id like to use stone flagstone slabs for the roof. Precast concrete harth slabs would work also.
16 ga metal studs will support it. Roof sheathing will be 1/2” concrete board.

How do I prep (waterproof) the roof to accept the stone? Some type of rubber liner or paint on membrane like redgard?

Here’s a pic of the roof type I’m talking about. Just need to fig. Out the underlayment.