Project OpenSolarMap aims (aimed?) to gather information on potential roof surfaces for solar power production. Taking into consideration that renewable energy production is one of the biggest challenge of the century, I wonder if there is some other project/mobile app that gather and publishes open data on the topic.
I think imagery available today is detailed enough to estimate roof:direction and roof:orientation; both can be mapped with no particular skills.
It would be nice if in the future public institutions could query OSM database and make planning on results… am I too naive? 
Maybe such tags can be an implementation of iD editor building object, or a quest for StreetComplete (BTW with StreetComplete you can already gather roof:shape).