That's not really much different than having a terrace or a green roof to deal with when roof problems occur. Sure you can make a plain roof but it is a ridiculous waste of outdoor space, which leads you to need to reserve other space.
I would like to see terrace materials with mediocre efficiency solar collection, similar to the solar streets idea, but dealing with a realistic amount of wear and tear.
If you are talking about a single family home then build a solar A-frame, or better yet don't build it. But most larger buildings need to have a significant amount of flat roof.
I would like to see terrace materials with mediocre efficiency solar collection, similar to the solar streets idea, but dealing with a realistic amount of wear and tear.