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Old 08-17-2018, 05:53 PM   #1
Tacoma88
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Kerdi Membrane over 1/2" Hardiebacker on Floor?

Hi all, I'm about midway through my bathroom renovation and really enjoying the process. So thankful for all of the knowledge shared in this forum.

I'm installing a curbless shower using the Kerdi pan on the floor and Kerdi membrane over 1/4" Hardiebacker on the walls. The shower area will be divided from the rest of the bathroom with a frameless glass shower panel, no door. My question pertains to the remainder of the bathroom floor outside of the shower area.

I currently have 3/4" OSB installed over which I'll be installing 1/4" Hardie, using thinset in-between and the appropriate screws. My impulse is to also waterproof this floor because everything I am reading says that a shower without a curb designates the entire bathroom as a "wet room". Do I think water will flood the rest of the bath? Of course not. Still, all this "wet room" talk makes me cautious.

I cannot use Ditra, the natural choice, because my tile is less than 2x2.

I have a surplus of Kerdi membrane, enough to apply to the entire remaining floor area overtop the 1/4" Hardie, so I'm tempted to use that instead. I know it won't serve as an uncoupling membrane like Ditra would, but it should at least waterproof the area, right?

In the Kerdi installation guide it lists floors as a location where Kerdi membrane can be applied. So why have I not come across anyone doing this? I contacted Schluter directly and got an email saying they would put me in contact with my local distributor or whoever, but that was weeks ago and so far no response. So now I'm turning to this forum's fount of knowledge.

So am I over thinking this and people actually do it frequently, or is there something I don't understand that will cause it to fail?

Or, is this all a moot point, and I shouldn't even bother waterproofing the floor outside the shower area at all?

Thank you in advance for your help
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Old 08-17-2018, 06:11 PM   #2
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Welcome to the forum, George.

A couple questions before we go to far:

1) Are you really only using 1/4" Hardibacker Board for the walls?
2) You may be going curbless, but you've got a slope going up from the drain to the level tile outside the shower area. How much of a height difference is there from the drain to the level area of the tile? (I'm trying to understand how deep your shower pan is.)

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Old 08-17-2018, 09:39 PM   #3
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Hi Tonto!

1) I'm sorry, that was a typo. I'm not. I'm using 1/2" on the walls, 1/4" on the floor, like Hardie recommends.

2) The pan I'm using is the 32x60 pre-sloped Kerdi pan with the offset drain, which is 1.5" in height. The floor under the pan has been recessed, so the perimeter of the pan sits level with the OSB & 1/4" Hardiebacker once installed.

I hope this helps, sorry I needed to clarify.
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Old 08-17-2018, 09:55 PM   #4
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FWIW, Hardie does allow 1/4" HardiBacker for wall applications, but most people prefer the stronger 1/2" nominal stuff as it makes seaming with drywall easier and provides a bit more margin for error.

While you can use Kerdi on the floor outside of the shower, other than waterproofing, it probably won't buy you much else. Maybe a little, but Ditra, if it would work, would be better. Sounds like you're carrying the same tile as the pan out into the room and their size won't work.

If you haven't put the Kerdi up yet, make sure to really wet down the Hardie before you do or you'll run into problems. It's not too wet unless there's standing water on it. If it's dry, it will suck a huge amount of moisture out of the thinset, making embedding the Kerdi hard to impossible.
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Old 08-17-2018, 10:35 PM   #5
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Hi Jim, thank you for your response. That was also my inclination with the Kerdi, I really wasn't expecting anything from it besides the waterproofing. Reflecting on it now, Ditra would have been the obvious choice and I could've easily done 2x2 of the same tile (American Olean Unglazed Porcelain) rather than the 1x1, but alas, I bought the tile before I fully understood the substrate. Lesson learned.

Thank you for the recommendation of watering the Hardie down too. I've read that time and time again on here.

In terms of unmodified thinset, I've seen guys recommending Versabond as a thinset although it's modified, claiming that they don't have any issues with it because it's only lightly modified. I picked up both Versabond and Mapei's Floor and Tile Mortar and I haven't fully decided which one to use yet. I know Versabond would void the Schluter warranty but I haven't seen a ton of people recommending Mapei's Floor and Tile Mortar for some reason.
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Old 08-18-2018, 09:43 AM   #6
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Another thought I had this morning: If I install the Kerdi membrane on the floors over the 1/4" Hardiebacker, should I still tape the seams with fiberglass tape and modified thinset like Hardie recommends before I install the Kerdi overtop (using unmodified thinset)?

I read here that there's no need to tape the seams when installing Kerdi over Hardiebacker on the walls. John said it's stronger than any fiberglass tape that I could apply.

Not taping the seams on the floor makes me much more nervous however, due to the stress of foot traffic, and all the reading I've done that said it's the cardinal sin of flooring installation to not tape seams. Is this fear unwarranted so long as I'm using the Kerdi? This is all new territory for me and I want to pony up the question to someone with more knowledge and experience than myself.
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Old 08-18-2018, 10:11 AM   #7
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Welcome, George.

