Artificial grass

Blues

I would like to lay artificial grass on top of my paved patio. There are a few problems that I would like your advice on. There is a sunken area about 2mt sq X 50mm drop at its deepest point. Could I get away with just screeding over or would you recommend relaying the flagstones The patio was laid years ago with no membrane and although its now settled and stable but most of the joints are open allowing drainage and of course weeds to come up. Obviously I need to maintain drainage and the artificial grass has holes in the rubber backing that allows for this.  Which brings me to the question is a membrane really necessary? The patio is on two levels and the flag stones on the upper level are laid flush with the step (stone edging) so I was thinking on relaying to give a 15mm edge for the grass to but up to. I'll have to do something similar with the entrance to the garage side door and the shed. So what do you think guys.  Appreciate your help thank you