This always worried me when I lived in an appartment. I always thought that buns had enough sense to stay on the safe side of a balcony, but apparently not!!:shock::shock:
I guess the first thing is to remove the hanging plants to avoid the ladder effect. Next to heighten the barrier above the garage. Then to put in a wire overhang (at this sort of angle \ or 45 degrees) to keep him off the roof (without interfering with drainage).
Just thinking about it brings out my own fear of heights, I don't envy you your task!