The incident happened in the northern Brittany town of Saint Malo when the man climbed onto the roof of the local coastguard building.
The roof, made of asbestos cement, could not support his weight and he plunged five metres to his death.
When he failed to return home his girlfriend became concerned and called his older brother to come and investigate.
The 26-year-old man climbed onto the same roof and also fell. He suffered a fracture to his head and back injuries and was taken by helicopter to a hospital in Rennes, reported newspaper Ouest France.
Investigators believe the first man died at around 1am on Saturday morning while the second was injured several hours later.