With a possibly unstable floor and settling issues, my first recommendation would be to use a marble look-a-like ceramic instead of natural stone on your floors.

If you're going to use a natural stone, the smaller the tile you use, the better. Regardless, for installation, simply follow the Ditra guidelines. Two layers of ply, etc.

For the shower, I don't see any real significant advantages to 9235 over HB. If you use Hydroban, I recommend you use Hydroban plus fabric due to settling issues.

Use Ditra instead of HB on the bathroom floors.

Thanks mongoct for the suggestions, but we already purchased the 12x12 marble tile, so that's a done deal.

I don't think our house has any current settling issues. During our home inspection we had two people look at the settling in the house and both agreed that the settling had taken place quite some time ago and it didn't appear to have any recent settling.

Again, while the settling may have stopped I still want to build our 2nd floor bathroom floor as sound as possible in light of the settling issues the house had in the past.

Based on what I have read it appears that Ditra will only warranty their product when used with natural stone if a double layer of wood (plywood/OSB) is used. If this is the case should we install two layers of plywood or one layer of plywood and one layer of OSB? Or does it really matter which is used? Is one stronger/better than the the other? Ditra requires a minimum of 3/8" thick underlayment and since OSB is either only 1/4" (too thin)or 1/2" (too thick). Since I am not looking to raise the floor any more than I need to I am looking at plywood.

So the order I am looking at is:
1st layer: Plywood (replace old with new. 3/4" thick)
2nd layer: Plywood (Ditra states a minimum of 3/8" thick)
Layer of laytex p.c. mortar
Heated floor system
Unmodified thinset mortar
Ditra
Unmodified thinset mortar
Tile

I guess my question is if the SLC is really needed in our case. Ditra's installation states "any leveling of the assembly must be done prior to installing Schluter�-DITRA"

How do I determine if the floor is level? The floor is not wavy, but there is a chance that the entire floor is sloped slightly. If this is the case is SLC required?

I am just concerned that our contractor has never worked with SLC, but has agreed to install it if we want it. If it isn't required or will not provide us any additional benefit I don't want to bother. But if it is worth the extra security I will have our contractor use it.

What you want is a flat floor. "Flat" doesn't necessarily mean that it has to be level. Flat means, as you wrote, not wavy. No high spots. No low spots. "Flat." A flat floor can be sloped...or it can be level.

If you are looking to minimize floor thickness, if your floor is not level the SLC will make it level. Realize that SLC will fill the low spots. If, for example, the low part of your floor happens to be near the entry door, the SLC will flow to the door. You'll end up with a certain thickness of SLC at the entry door. Could be 1/4". Could be 3/4". But it can ruin your floor height at the threshold.

So with SLC, you need to know where it's going to flow to, and how thick the the SLC will be at the lowest part of the floor, before you consider using it. At the highest part of the floor, the SLC will feather out to nothing.

The easiest way to find out if your floor is level is to use a level. A straight edge or a long level can show any high or low spots in the floor.

If your floor has high and low spots, that could mess up your tile installation. Example, let's exaggerate and say you have a hump in the floor, a large tile could "rock" over the hump. The center of that rocking tile might be right on the center of the hump. The edges of that rocking tile might be floating above the floor a bit due to the hump. When you tile, it'll be tough to match the edges of the rocking tile to the edges of the tiles adjacent to the rocking tile. Some will be higher, some lower. That's what we call "lippage". Or a toe stub waiting to happen.

Flat floors can handle large or small tiles. Non-flat floors that have high spots and low spots are better served with smaller tiles. You can envision sheets of mosaic simply conforming to the ups and downs of a wonky floor. Mosaic will conform to a hump instead of rocking over it. Smaller tiles, they go with the flow, so to speak.

If you're stripping the floor down to the joists, you can take advantage of the open access to the joists to level the floor at that time. You can shim and shave the existing joists (painful), or you can sister new flat and in plane joists to the existing joists (less painful).

Do that and your subfloor will be flat and level, and the remainder of your construction will be a breeze.

Your subfloor, I'm a plywood guy as well. I use 3/4" T&G for the subfloor, then usually 1/2" for the underlayment. But yes, as Schluter specs out, you can use 3/8" for the underlayment.

You don't put SLC over Ditra, which is why if you are going to level the floor, Schluter wants the floor leveled prior to the Ditra going down.

So: If, as you wrote, your floor is flat but slightly sloped, you can build over that. You'll still have to contend with the slope when installing the fixtures, the cabinetry, trim, etc.

If you use SLC, understand that it'll flow where it can. Crevaces, cracks, it'll find them. And flow through some of them. So contain any exit points before you do the SLC. Pouring SLC is when you want to have your ducks in a row. Your work flow thought out. Your work space prepared. Your bags of mix, your water, etc. You don't take a break in the middle of an SLC pour.

My favorite SLC story of woe was someone who simply stood at the door and poured bucket after bucket of SLC at the doorway and used a squeegee on a push pole to push it to the back of the room. But despite how many buckets were poured, the SLC never filled up the floor to the screed line. Because it was all flowing down the toilet waste pipe that was out of sight behind a cabinet.

Depending on what floor heat you use, it can sometimes be tough to Ditra or tile right over the RFH wires or mats.

That's where SLC comes in handy. Install the RFH wires on the subfloor. Prime the mat and subfloor with SLC primer. Pour just enough SLC to fill in around or encapsulate the RFH wires or mesh. Now you'll have a nice flat and level surface to Ditra and then tile over.

Thanks so much mongoct. That was very helpful.

I will discuss sistering or shimming the joists as well as SLC with my contractor. I took a level to the floor tonight and it is hard to determine how level the floor is because the tile currently on the floor is glazed and isn't flat. Overall it doesn't appear to be perfectly level, but it isn't as bad as I thought it would be.

When installing the plywood underlayment to the subfloor...are the two just screwed together? Does any mortar or modified thinset need to go between the two? It doesn't appear to needed based on the Ditra installation guide.

The subfloor gets glued (construction adhesive) and screwed (or nailed) to the joists.

The seams of the underlayment get offset from the subfloor seams, and the underlayment gets screwed (not nailed) onto the subfloor. No adhesive or anything between the two. When screwing down the underlayment to the subfloor, screw into the subfloor only. DO NOT screw through the subfloor and into the joists.