Deedles - what's the unsupported SPAN of your floor joists? This will affect the rigidity of your floor and is the first thing to look at/understand when dealing with sub-floor stiffness issues related to upcoming tile work.

Dee, I recently had a suntouch floor put down and my contractor did put cement board over the subfloor.

I don't know what brand you have, but you might look at manufacturer instructions. I've pasted below what it says on the suntouch website.

Yes, SunTouch can be installed directly on plywood if it is covered with at least 3/8 inch of mortar. It is a good idea to coat the plywood with a primer that is compatible with your brand of polymer modified mortar. If you are installing a thinset tile floor over plywood, consider first putting down a layer of cement "backerboard". Then, secure SunTouch to the backerboard and cover it with a latex or polymer modified mortar and then a finished tile or stone floor. In all cases, install the tile floors according to the Tile Council of North America guidelines.

Here is a link that might be useful: Suntouch

What about using a product like Schluter Ditra?

Dseng: The unsupported span of the floor joists is 8'

Flicka: The question wasn't so much about the heatmat, it was being told we needed 1 3/4" of underlayment. The guy misspoke I am now told and we need 1 1/4" of underlayment. Which is better but still will leave a 1" transition from the bathroom tile floor to the little hallway (like, really little 3' sq. hallway) that it opens onto. Eh. my whole MB tile floor is not going to be doable without impacting everthing else that comes behind it... other flooring, baseboards, door jambs and casing.. the DOORs will have to be cut down. :>(

PRickly: DH looked up that ditra and thought it was the ticket until he saw that it couldn't be used for tile smaller than 2". Lots of the pebbles are smaller than that. Our flooring guy concurred. So that's out.

dang.

Given the information you've presented, your floor is more than stiff enough for tile or natural stone. Any additional thickness added is going to be for the actual flooring, and is not needed for structural purposes. I cannot see any need for adding additional thickness to the underlayment. Your layers may end up looking like this: 3/4" plywood subfloor, SLC primer, heatmat (no thickness added because it is encapsulated in SLC), Self-levelling compound (unknown thickness - guessing ~5/8"), thinset 1/8", antifracture membrane (e.g. Noble CIS), thinset 1/8", floor tile 1/4"?? At best you'll be adding approximately ~ 1.25" to your existing plywood. IF you can find someone who will do it that way. Tile installers all tend to have their own preferred ways of doing things - and that's usually fine because their work should be guaranteed for years.

Sorry, I wasn't meaning to point you to the info on the mat but rather the info where they talk about the underlayment (which for suntouch is 3/8 inch of mortar over the plywood). I figured your manufacturer would have similar info. It seemed to be consistent with what your first guy was saying.

Thank you all for your input. We're trying to problem solve this and will use the suggestions here to help us in doing that. Hopefully we'll be able to make this work!

I went through all of this. Mongoct and Bill V. are very helpful.

Joist. you may or may not have the ridgidity to tile with stone. It requires a stiffer floor system than ceramic. Check the JohnBridge web site and look at their "deflectolator" to determine, roughly, your capacity.

Subfloor. If you use cement board it has to be thinset inplace as well as screwed down. The thin set is not to glue in place but to fill voids that may flex and cause cracking of the tile system down the road. If you use plywood as the underlayment it needs to be screwed down on center every 6" not glued down.

Ditra is a decoupleing membrain that goes down after your self leveling compound is installed.

Check out my post on floor joist on the remodeling forum and here. There is a link to the remodeling forum that is very detailed and time consuming to read and interpret. I provide a link below.

Check out Bill V's. post on frequently asked questions. bathroom tile FAQ's-bathrooms

Check out Staceyneil's post on " Self-leveling compound & radiant heat: what I learned..."

Mongoct has very detailed information throughout different posts.

Here is a link that might be useful: Progress on floor prep for slate tile