Can the top of a cedar tree be exposed to rain?

No.

Fir is afraid of water and is prone to mold. Wood mold is mainly caused by moisture absorption and is related to weather and climate. To solve this problem, first, dry it to keep the material dry. The second is to do anti-corrosion and degreasing treatment. The third is to apply finishing paint to seal after drying. The fourth is to use wood antifungal agents. Apart from these, there is almost no other way.