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Unread 12-06-2020, 11:21 AM   #1
burn123
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Ditra over plywood and Henry 549

Hello,
I’m putting Ditra over plywood in a small bathroom remodel. After opening the floor and checking, and correcting, the structural requirements for deflection, I discovered the floor has an area with a dip. So I have leveled the area using Henry 549. Now I’m ready to install Ditra over the floor. My question and problem is, what thinset to use for applying the Ditra. Part of the floor is plywood and part is plywood covered by the cement-based 549. So I don’t know if using modified thinset is appropriate for this????
Can someone please help with this issue.
Thank You
John
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Unread 12-06-2020, 11:39 AM   #2
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Welcome back, John,

According to Schluter: "For example, to set DITRA over plywood or OSB, a modified thin-set mortar meeting the requirements of ANSI A118.11 is used. To set DITRA over concrete or gypsum, Schluter-Systems recommends an unmodified thin-set mortar meeting the requirements of ANSI A118.1."

Since you basically have both you'll need to use a A118.11 mortar. It will stick just fine to the 549 but, being under Ditra, it'll just take a bit longer to cure.
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Unread 12-06-2020, 11:47 AM   #3
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Thank you, Dan
Since the modified will take longer to cure over the 549, should I wait and allow it to cure before tiling over the Ditra?
Thanks
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Unread 12-06-2020, 11:59 AM   #4
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You can tile over Ditra immediately after installing according to the manufacturer, John. No need to wait if you don't wanna.

My opinion; worth price charged.
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Unread 12-06-2020, 12:01 PM   #5
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It's not so much that the modified will take longer to cure over the 549, it's more that it'll take longer to cure under the Ditra, regardless of the what the subfloor is.

A blanket of Ditra all but eliminates the mortar from being exposed to air and unmodified mortar doesn't care about that since it cures by chemical reaction. The modifiers in some mortars have to dry and Ditra slows that process down.

I'd give it a day, maybe two, but I'm super conservative with this stuff due to my lack of experience.
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Unread 12-06-2020, 03:17 PM   #6
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Thank you CX and Dan.
Do either of you have recommendations for:
1. modified thinset for applying the Ditra over the plywood and 549
2. unmodified thinset for setting 12”x24” tiles over the Ditra
I’m in Dallas and have access to most products.
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Unread 12-06-2020, 03:42 PM   #7
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1. Versabond, available at HD. Meets the A118.11 specification.
2. Don't know about an unmodified. But, if you are installing porcelain tile it's generally recommended to install those with a modified thinset mortar. Here again Versabond will do the trick.

It's a pickle; though using a modified goes against Schluter's instructions it is in keeping with the recommendation for porcelain tile.

Using modified on Ditra has been debated here before and the consensus is always the same; it will, and does work just fine.

I used Versabond for nearly my entire bathroom. Both walls and floor are clad with 12X24 porcelain tile, and the main floor tile was installed over Ditra (Heat). I even had to remove a floor tile after it had been set for a day or two and the Versabond under it appeared fully cured (even though it technically wasn't).
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Unread 12-06-2020, 07:37 PM   #8
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Many of today's modified thinsets can achieve stability without drying. The one that can't is modified with latex. There are at least four classes of modifiers used in thinsets. As mentioned, the cement cures regardless, but to achieve the desired results, a latex modified mortar also needs to dry. Both the Henry's material and the plywood will allow the material to dry, so isn't an issue. The issue on top of Ditra is that it is essentially waterproof and a typical porcelain tile is as well, so the only path to dry is through the grout joint. With a larger tile, that path will be very tortuous and long and if you chose an epoxy or acrylic grout, nearly impervious as well. Tests have shown nearly 3-months! Since the ANSI specs do not indicate what modifier is used, Schluter, knowing a latex modified one has issues, doesn't want you to use one. Forcing the mortar to hold the moisture also means that the cement in the mix can fully hydrate, which also means it can end up stronger than a more typical air drying.
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Unread 12-07-2020, 11:26 AM   #9
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Thank you for the replies.
So for setting porcelain tiles over Ditra, the use of a modified thinset is good as long as it’s not latex modified?
Thank You
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Unread 12-07-2020, 11:40 AM   #10
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John,

If you can get ahold of some Schluter All-Set mortar, you can use that to set the porcelain on the Ditra. It's twice as expensive as Versabond, but then there is no question about it's use over Ditra as it's one of the two approved modified mortars by the manufacturer of Ditra. I believe the consensus is that Versabond will work just fine and many use it to set tile on Ditra without issue however.
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Unread 12-07-2020, 02:03 PM   #11
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Thinset does not bond to Ditra on the top (well, not the bottom either). It's held into the Ditra by the shape of the pockets, so the only thinset bond is to the tile. That bond with a premium dryset mortar should be in the order of 2-300psi. A 12x12 tile, that's 300*144= over 20T. A good modified might be up to a 100 psi higher. If you were bonding directly to say plywood, you'd need all of the bond strength you could get. Over a membrane like Ditra, it's kind of a moot point. As was said, you don't want to use a latex modified mortar and you don't want to use a price leader dryset one (lots of sand, not much cement!). If you want their warranty, you should follow their instructions. You DO need a modified underneath, not so much to bond to the Ditra, but to the floor. The bond to the fleece is because the thinset flows around the fleece and cures, not because it sticks to it...sort of like pouring liquid wax on fabric...once it hardens, it's a bear to get out. FWIW, the bond to the fleece is >50psi (typically tests out at about 75psi). IOW, the fleece will debond way before the tile will if you have any kind of decent workmanship and choose a decent product.
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Unread 04-16-2021, 11:10 AM   #12
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I’m going to put down the Ditra this weekend but really need some advice beforehand.
1. I can’t source a Ditra trowel, so I picked up two trowels and need to know if one will work as a suitable replacement. 1/4x3/16 v notch and 3/16x3/16 square notch.
2. I’m using Schluter all-set to bond the Ditra over osb, but I’m leery about how thick or loose to mix it. The last thing I want is to mix it too dry and not get a good bond. So I’d appreciate some learned advice on mixing and processes.
3. It’s only 40 sq ft, and I have two sections to put down. So will 1/2 bag do the whole application?
Thank You
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Unread 04-16-2021, 11:31 AM   #13
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1, 2. John, you really wanna do yourself a favor and go to the Schluter Ditra website and download the Installation Handbook for the product. Might be a number of other things you're missing, so I recommend you read through the whole thing before you proceed.

I believe about page 18 you'll find the manufacturer's recommendations for the correct trowels for installation of the product. I'm sure there are water quantity recommendations for their own All-Set mortar in there somewhere as well.

3. I dunno.

My opinion; worth price charged.
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Unread 04-16-2021, 11:32 AM   #14
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John,
1) According to Schluter: use a 1/4" x 3/16" (6 mm x 5 mm) V-notched trowel, 5/16" x 5/16" (8 mm x 8 mm) V-notched trowel, or the DITRA trowel, which features an 11/64" x 11/64" (4.5 mm x 4.5 mm) square-notched design, is recommended. The 3/16X3/16 will be too small.

2) The mixing instructions on the bag will provide the correct water/powder ratio and mixing instructions. Given my limited experience with Ditra (heat) I'd want the mix a bit loose - just barely able to hold a ridge. You will also want to use the flat side of the trowel to first burn in some mortar on the OSB before combing it out.

3) 1/2 a bag should be plenty.
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Unread 04-16-2021, 12:48 PM   #15
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Am I going to save myself possible bonding issues by using the Ditra trowel?
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