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Unread 01-02-2020, 02:25 AM   #1
bob53
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Advice sought on new construction project - Ditra size, ThinSet vs LHT...

Hello all and happy new year -

It's been some time since I last posted...

I am in the middle of building a new home and am playing an "active" role in the project as the project manager (not entirely by choice but that's a different discussion!). I have a few questions about the tile work and wanted some expert opinions...

The home is a 2 story "ranch" with a walkout basement. The home is over-engineered (my choice) with oversized footings, engineered lumber, and floors designed to meet L/720 or better for deflection. I'm only going to build once so I figured I would do it right... The main floor (over the finished basement) is comprised of 16" TJI 560s spaced 12"-14" OC as needed to meet the deflection noted above. The subfloor is Huber Advantec, and the subfloor is covered with 1-1/2" thick of gypcrete for the hydronic radiant floor system. After gypcrete, the floor is now quite flat, with minimal (1/16" to 1/8") height variance over long distances (40-50').

The main floor is primarily going to be covered with 3/8" thick 12 x 24 white travertine tiles (2000+ sq ft). The tiles were all quarried at the same time and show little if any variance in thickness. I plan to have the tiles set over ditra as a final insurance against movement. Due to several design issues, the finished floor cannot exceed 3/4" thickness over the top of the gypcrete.

My questions are severalfold:

1. Ditra vs Ditra XL? - Given that the floor (TJIs, subfloor, gypcrete) was engineered for stone floors and that I am using 3/8" stone tiles, can I use Ditra instead of Ditra XL? The finished floor needs to be 3/4" thick and so using Ditra XL would make it difficult to hit that target.

2. Mortar beneath Ditra - probably going to specify laticrete 317 beneath the Ditra. Concerns?

3. Mortar over Ditra - Can I use thinset or should I be looking at one of the large tile medium set products? Any advice on a product? Can I get away with an unmodified product here or would it be better to "cheat" and disregard the Schluter instructions and use a modified product here for better bond strength? Laticrete and Mapei are easy to source where I live. Ditraset is not.

I figure I have a maximum of a 1/4" thick bed of mortar between the ditra and tile base to hit my 3/4" thick goal and so I might be able to get away with the thinset but I know that medium bed mortars might be a better choice even if they are applied thinner than their typical application.

4. Trowel size - what size would you recommend here above the Ditra keeping mind the larger tiles will need to be back buttered...

Thanks for your help,

Bob
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Unread 01-02-2020, 04:04 AM   #2
Tool Guy - Kg
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Welcome back, Bob! In honor of you posting after nearly 10 years, I’ve spent just about as long composing a book for you.

1. Yes, you can use regular Ditra (though more on this in answers #2 and #3). The XL was designed to add thickness, not do a different or better job (though there’s nuances that could be argued/discussed if you want to ‘go all geek’).

2. Concern is that gypcrete and your setting mortar will react to each other negatively and result in a loss of bond due to the formation of crystals between the two layers known as ettringite. My preferred method (and approved by the manufacturer) is to skim the gypcrete with a thin layer of Laticrete 254, allow to thoroughly dry, apply Laticrete Hydroban to spec, then set the tile. So, the first and second thing to do is decide on a mortar and figure out an approved method for that manufacturer. That means Ditra may or may not end up as part of the tile assembly.

3. If you end up with Ditra as a membrane (depending on #2 above), that’s up to you. If you stay simple, it’s easy to say: properly executed, a high-quality unmidified mortar is more than sufficient between the Ditra and tile. If you want a modified thinset mortar that is stickier than the unmodified and desire to keep Schluter’s warranty, find a place to purchase their All-Set Mortar. If you don’t care about Schluter’s warranty, Custom Building Product’s Versabond will work just fine for you.

4. Typically, tile of this size ends up using a trowel with notches of 1/2” height, BUT...
You’ve got something a little unique going for you: an unusually flat substrate and very flat tiles. The object is to obtain as close to 100% uniform mortar coverage between your substrate and tile. If you mistakenly use a trowel with notches too short, you’ll reduce coverage to less than the 95% required for stone tile or less than the 3/32” minimum mortar thickness commonly required by the mortar. Mistakenly use a trowel with notches too tall and you’ll have excessive mortar that’s sloppy (excess oozing from joints), possibly exceed the max mortar thickness as defined by each individual mortar product, and/or introduce excessive thickness that makes it slower to obtain a flat/flush finished tile surface. Normally, a little play in mortar thickness helps, but with your nearly dead-flat substrate and tiles, it can work against you. You need to experiment a bit...you can start with a 1/4” or 3/8” deep notched-trowel and adjust up or down as needed to get proper coverage. I just hope your tiles are as glass-flat and as uniform in thickness like you think they are, or you’ll be fighting to keep a finished surface that’s flat.

Oh, and large format tile mortars are designed to support heavy tiles (with less than perfect coverage IMHO). They are made with larger aggregate and shrink less than thinset mortars upon curing. With your flat substrate and tiles, I think you’re going to obtain great coverage that’s very uniform in thickness. I don’t know if you can say it would add any benefit to your install to use one...though I’d imagine every tech department is required to tell you that you would.

