I'm also considering making a green roof. Apparently, since the plants (sedum/stonecrops and mosses) help cool the roof, the solar panels on top of it are a bit cooler too increasing their efficiency.
It also helps retain some water so it does not go to the sewer directly after it rains of the roof and keeps the sewer from getting overloaded.
Another claimed benefit it is that a green roof helps reduce fine particulates in the air.
And biodiversity etc etc.
Its kinda expensive to do right though, so maybe some time.
Depending on the style of house, it also makes better use of space if you're otherwise trying to balance space for gardening or for your children to play, etc. I would love to be able to grow my vegetables on the roof and free up the ground level of a basketball ring or more lawn for the kids.
It's a bungalow so all ground level living (not just handy when you're old but apparently also when SO just delivered a baby) so the roof is also mostly flat.
I've considered adding a vegetable garden on the roof too, or maybe just herbs. And thought about a terrace on the roof too, that would be cool but the ground-level garden is nice already as it is.
It also helps retain some water so it does not go to the sewer directly after it rains of the roof and keeps the sewer from getting overloaded.
Another claimed benefit it is that a green roof helps reduce fine particulates in the air. And biodiversity etc etc.
Its kinda expensive to do right though, so maybe some time.