I have a 3/4" maple floor over a 3/4" plywood subfloor on the second story of my Chicago home. I would very much like to install a solid granite hearthstone in front of my gas-ignited woodburning 36" fireplace and have it be flush with the floor, but I'm having difficulty thinking of a way to do so that wont have major risk of the granite cracking. I plan to defintiely use Ditra to uncouple the expansion of the floor from the expansion of the hearth stone. However, I can only use 2cm, not 3cm in this space, and I can't fit any Hardiebacker / Durock / etc. nor any mortar depth without pushing the 2cm stone above the surface of the floor. I'm assuming it would be a very bad idea to cut out the subfloor and try to attach a piece of supporting wood beneath it, since then I'd have granite hanging by screws. There's not much general foot traffic on the hearth, but there is a cabinet above it, and sometimes I stand on it when moving fireplace logs, so I don't think I can assume no one will walk on it.

The dimensions of the stone will be about 55" wide by 16" deep. The firebox opening is a little over 8" up from the floor.

Do others think I can get away with installing just Ditra and thinset and 2cm granite directly over plywood?

Also, if I use Ditra, does anyone know how much thickness that will take up? And what is the thinnest layer of thinset I can get away with to bed the Ditra and the granite?

Also, is there a particular type of thinset / mortar that would be best for this?

Thanks!