What does it mean to have a double dragon ball on the roof?

A decoration on the ceiling of traditional buildings in China. It is called "algae well", which means five elements to prevent fire with water. Usually on the Buddha's seat in a temple or above the throne of a palace. It is a flat top with a concave part, which is square, hexagonal, octagonal or round, with carvings or paintings on it, and the common one is "Shuanglong Play Pearl".