Extending framing over concrete slab

[rbperrie]

I'm considering expanding our kitchen floor which is 2x10 joist with 3/4 plywood subfloor and 1/4 underlayment 16 inches into our den which is a concrete slab. There is about a 6 inch stepdown to the slab. As I've learned in this house nothing is square or level and neither is the current kitchen floor or even the concrete slab.
The plan is to put tile down in the kitchen so I know this is going to have to be rock solid. My questions are:
1. Give up the tile floor as there is no way to keep the two areas from moving over time. Only consider more flexible floor materials.
2. I can contour the framing to match the exisiting floor but I'm concerned that it would be difficult to exactly match the contour of the slab and result in odd pressure points that will eventually cause too much movement. I am thinking of just building the frame about a 1/2 to 1 inch low and setting it in a mortar/concrete bed to assure even support of the framing.
3. Since the framing will be sitting on the concrete slab or in the bed of concrete should it be pressure treated lumber?

All thoughts and comments are greatly appreciated.

[rwanders]

I would use PT myself---the small extra cost involved would be cheap insurance against premature rot. Use non-shrink grout mix.

[gkdesigntech]

You are correct - the wood floor is going to much more dimensionally unstable than the concrete and will thus "move" more, making a rigid floor spanning over the two different materials more likely to fail.

You could cantilever the new floor over the den slab by installing 2"x6" joists glued and nailed to the existing joists - make sure the new joists extend at least 48" (3x the cantilever) into the existing floor. This way you're not really bearing on the concrete slab so any movement that occurs will be consistent.

Also, if you're still leaning towards bearing on the slab, you definitely should use a treated lumber plate - preferrably either SBX (Sodium Borate - blueish in color) or MicroPro (natural color) treated wood. They are more dimensionally stable and true than ACQ treated lumber.

If you'd like more detail on the cantilever idea feel free to email me at design_tech@windstream.net. Good Luck!