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Unread 01-02-2020, 09:41 AM   #3
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My questions are severalfold:

1. Ditra vs Ditra XL? - Given that the floor (TJIs, subfloor, gypcrete) was engineered for stone floors and that I am using 3/8" stone tiles, can I use Ditra instead of Ditra XL? The finished floor needs to be 3/4" thick and so using Ditra XL would make it difficult to hit that target. I think Tool Guy answered this well. No comment.

2. Mortar beneath Ditra - probably going to specify laticrete 317 beneath the Ditra. Concerns? Ettringite, as Tool Guy mentioned. Before any mortar, I would clean and wipe down the gypsum mortar bed and apply a Primer. In our case, Primer L, Dilute 1 part of Primer L with 3 parts of clean potable
water. Allow to dry, than apply a non-modified or modified mortar (MAPEI will warrant our modified under and over uncoupling membrane).


3. Mortar over Ditra - Can I use thinset or should I be looking at one of the large tile medium set products? Any advice on a product? Can I get away with an unmodified product here or would it be better to "cheat" and disregard the Schluter instructions and use a modified product here for better bond strength? Laticrete and Mapei are easy to source where I live. Ditraset is not. I would consider a LHT mortar due to the potential different thicknesses of the stone, size and weight of the tiles, again you can use non-modified or modified, a product like our LFT. LFT can be applied as thin as 3/32" (minimum thickness of thinset mortar per ANSI) up to 1/2" thickness, after beat in. You do not have to use an LHT mortar, if regular mortars provide adequate coverage and they are used from 3/32" to 1/2" thickness.

I figure I have a maximum of a 1/4" thick bed of mortar between the ditra and tile base to hit my 3/4" thick goal and so I might be able to get away with the thinset but I know that medium bed mortars might be a better choice even if they are applied thinner than their typical application. As mentioned in 2. consider the LHT (large and heavy tile mortar, formerly known as medium bed) mortar like our Ultraflex LHT or Ultraflex LFT.MAPEI will warrant their modified under and over uncoupling by Schluter.

4. Trowel size - what size would you recommend here above the Ditra keeping mind the larger tiles will need to be back buttered... Again, I agree with Tool Guy. The trowel that gives you adequate coverage. Sounds like you have a substrate within industry tolerance, so more than likely you should not need a big notched trowel. Install a stone tile and check the coverage.

Early LHT (large and heavy tile mortar, formerly known as medium bed) types of mortars may have used larger aggregate. Today, that is not typically the case which is why our LHT's can be used a thin as the minimum requirement for all thinset mortars, 3/32" after beat in.

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Unread 01-02-2020, 11:39 AM   #4
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Hi Bob, Bubba and Jim

I have a couple points more to muddy up the waters than to solve any problems. One, I have never liked the idea of putting tile over lightweight concrete, but I also know that products manufacturers will warrant it under certain conditions.

Two, I have never depended on product warranties. The failures I've experienced have been my fault. But it sounds like Schluter is not going to go along with any of this, so don't look for a warranty there.

Glad you jumped in, Jim.
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Unread 01-02-2020, 04:15 PM   #5
bob53
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Thanks all for responding -

Just to clarify:

1. Gypcrete - I am in SE Minnesota and had Maxxon (based out of Western Twin Cities in Hamel MN) pour the gypcrete. The guy who invented gypcrete and his grandson were both at the site - neat story. They provided me with 10 gallons of their overspray product. I will apply this primer prior to Ditra installation. I was told this will properly prepare the surface to accept thinset and prevent ettringite formation. Concerns with this?

2. Thanks for the info on the Mortars - sounds like I should go with the LHT product above the Ditra as it should be more stable for heavier tiles.

3. Warranty - I'd like to keep this work warranty friendly but also understand that there have been pros who have self-modified the Schluter rules now for well over a decade successfully without ill-effect. I just want to arm the installers with the best info and materials I can so that the job holds up for 30 years...

Also, if anyone is in the region and wants to bid the job, please feel free to reach out. My GC's local installer sadly was diagnosed with a terminal disease and is no longer able to work.

Thanks again!

Bob
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Unread 01-02-2020, 06:06 PM   #6
jadnashua
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Two things to consider...
1. I'd use a slant-notch trowel. This presents a much flatter, properly gauged thinset layer that makes it easier to get the coverage you need. Very popular in Europe, but available here, too. AFter combing the thinset, the slanted notches tend to fall over, creating a smoother, more even surface without the gaps that need to be flattened when then setting the tile to achieve the required coverage. The goal is 100%, but 95% of the overall and 100% of the edges is the minimum.
2. Given the large surface area, and the hydronic heating, you'll probably want to use the expansion joint spacing called for on an exterior surface, especially if there's some large window areas that could add solar heating to the mix. There's more than one way to achieve that, but I'd look at the Schluter profiles designed for that.
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Unread 01-02-2020, 07:07 PM   #7
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Welcome, Bob.

If you don't add that geographic location to your User Profile the information will be lost before we leave this page.
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Unread 01-02-2020, 10:46 PM   #8
bob53
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CX - fixed. Thanks....
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