Built Around the Existing Structure
Stone Raises the Standard the Floor Has to Meet
Marble, limestone and large-format porcelain behave differently from a six-inch ceramic tile. They are heavier, they span further, and they have far less ability to absorb any movement in what is under them. A floor that would carry small ceramic tile for twenty years can lip or crack large-format within a season.
Flatness is the measurable part. Large-format needs a floor within a tight tolerance across its whole span, and on a floor that is merely acceptable the corners of the tiles stand proud of each other. That is not a finish problem, it is a substrate problem, and it cannot be grouted away.
Stone brings its own chemistry too. The setting material has to suit it, some stones will stain from the wrong sealer or react permanently to the wrong cleaner, and a marble shower needs to be specified knowing how it will be looked after.
So an Osterville job is quoted against the material, not just the room. Deflection and flatness are measured, the floor is corrected to the tolerance the material needs, the shower is built on Schluter Kerdi-Board, waterproof through the panel, and a bonded membrane is dressed over every seam, corner and curb.
With stone, the floor underneath has to be better than the tile going on it.