One of the lads at the design tile place mumbled something about no sag mortar for setting ceiling tiles.

That's exactly what I was going to say. Any time I do a Kerdi shower, steam or otherwise, I'll use non-sag thinset on the walls and ceiling. It's alot easier to avoid a headache! I still use unmodified on the floor (I don't want to wait a week to get in and grout), but everything else is non-sag, usually Laticrete's 255 thinset.

Thanks Bill!
Sally

I don't have a problem using lightly modified thinsets over Kerdi with "standard" sized tiles.

But with large tiles like 12 by 24s, I recommend staying with unmodified.

If you do use modified, I recommend giving the thinset several days before you grout. I'd probably wait a week.

I did sample boards years ago and then ripped them apart for inspection. It did make a difference when giving the latex the extra drying time prior to grouting.

so...is the idea to put unmodified over the Kerdi, let it dry then use modified on top??? and to make sure to leave lots of time before grouting? (for walls and ceilings that is - floor will be 2x2 and unmodified only)
Sally

If you do it by the book, use unmodified only. If you use the non-sag, that's all you should be using-- not combination of the two under the same tile. The reason why they recommend the unmodified isn't because the other won't bond well. It's because it'll take much longer to dry out completely. The thing is, the only place I've seen a problem with that is on the floor-- you wouldn't be able to walk on it for several days to even get in and grout. Otherwise you'd break them loose as you walk on them. That's not a problem on the walls or ceiling.

"so...is the idea to put unmodified over the Kerdi, let it dry then use modified on top???"

No. Either use modified or unmodified. No need to skim coat with one then go over that with another.

I just did a kerdi shower ceiling with 12x24 tiles. I used unmodifed thinset per kerdi directions. I used laticrete 317. I know versabond will work (but its lightly-modified) and would void the Kerdi warranty if used.
The 12x24 will stick just fine. It's no different than doing a 12x12 because even though it is twice as heavy, it has twice the surface area to hold it.
I did back butter each tile. When I tried to pull one down to check coverage it was really stuck up there!
My biggest problem was that I started tiling from the center to the sides and i have a slanted ceiling so the tiles wanted to slide out of postion. if I had started at the wall and used spacers it would have been easier.

Many thanks for the clarification.
Sally