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Modeling shows green roofs can cool cities and save energy
(techxplore.com)
This is a most excellent place for technology news and articles.
It also takes a toll on the sunlight below.
I still somewhat like the idea, but maybe it should be some carcasses above the actual roofs, so that roof maintenance would be easier (maintenance of these gardens as well, I think).
And no trees, just grass, moss, plants like wild grapes.
In that case maybe it won't have to be flat roofing.