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Old 02-10-2019, 03:11 PM   #1
sfklaas
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Using Modified Thin Set for Marble Tile Over Schluter Ditra Heat

Hello,

I have recently installed the Schluter Ditra Heat panels in our bathroom using a modified thin set to adhere it to the plywood subfloor. We let it dry and it worked great. We then installed the heating cables. But now i'm stuck on what to do next.

We are installing 12"x24" marble floor tile over the heated floor. From what I have read, a good quality modified thin set is recommended for marble tile. However, Schluter wants an unmodified mortar to be used over the ditra heat panels. I went to Home Depot and talked to the tile guy there. He recommend "Natural Stone and Large Tile 50 lb. White Premium Mortar". But I don't think he understood the schluter floor system. I went ahead and bought a few bags anyway.

Will this home depot mortar work? Any of you experts suggest any other thin sets that would work that I can buy at a local store? (Rather buy today, then wait to ship the Schluter Thin Set or any other special order mortars.) Thanks in advance.

PS. I might prefill the holes/wire and then let it dry a few days. Then go back and tile over the dried mortar. (Maybe that would help with the drying issues, keep the wires in place during the tile installation, and offer a flat surface when it comes time laying the tile.) Just hoping the "Natural Stone and Large Tile Premium Mortar" from Home Depot would do the trick
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Old 02-10-2019, 03:49 PM   #2
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I've used that thinset many times over Ditra, I like the way it holds up the stone. I'd give it extra time to set before walking on it and not worry about their warranty. Here's a link. https://www.custombuildingproducts.com/TDS/TDS-309.pdf
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Old 02-11-2019, 07:19 AM   #3
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Having recently completed my own Ditra Heat installation as a generally competent DIY'er but relatively novice tile setter, Steve, I'd recommend you pre-fill the Ditra. The mortar is going to be thick in the Ditra, and it's gonna shrink. After I pre-filled I skimmed it again. But I'm a little OCD.

I used Custom's Versabond white (lightly modified) to prefill and skim, then used Custom's Pro Lite white to lay my 12X24 porcelain tile. The Pro Lite is also modified, and meets the same specs as the mortar you picked up.

Hope your floor is sufficiently flat. Don't forget to take readings on the heat cable.
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Old 02-11-2019, 02:54 PM   #4
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Davy and Dan,

Thanks so much for your comments. I feel more confident that this mortar will be sufficient. I'll be sure to give an update once everything is installed!
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Old 02-19-2019, 02:04 PM   #5
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As an update...

I decided to call Schluter to see if I should just use their thin set for my 12 x 24 marble tile. Long story short, they wanted to me to call the tile manufacture to see which thin set they would recommend for the large marble tile. After talking with the manufacture, they told me that they did not have any recommendations for over the Schluter system and they recommended to call a tile installer or Custom's support hotline regarding their "Natural Stone and Large Tile" mortar that I already bought. When calling "Custom"s support line, they said they didn't recommend the Natural stone and large tile thin set because it is a modified thin set and Schluter recommends unmodified thin set.

So what the heck am I suppose to use lol. It is comforting to know Davy and Dan recommended the Custom "Natural Stone.." thin set, but I thought I would just try one more time if anyone else out there has recommendations. Thanks!
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Old 02-19-2019, 02:13 PM   #6
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If you want to stay within Schluter's warranty and use an unmodified, I'd recommend DitraSet.
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Old 02-19-2019, 02:21 PM   #7
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Custom is executing a CTA move, Steve. They know Schluter recommends unmodified over Ditra. They don't want to be liable.

Call the tile manufacturer back and ask what type of thinset mortar they recommend for setting those tiles on, say, Durock cement backer boards. Then use that.
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Old 02-19-2019, 04:13 PM   #8
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The issue with modified over a waterproof membrane is that not all of them work until they dry out. The cement cures, regardless. The big offender in this scheme is a thinset that uses latex based modifiers...those need to dry to attain full strength and performance, and over a waterproof membrane and a large tile, that can literally take months. There are some modifiers that attain their strength while still being wetted.

If you want a modified that you know will work, you can buy Schluter's https://www.schluter.com/schluter-us.../p/SET_ALL-SET

There are certainly others that will work. You need to carefully read the spec sheets, or talk to the manufacturers.

If the floor is flat and the backs of the tile are, too, you don't need a particularly deep notched trowel, and especially if you choose to use a leveling system, most thinsets will work.

