Just because it's more frequently traveled, and time is of the essence, I crossposted this on the JB Forum. Bob Camp wrote back:

1) If you are using Ditra, use more plywood and no backerboard. It's all about stiffness.

2) Laticrete 255 is a modified medium set mortar. You don't use it to set the Ditra on the floor, and if you use it to set the tile on the Ditra, you won;t get Schluter's warrenty. I would not worry about that, though.

3) Place all your plywood before you level with SLC. Don't forget the primer! Set Ditra over SLC just like you would a slab.
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Bob

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Pretty much confirming what I'd thought, which is good, since I did exert a bit of quiet force (in a Gandhi-MLK kind of way, you know :)) to get him to use the plywood

What do you all think of the answers? Anythign else to add? So I use an unmodified thinset for the ditra/tile--any favorites? TIA. I'm so used to using modified for mosaics I don't have any plain stuff in the house!

I'm typing dyslectically today. Sorry.

Pretty much confirming what I'd thought, which is good, since I did exert a bit of quiet force (in a Gandhi-MLK kind of way, you know :)) to get him to use the plywood

Fly- that's funny!
DH needed me to look up the Plywood association of America ratings for the plywood when he came home with the wrong type to back up my claim that we needed to return for the correct one!!

I know that we used 2 layers of ply for as much rigidity as we could get on the floor, needed SLC for crazy out of level floor, then set the Ditra over SLC (with I think was unmodified too), followed with tile using unmodified to keep the Schluter warranty.

Cement board I understand confers no structural addition to the floor, and one uses Ditra or cement board, not both, as an underlayment.

I love loved the Ditraset thinset by I think Hydroment- it was like butta'! I know that folks like the plain unmodified they sell at Lowe's too. I did NOT like the unmodified at HD, though, FWIW.

Let me clarify Bob's post a little bit. First, as he said, you don't want to use it to set the tile over the Ditra. And although I wouldn't use that specific thisnet to set the Ditra over plywood, I DO disagree with his point of not using modified thinset to set the Ditra over plywood. You use unmodified thinset (mixed with water) to set the Ditra, and you'll pick that Ditra up the next day. I WILL agree with it mixed with liquid latex, though, and I think I'm going to go looking for your post over at John's and give the same clarification over there.

That is so strange, Bill! I read the Ditra installation instructions and it says that if the substrate is wood, use modified, if it's completely SLC/cement/etc., use unmodified.

"...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."
(from their faq)

So you think no matter what we should use modified? Installer said he's going to SLC the entire floor, so there will be no wood showing at all.

headed over to JB...

oh, and thanks for your post, pepperidge farm -- wonder if it's too late to get Ditraset...I don't even know who carries Hydroment around here!

Fly, I know what Schluter's directions say. But if it were me, I'd be using Laticrete 253 to set the Ditra over the SLC.

I can't find the thread over on jb. If I was doing your job today:

1) Durock does have some dimensional stability, but offers no increase in structural/flexural floor strength. So if using Ditra I'd forget the cement board and add more plywood.

If I was installing Ditra over plywood, I'd use dryset modified with a liquid admix.

Ditra over a concrete slab or over slc? I'd use a modified thinset and be quite happy. If you want the comfort of the Schluter warranty then unmodified would be fine.

2) Large tiles like your 12 x 24s over Ditra on a floor? I'd use unmodified thinset.

Thankfully I have none of that to install today. Today I get to sit out under the warm sun and watch my son step in front of 80mph lacrosse balls. Ouch!

Thanks, bill and mongo! I'll get them solid on the first (SLC/Ditra) thinset choice right now.
mongo, fabulous pic! Is he the goalie? Love that double-twisting-underhand-warrior-pose-save :) Such style! "Be our guest, be our guest, put my goalkeeping to the test" :) DD once almost got nailed by lacrosse balls trying to do a track workout while the HS lacrosse team was in the field...never again. They get the whole stadium.

Yup, he's the goalie. Some people choose to inflict pain. Others chose to receive it.

