I have taken over a flooring project and need some advice. I have been installing tile, stone and porcelain since 1982 and have come to a delema. We have a 6" structural slab, a 4" slab over to house heating tubes, the original contractor used ditra over this, which was over kill, but none of the less, I am oblidged to continue. I have already installed the 18" polished marble with a combination of absolute black granite for borders and dots, I sloped from the other tile setter at the hallway to get to a reasonable height for the thinset to work. The tie into areas that I need to lay this stone at the other section of the house is 3/4" - 13/16" above the ditra. this contractor has decided that floating out the stone using thinset was the way to go. should also note that the slab is very flat to begin with, so this should not have happened. My thought was to burn thinset over the ditra and install a fat mud morter bed over to the appropriate height and since no wire can be used, finish surface with redguard, or install a 1/2" wonder board over the ditra and deal with the differences of height for finished marble the old way, pull and back butter, pull and back butter, on and on and on. Any suggestions would be appreciated.Note, we have already tied into the hallway by using a 1/4" square notch trowel and thinset for the build up in this area

Couple of problems I see with your plan.
1- Mud needs to be a minimum 1" thick to work properly. It can't be feathered out to make the slope.
2- Cement board can not be used on a slab. Plus, if you could use it, you need to mechanically fasten it. Not a good idea with radiant floor heat.

3/4" is a lot of filler, even for a medium bed thinset.

I know you said that you think the slab is flat, but I don't see how that's possible if it's out by 3/4" or more. Have you shot it with a transit or laser level?

If it was way off level and the low point was away from where you started tiling, then you might consider using a SLC for the build up. Unfortunately this would mean ripping out the Ditra. Problem is you would need to place another antifracture membrane over the SLC.

The only type of CBU that you could install directly to the slab without mechanical fasteners is WEDI board. Maybe you could bond 1/2" or 1/4" WEDI board in the low areas and then float a skim coat of medium bed to transition the height differences.

Or, like the other installer, run away. Just kidding.

There certainly won't be any industry guidelines to go by to fix this issue so ultimately you will be on the hook for any problems after the install. I would make sure the GC signs off on any method that you come up with. IE: no warranty,etc.

IMO a mud bed can be thinner than 1" if it is not being used for structural reinforcement. If the mud bed is simply a filler, then it can be as thin as 1/4". i also think that with the ditra uncoupling things from the heated slab you should not need redgard over top. I would proceed as follows: rip several strips of 3/4 inch plywood and lay them flat on the ditra to use as screeds. Mix up a bucket of thinset and a bucket of dry pack deck mud. then use a v notch to skim the ditra with thinset and immediately put the deck mud on that. Use the plywood for screeds and yank it out when done and fill in the gaps with mud.

Set with modified white thinset directly on that.

Thankyou. This solution was my thought also since the mortar is simply a filler and would not cause structural cracks. My only other thought was the hardpack will crack and the redguard would possibly hold it together while the marble installation was proceeding.

Also one more item, thankyou to the senior members for forgetting more than I know.

Pistol Pete !
We are in the process of installing the hard pack filler over thinset over the ditra and this seems to work just fine, nothing like a machine mixed hardpack. I still might kerdi band the cold joints. Thank You! We still have another yard of mix to install.

Pistol Pete!
We started the marble and granite installation over the hard pack filler to get to where we needed to be, thinset thickness is more than accesible, 3/16", we are using a 9/16 half round trowel and of coarse we still need to pull, back butter and reset marble as we go, but not as thick. I did not wan't to take any chances so I mixed in water to the redguard, not advisable so that this solution would penetrate the hardpack and bonderize the entire fill, seems to work just fine. I have never had to create this system and hope to not duplicate this in the future. Will take picture and send along after completion, we still have 1 yard to go. Thanks again. Rick Hewitt

This system with the diluted reguard is the way to go, works perfectly, the reduced reguard did bonderize the thin hardpack
and we are ready to proceed with the installation of the marble
Time involved worked out to be 2 people 4.5 days to install approx 2 yards of hard pack. Just a note, we filled the screed boards as we went even though there was no thinset under, figured that the hardpack would adhere to itself.