Mortar in Kerdi Shower

[Kipley]

I’m installing my first Kerdi shower and am reading my recently purchased Kerdi Shower eBook. Hopefully you can help; I’m not sure of what mortar to use. The local supply stores have the Custom Building Products Masterblend, Customblend, Flexbond, and Versabond and the TEC FullSet, SturdiFlex, and SuperFlex mortars. As far as I can tell, I need the following:
1) dryset mortar for setting the Kerdi shower tray
2) thinset mortar suitable for the substrate (drywall here) for installing the Kerdi and
3) some type of mortar for installing the tile (which is ceramic, 3x6 for the wall and 2x2 for the floor).
What is the best mortar for each of these 3 applications?
Thanks in advance for your help!

[muskymike]

Hi there Kipley, you can use the TEC Full Set or if you wish you can use Versabond. JB uses it alla time and never has had any problems. The Versabond is slightly modified where the Full Set is unmodified.

[cx]

If you're using Cutom products, Schluter wants you to use the MasterBlend for everything you've described.

John Bridge will use white VersaBond for everything you've described.

If I were doing it and I wanted to make Schluter happy, I'd seek out Laticrete 317 for everything you've described ('cept I wouldn't be usin that foam tray thingee). If Schluter weren't lookin', I'd likely use VersaBond on accounta John says it works real well and I believe him.

My opinion; worth price charged.

[PhilUpNorth]

The advice from CX and MuskyMike is no doubt dead right, but I'll share with you what tech support at Schluter told me just today just in case it adds anything to the discussion.

Although Schluter states that an ANSI 118.1 (unmodified) is what is needed to bond the membrane to the structure, a latex-modified (but not too modified) ANSI 118.11 is also OK, especially if you've got some stuff made of uncovered plywood such as seats, niches, etc. where the unmodified thinset wouldn't work very well.

However, wherever the membrane overlaps or wherever you use KerdiBand, I was told that you have to use unmodified because the modified thinsets need some evaporation to set OK (comment that made the rep at TEC smile...) and Kerdi membrane on both sides of the thinset is so vaporproof that it prevents that from happening, at least in a reasonable time. Anyway, they say that, for the same reason, you should use unmodified thinset to set the tiles to the Kerdi.

So there you have it... The word from Schluter. Now, even though the rep seemed to play it really conservative (which is quite understandable), I must admit that he was excessively knowledgeable about his product, and managed to answer all of my questions and educate me in a very professional way. In my view, that is how you establish customer trust and loyalty. Sure beats tech support in a different hemisphere where all they can do is read back to you the FAQs from their web site. But that doesn't happen, so why do I even mention it?

[Kipley]

Hey guys, thanks for your input. So I think it's down to using either (or both) Versabond or the unmodified MasterBlend/Fullset. What are the advantages/disadvantages of using one over the other?

[muskymike]

None really, just what's available for you.

[Kipley]

I meant to ask what the advantages/disadvantages of using Versabond (modified) over MasterBlend (unmodified).

[slb]

One advantage to Versabind is longer working time. If you are a meticulous worker (meaning slow) like me, Versbond gives you quite a bit more time before it skins over and starts to really set-up.

[Kipley]

I finally went out and bought a bag of VersaBond during the holidays. I mixed up half the bag and started installing the Kerdi Band over the vertical corners and the horizontal floor to wall junctions. I thought I was being pretty diligent about getting full contact and removing all the air bubbles but the next day I found a couple of places where the Kerdi Band had bunched up making ridges at the edge. I couldn’t think of a good way to address this so I went ahead and installed a Kerdi membrane over it. I figured getting the 2” overlap would be good enough to provide a water tight seal.

One thing I’m not sure about is the amount of pressure to apply when removing the air bubbles and any excess thin set. John’s book indicates I should press the membrane into the thin set ‘in earnest.’ I’m not sure what that means. I don’t know if I should apply just enough pressure to remove the air bubbles and get good contact or if I should apply enough pressure to remove as much thin set as possible. Do the combed ridges of the thin set need to be pressed hard enough to eliminate the ridges entirely? In other words, should I be using 5-10 lbs of pressure or 50-100 lbs of pressure to press the Kerdi and/or KerdiBand into the thin set?

[muskymike]

Moderate pressure, you want to smooth the Kerdi to the mud, and work any air bubbles out. I use a wallpaper smoothie.

[cx]

Originally Posted by Kipley

Do the combed ridges of the thin set need to be pressed hard enough to eliminate the ridges entirely? In other words, should I be using 5-10 lbs of pressure or 50-100 lbs of pressure to press the Kerdi and/or KerdiBand into the thin set?

When finished, you do not wanna see any of the ridges in the thinset through the Kerdi - and you will see them if they're there.

I don't know how much pressure we're talking about, but it's prolly between your ranges. More like 25 lbs or so would be my guess, but how you fixin' to measure it anyway?

Pewsh vigorously, or smartly, or earnestly, or git after it, or moosh it pretty hard, or whatever. You wanna end up with thinset between, but not much. No lines. None. Just try some, then pull it off. Shouldn't see any lines, should see thinset on both the wall and the Kerdi.

It's handy to have been present when some Schluter guys showed everybody how, but it's really pretty intuitive once you do it a little. Really it is.

You do believe me, doncha?

My opinion; worth price charged.

[Kipley]

I certainly don't intend to measure the amount of pressure applied, I'm just trying to get a better feel for John's "in earnest." If I press really hard, it seems I can get almost all of the thinset out from between the Kerdi and the drywall. If I press moderately, I can eliminate all the ridges and air bubbles but leave most the thinset between the membrane and wall. If I press lightly, I can still eliminate the air bubbles and any 'excess' thin set (but there really shouldn't be any excess thin set if it's properly placed and combed).

I gather I should be pressing hard enough to mush the combed thin set ridges down to get a relatively consistent thickness of thin set coverage under the entire Kerdi sheet. To put it more mathematically, assume a 1/4 x 1/4 square notched trowel were being used (to make it easier) and it were perfectly combed on. Enough pressure should be applied to mush the 1/4" ridges down to fill the combed voids resulting in an overall 1/8" thickness throughout. Right?

[Kipley]

OK, another question has come up. I’m transitioning from tile to dry wall at an outside corner. Do I run the Kerdi around the corner a bit and cover with drywall compound, run the Kerdi up to the corner and stop right at the corner or do I stop shy of the corner to give a finished look? Also, do I finish the corner of the wall with drywall compound over the Kerdi membrane or beneath it?

[Kipley]

The weekend is here again so I did some research and found in John's book where he recommends wrapping around an outside corner with the Kerdi membrane. That way, a completely waterproof corner is assured and the membrane can easily be finished over with drywall compound. Thanks John!

[muskymike]

Answering of your own question isn't allowed.