Rain water green roofs

These are starting to pop up more and more on rooftops in the city. It's fantastic to see. Also here in the city we need green spaces/rooftops more than anywhere else for obvious reasons.

Rainwater catchment roofs have to be non-toxic and not have any dirt or decaying vegetation in the water. Some use expensive roofing types like aluminum, tile, poured Bermuda style, and rubber. We used the much cheaper acrylic roofing. We spent $300 for a 700 sq. ft. roof. One inch of rain fills our (recycled) 425 gallon propylene tank, and keeps our 100 sq. ft. of interior gardens well watered. Overflow goes to a floor drain hooked to the foundation drain system. Water for drinking goes through a complete filter, with a ShurFlo 12VDC pump on a manual switch. Under the tank (custom cab by husband) is the battery, charge controller for PV panels, and inverter, plus storage.
So the rainwater catchment green roof is totally different than the green growing roof type. These can be very heavy and require extra strength in the structure to hold the massive wet dirt and added occasional heavy rain or snow loads, and live loads of people walking around. The runoff on these is generally unusable for consumption. They can be just grasses for insulation and cooling, or food growing types.
This thread was started about the rainwater catchment types, and hijacked by a growing green roof salesman. Always remember, that in green living, salesmen and not doing it yourself add, too often, extremely excessive costs.

Yes these rainwater roofs seems like a good idea, but how well do they do with insulation and will they require maintenance? Will I need to get on the roof and periodically maintain? What about living in a a very dry area, will I need to start watering the roof? Couldn't I just collect roof water and filter it with a separate filter? This would probably be a cheaper configuration compared to the floor depot?