Home DIY

Should have thought to come to the forum for my quandary.
I have a 15m2 balcony with a disaster of a concrete finish, which has waterproof membrane underneath.
I want to put synthetic grass down which the manufacturers tell me will drain ok.
Our building inspector says rain will pool and I'm inclined to agree.
Does anyone know of a draining underlay that could work and doesn't cost a fortune?

Drain to where? Assuming you're leaving the concrete in place, will it just run through the grass off the concrete off the side of the balcony (which is what I'm guessing it does now)?