But, in reality, unless the back of the tile is coated with something, a premium unmodified will work just fine. Keep in mind that the thinset doesn't actually stick to the Ditra...it is locked in place because of the shape. This means that there is very little stress on the thinset/tile bond, which still could be in the order of 250+ psi, way more than what it would take to tear the Ditra off of the floor. A modified is a good idea when bonding a tile directly to say a backer and required when doing it to plywood, but much less so when bonding to something like Ditra. 12x24=288sqin x 250psi = 72,000 #. Do you really think you're going to need more? WIll a modified have more? Yes, and with the heating, the variations in thermal expansion might buy you some performance. Personally, I don't think you'd have a problem using either. For peace of mind, go with a modified, the cost isn't all that much more, but IMHO, you don't need it.
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Old 02-19-2019, 05:00 PM   #9
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For whatever it's worth, my first hand experience proved, in my situation, that the modified I used (Custom's Pro Lite) dried completely in 3 or 4 days under my 12X24 porcelain tiles over Dirta Heat. This I know because I had had to remove one that I chipped. There was no wet mortar to be found.

But my situation was that I'd pre-filled the Ditra Heat first, using a lightly modified (Versabond) thinset mortar, which I then skimmed over with a thin layer of Versabond to fill in low spots, all of which had completely cured before I set tiles using Pro-Lite.

Perhaps things would have been different had I filled and then set tiles, using a modified, all in one shot.
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Old 02-19-2019, 07:00 PM   #10
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The existing thinset became a sink for the extra moisture in the thinset you added to set the tile, raising it's moisture content, and lowering the new stuff's.

More than a few modified thinsets will work. You want to avoid one that needs to dry, as that gets slowed down radically between an impervious tile and a waterproof surface (unless you have a sponge in there like a bunch of cured thinset).
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Old 02-19-2019, 07:19 PM   #11
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Why complicate things?

Will a modified work? Probably. But I assume you followed schluter’s directions for installing ditra heat mat, cable, thermostat, wire spacing, etc, etc. Why not just continue to follow their directions for the setting material? Laticrete 317, Bostik Ditra Set, Mapei Kerabond...all are great unmodifieds.

And just so we’re not confusing OP, versabond isn’t a lightly modified thinset, it’s just a modified.
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Old 02-19-2019, 10:03 PM   #12
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I want my thinset to bond well to the porcelain and modified is what the tile manufacturers recommend. Just about any thinset will lock into the Ditra, it's just a matter of drying time. I don't believe it takes months for thinset to set under the tiles, it's not mastic. The moisture will take a while to completely leave even though the thinset will be fairly hard.

The way I see it, if I use modified and give the floor extra time to set, I won't need Schluter's Warranty.
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Old 02-19-2019, 10:18 PM   #13
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Last class I had at Schluter showed progressive pictures of a piece of glass they set over Ditra using a modified thinset. Worst case, but a good porcelain is only a little more permeable. IT was obvious from the color that it took over 80-days to fully dry out and that was without it being grouted which left the edges fully open for the moisture to escape. It's real. The cured crystals in thinset don't get the benefit of the covering of the modifier if it can't dry, at least with a latex based one. There are at least four classes of modifiers used, and each one of them will have numerous variations in their characteristics and the ratios of modifier, aggregate, cement, and other additives. You want one that achieves its strength before things fully dry out. Most rapid set modifieds fall into that group, but there are certainly others.

Most tile are stiff enough such that they probably won't flex in the middle where it may still be soft if you wait long enough for the edges to dry out. Use too much water during cleanup, increase the amount of moisture that has to dry out. Try to put a significant point load on it, you might have problems.

FWIW, I can dig out a reference to a test that showed a quality unmodified had a stronger shear bond to a porcelain than a modified...it all depends on the quality of the products involved.
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Old 02-20-2019, 11:02 PM   #14
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Modified vs Unmodified

If you would like to stick within Schleuter's warranty, stick with an unmodified. Most marbles (unless they are moisture sensitive stones like black marble and green serpentine) an unmodified thinset is more than adequate. The reason for this is as follows:

1. Thinset is a combination of Portland Cement, Sand (aggregate), Cellulosic ether(think of this as a sponge to hold moisture in) and water.
2. During the curing process hydration occurs (chemical reaction where the oxygen and hydrogen are torn apart. Hydrogen is left behind and oxygen escapes) . Essentially what is happening from this reaction is that there are hydrate crystals that are growing (you would see it under a microscope).
3. Since marble is very pourous, these hydrate crystals grow into the pores of the marble creating a mechanical bond between the substrate and the stone.
4. Does a modified thinset offer additional adhesion properties? Yes absolutely; however, it is not necessary for most stone installations.

Although, I doubt you would have any issues using a modified thinset, why run the risk of voiding the warranty given by schleuter when an unmodified thinset will function absolutely perfect on a molecular level.
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Old 02-20-2019, 11:38 PM   #15
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The recipe for Portland cement is not a fixed thing...thus, the shape and size of the crystals it grows as it cures will vary. Here are a few pictures. The size, thickness, and shape of the crystals can be customized, and is one reason why there are so many different thinsets out there that perform in different ways. Throw in the mix of aggregates and other additives, and the actual variations available is really quite huge. IOW, they can be customized for specific purposes. That one with the thick, stubby crystals probably wouldn't make a great cement for thinset, but would be great for a high-strength cement. Schluter knows their products and what works.
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