I've just seen bits on tv about green roofs and I would guess that the asphalt shingles would have to come off, a water-proof membrane installed (maybe like the ice shield stuff we use here) and then some kind of shallow, lightweight grid system to keep growing medium from washing away. Yeah, we do have to get rid of the moss on our shingles every couple of years. We've thought about installing copper strips on the roof but have never actually done it.
