Usually the added weight of the SLC which has got to be thick to cover the in floor radient heat, is not recommended over 16" center joists and just ¾" plywood. It is not stout enough, and then your adding more weight with the thinset and tiles.

Usually the area is 12" center joists or at least 1¼" of subfloor, or the joists are blocked from below. It doesn't sound as if your GC is educated in proper substrate requirements for natural stone, which are double that od clay ceramics.

Sealing the surface only prior to setting is a very good idea, as a texture tile surface is impossible to clean, thinset or grout from. It should be sealed before setting, and then come time for grout, it is already sealed and ready to go for the grout to be easily cleaned and wiped down.

It depends on the size of your grout joints as to whether to use sanded or unsanded. 1/8" and smaller is unsanded territory, and wider then that up to 1/2" is sanded grouts territory.

I thought the SLC will form part of the "substrate", so it'll be 3/4" plywood + the SLC, no? Or is that still too little? How much does the SLC add? Would a layer of Sleuter-Ditra over the SLC do the trick?

Darn, I was hoping to keep the floor nice and low for the transition.

Thanks,
Elma

Elma,

Read the first two short paragraphs, very slowly in my reply above. Your adding a lot of weight over a substrate that is not engineered to have that much weight.

An excerpt from http://www.custombuildingproducts.com/docs/tech_bulletins/TB11%20LevelQuik%20Subfloor.pdf?user=diy&lang=en : "The installation system consists of a LevelQuik® Self-Leveling Underlayment installed at 1/2" (13 mm) thick over the 23/32" (18.2 mm) plywood subfloor with metal lath installed. The metal lath must be a minimum of 2.5 lbs./sq. yd. (1.13 kg/.83 M2). This system provides a more stable substrate on which to set tile than the double layer of plywood method and is less expensive and labor intensive than the uncoupling system."

This installation isn't the same as the one I'm planning, but still, to me that sounds like the SLC forms part of the substrate, instead of being a liability on the substrate. Would it having heating cables in change that?

Sorry for for the persistence. I'm learning every day. :)

Thanks,
Elma

You are adding an underlayment (Ditra or cement board) in addition to the subfloor, right? Setting travertine (or any tile) directly onto the subfloor is a bad idea. And this is a separate question from whether the subfloor is strong enough. Check out the John Bridge deflectolator to see if your construction will effectively support your tiling.

Here is a link that might be useful: John Bridge Deflectolator