Flagstone over Waterproof Membrane

I have a 400 SF "deck" that is precast concrete with a topping slab and an exposed abrasive resistant waterproof membrane, all over conditioned living spaces below. My client would like to have a flagstone patio installed on the deck. Was wondering if there is a glue or adhesive that would work to adhere the flagstone to the pvc waterproof membrane system? The other issue is that we need to keep the profile lower so that it does not affect the threshold height to the entry door.

What is the best way to place the flagstone in a high altitude mountain environment like Breckenridge, CO?