Want to lay a stone floor in dining room

Assuming this is a wooden floor you will be tiling onto as there has been mention of joists. The joists in a house will be able to support the weight of any floor. It does need to be flat though. When tiling onto a wooden substrate it must comply with British standards. A minimum of 15mm ply must be used but may I suggest using 9mm no more ply product instead of 15mm ply. It's not as thick and a stronger product. If there is any movement in the floor prior to laying these boards the floor will have to be strengthened by removing the floor boards and fitting further support. Don't cut any corners when fitting stone. It's not cheap and when it goes wrong, it goes horribly wrong.