Slate tile outdoor installation subfloor

[TJ]

I have a balcony with 12 inch on center framing, 3/4 inch cured CDX plywood, with PVC sheeting glued down on top.
My question is:
What do I need to put on top to set my 12X12 Slate tiles?
Mortor Bed? If a mortor bed how deep?
Cement Board? Hardi Backer Board?
My concern is NO LEAKING through to my deck below. I want a water TIGHT seal.
Please help.