Green roof and rainwater catchment system? Design, planning...

I am trying to start a little intentional community and am considering how to go about this. I would prefer to use green roofs and I am also seeking to catch rainwaternfor most of our water needs.


Also, due to our design limitations as a result of planning and zoning, the buildings have to be interconnected. I am thinking to do this with a utility room that houses the water filtration and cistern for each unit. (Probably also solar battery banks, cleaning supplies, tools, etc, haha.)
This is my design idea, attached. Does this look like a sound design to you? What filtration system would you recommend? I am considering the rocks and sand method since I already have a green roof in place anyway. But then I must consider also the potentially structurally weaker point here. Most of my building design involves minimal cement use but I would consider some here to reinforce and create a funnel/trough for the rain catchment system if that is deemed best!

Thoughts?
Thanks in advance for any ideas or input!

P.S. I am wondering what the simplest filtration system would be. I am wondering if just adding an inverted roof shape in between the building units to direct water to a cistern, and filling the trough with sand, would be an acceptable filtration system gor potable water? Or would we need to filter water in smaller amounts for drinking, on demand?
I saw this interesting video about a sand based filter...
https://youtu.be/P5TrNBOy-hA