While using the Kerdi as a waterproofing membrane on your floor is not a manufacturer recommended application, I wouldn't hesitate to use it if I had it on hand and wanted to waterproof a bathroom floor.
Quote:
Originally Posted by George
...it's the cardinal sin of flooring installation to not tape seams.
Actually, the greater sin is to fail to bed the CBU in thinset mortar as required by the manufacturer. Since you did that part, and if you elect to cover the entire Hardibacker floor with Kerdi, I would not fill and tape the seams before installing the Kerdi.

Keep in mind that Schluter requires that you use a thinset mortar that is appropriate for the substrate. In the case of Hardibacker, the manufacturer requires a modified thinset mortar be used and, since it is the substrate for your Kerdi, the modified mortar would be appropriate.
Quote:
Originally Posted by George
I've seen guys recommending Versabond as a thinset although it's modified, claiming that they don't have any issues with it because it's only lightly modified.
That "claim" of Versabond being only lightly or slightly modified got started here on these forums many years back and I have no idea just how. I doubt there has ever been more than one member of this website that knows just what is in versabond and I don't recall him ever saying it was only slightly modified. Versabond meets the requirements of ANSI A118.4 and A118.11 and is therefore a modified thinset mortar. I know from personal experience that it will work just fine with any Schluter product and Schluter also knows that, but that still won't give you your warranty. I also know that you don't need no steenkin' warranty on accounta any failure you'll have with those products is gonna be an installer error. Take that to the bank.

My opinion; worth price charged.
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Old 08-18-2018, 10:58 AM   #8
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You know CX, now that I think about it, I'm nearly sure I picked up that "intel" about "lightly modified" right here on this forum. And repeated it more than once. Could well be I've been talkin' out my rear end all this time.

FWIW, it made some sense to me both from a performance characteristics and price point perspective. I too have used in many, if not most, Kerdi installations. I like the way it handles and am familiar with it. It's also easy to come by in this area because of Home Depot, who's hours and locations are better than other suppliers. That means I can go thinset shopping in the evening. What more could one wish for?
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Old 08-18-2018, 01:10 PM   #9
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Great, thanks guys. Versabond under Kerdi without taping the seams it is. I really appreciate the help!
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Old 08-18-2018, 01:23 PM   #10
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And you do understand, George, that Schluter does not recommend that Kerdi membrane for floor use at all, other than in a shower application, right?
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Old 08-18-2018, 02:55 PM   #11
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Hi CX,

So I guess that's where I'm confused then. That's what my original question was referring to. In the Schluter installation manual they list possible applications for the Kerdi membrane as "Over wood or concrete subfloors". Further down it specifies that subfloor as being among other things "cementitious backer unit." I attached a screen shot below, and this is a link to the installation manual itself. The page I'm referring to is page 7.

So to me that implies that it can be used for floors. But like I said before, I haven't come across anyone using it on floors. Am I misreading the installation guide somehow?
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Old 08-18-2018, 03:08 PM   #12
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That's under the heading of "showers", at the top left you'll notice.
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Old 08-18-2018, 03:34 PM   #13
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What is the distinction between the shower and the rest of the bathroom then? Especially if the shower is curbless and therefore the entire bathroom is considered a "wet area" that must be waterproofed (according to CA code)?

If Kerdi membrane is a no-go and Ditra doesn't accommodate my tile, what options do I have? Redgard? I was hoping to keep everything within the same system, as everyone recommends.
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Old 08-18-2018, 03:43 PM   #14
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Not quite sure where you see that implication, George, but I don't believe it's there. That particular section begins with Areas of Application and specifies Interior Showers and invites you to "see Base information under requirements (below) for details," where it tells you you can use one of the various Schluter bases or a mortar bed. Again, for your shower floor.

You'll note also that it points you to page 12 for disabled access or curbless showers and shows the use of Ditra on the floor outside the shower.

Once again, entirely up to you how you wanna use the product in your application. Wouldn't scare me at all to see it used over CBU on a bathroom floor, but that doesn't make it right in the eyes of Herr Schluter.

Again, I don't think you require waterproofing on your entire bathroom floor (I suggest you check with your local code compliance authority), but if that's what you want and you want something with a warranty, which you'll not get to benefit from, there are other membranes to choose from. Some of the very best are from the Noble Company. The RedGard could be used, too, I suppose, but I'd rather have a sheet membrane myself.

My opinion; worth price charged.
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Old 08-18-2018, 04:00 PM   #15
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Alrighty, I see. I think I was crossing certain bullet points with others, seeing what I wanted to see I'll go back and read through the codes again, (especially now that I'm questioning my reading comprehension skills) and go from there. Thank you all again for all your help!
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