I don't see any reason not to create an artificial garden using artificial turf and those realistic fake plants you can buy. However, I have seen some excellent roof top gardens, and even gardens on top of barges and canal boats which use real plants. This is achieved by growing them in compost rather than soil, because compost is lighter and the roof can take the weight, whereas soil becomes very heavy when watered, which would be dangerous on a roof. If you want to use large pots, then you might want to partially fill them with polystyrene packaging chips before adding the compost, as this again keeps the weight down to a minimum. Hope this helped.