He's such a knucklehead! A brave one though! lol

He's obviously extremely coordinated, though--you can tell just in that photo, seriously--so he must be highly valued by his team. And a Dad has to highly value that mask, I bet...

And so seriously, you'd use unmodified for the 12x24s? Not modified? Just to be clear before I go out and buy what I want them to use. They'll use modified to set the Ditra, we got that established. But I need to make clear what I want them to use on the floor tile. Wall tile gets the 255.

Thanks again!

Wall tile gets the 255, but the floor tile should get 317.

Mixed with water.

Thanks, Bill! Unmodified it is. I'll get it tomorrow.

You guys are the best.

"And so seriously, you'd use unmodified for the 12x24s?"

You'll have a continuous sheet of plastic (Ditra) under the tile. You'll have large 12" by 24" porcelain tiles over the thinset.

Latex modifiers need free air to dry. They don't chemically cure like straight portland cement-based product. So for the most part, unmodified doesn't need air, modified does.

The Ditra and large tile will suffocate the modified thinset resulting in a slow, slow, slow cure. Walk on the tiles 2, 3, 5, 8 days after install to grout, who knows if you'll break the bond due to insufficient curing?

It's the yin yang of the tile world. Use a modern technology like Ditra and you have to step back and use an older technology like dryset.

The world is in balance.

Embrace the Taijitu.

The sensei has spoken; let it be so :)

Ha! lol

Seriously though, that's one place I get nervous, using a modified between Schluter and large format tile. You just don't know how long it'll take for the thinset to sufficiently cure.

Thanks, mongo. I was able to get them to go to Lowe's to get the unmodified Laticrete product there. The guys are being very nice about being managed so much; their attention to detail is amazing but their product choices need a bit of guidance :)

The ONLY time I'll use a modified thinset on top of Ditra is if I prefill the waffles, and then use a rapid set. Anything other than that, and you're asking for trouble.

That's fascinating, Bill. Do you do that on jobs you need to get done fast? My installer is using the Laticrete unmodified. We'll start with the 255 tomorrow with the wall tile and ***mosaic***!!! The dream is finally becoming reality! I hope I don't hate what I've taken all this time to do...

He also was using a tool I really like--it's a score and snap but on a much higher level than I've ever seen before--by Ishii. What a lovely thing that is--worked cleanly even on thick porcelain.

They're good cutters. I have their 26" version for large format tiles. funny-- we're talking about that in the Pros Hangout right now-- in the works in progress thread-- Laz is doing a job with Cerdomus tiles-- notoriously hard tiles to cut because of their density and extremely deep grid pattern on the backs of the tiles-- and he's using an Ishii with no problem.

And to answer your question, yes. I did a hotel about 2 years ago, where all the floors were limestone over Ditra, and we prepped all the floors in just that manner-- setting the Ditra and prefilling the waffles with fast set, and then tiling them with fast set the following day.

I was hoping to borrow it to cut some long pencil-type pieces out of the wall tile to finish off the mosaic running around the room, but he did point out to me that the edge that the cutter makes is sharp to the point of dangerous, and not at all smoothed like it is with a tile saw.

I'm wondering if I could just edge the pieces with a polisher but cut them with the Ishii (I couldn't find that thread,but I did check in on the mighty mighty 'craziest custom tile job' thread--I love that it just keeps growing :))? I have trouble getting the carriage on the 10" saw really straight enough for long pieces (24"). I might just make 12" slices instead. Not sure which method would be best, but I have definitely discovered another thing to check c's list for now :)

Here it is:

Here is a link that might be useful: Works in Progress P. 309

Thanks, Bill! I love to read the pro threads.

The tile's in and awaiting my grouting (Spectralock on the shower at least, prob. on the floor too). Space has been left for my mosaic additions...I can get to it probably on Sunday...I'm so darn excited I can barely make myself wait, but the field tiles have to be good and set before I take the supports down. We held them up in place and they looked cool, I just hope it still looks cool when it's *all* up.

I can't wait to see how this all come together!