards? Our house was built in 1939, this was the subflooring for the whole of the upstairs. And apologies for the mistake, I did mean to say exterior glue plywood. That is what we put down. Do these two count as one of the combinations of mandatory two layer subflooring or is the T&G not included?

As for the joists... I had planned to go with what I had, as they weren't accessible without tearing out all the above described subfloor (the first of which was there when we inherited this partial project). Is that a fatal error? I did just look at the calculator again and I am not sure I had the correct joist length, so I need to figure that out and re-calculate the deflection. I had just left the length at 10 feet but I am not sure that's accurate.

Edited to add: I believe I was going off of this, an F-250 diagram I found from the TCNA handbook: https://www.fcimag.com/ext/resources...jpg?1444847442
Though this says plywood T&G...
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Last edited by ZoeJR; 03-03-2021 at 03:21 PM. Reason: Edited to add link to diagram and deflection update
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Unread 03-03-2021, 03:24 PM   #12
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While the sawn board subfloor doesn't meet the letter of the NSI requirement, we've always held that it complied with the intent. Back when they were still the MIA I attempted a number of times to get an official reading on that without success. I consider it adequate. See my warranty information below.

The reason for the requirement is to eliminate any places in the subfloor where a joint above a joist top extends all the way through the entire subfloor. Your second layer of plywood, properly installed, will provide that.

As for the joist deflection, there's a good story behind that that says a group from the MIA was sitting about considering the requirement and one or more suggested that it needed to be at least twice as rigid as the requirement for ceramic tile. I'm told alcohol was involved.

Will your L/480 deflection be fatal to your stone installation? I dunno. Laticrete allows that when one or more of their products is used for bonding, but I don't know the basis for that, either. Come back in 20 years and report on your installation. Unfortunately, I'll not be able to attend.

But don't cheat on the subfloor requirement.

My opinion; worth price charged.
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Unread 03-03-2021, 04:05 PM   #13
ZoeJR
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Ha, well that is all very good to know. We were very careful about the plywood install avoiding exactly what you describe - we made sure not to place the plywood joints over any of the tongue and groove joints.

And it looks like we are good on the deflection, too. I just went back through some old photos etc. to figure out my ACTUAL joist length rather than arbitrarily guesstimating them at 10 feet (what a concept, right?) and it looks like our deflection is actually somewhere more like L/842 or L/986, depending on whether its 6.5 or 7 feet length (one side of the room is slightly longer).

Thanks again for your advice! Very grateful to have found this place.
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Unread 03-03-2021, 07:28 PM   #14
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Zoe, the size of the room to be tiled has absolutely nothing to do with the unsupported joist span. They might be the same, but that would be coincidental. The unsupported span of the joists is what you need to know, along with the size, condition, spacing, grade, and species.

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Unread 03-03-2021, 08:08 PM   #15
ZoeJR
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Yep, sorry, what I said was unclear. I just meant that the support beam doesn't run down the exact middle of the area, so the unsupported joist length of the joists on one side is slightly longer than that of the other side. I had all the other info ready and accurate, but I was just guesstimating and rounding up on length rather than going back to confirm. But, looks like we are at 7 feet unsupported length, which gives me a far better deflection rating than the original 10 I had punched into the calculator. Thanks again!
-Zoe
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