My yard is mostly Buffalo grass-the Comanche hybrid variety-very drought tolerant, I love it-but the problem with the customer’s garage roof is that-for reasons too convoluted to be understood by reasonable minds-it backs up on a cliff that overhangs about 40% of it. Hence the flooding/washing out. If it were me, I’d tear the thing down and rebuild it elsewhere on the property.
It’s hard to convince people to do something reasonable once they get a crazy idea in their heads.