First I would ask where are you located. This will determine the weather effects on the wood.

I live in northwest Connecticut. Moisture is more of a problem than sun damage. The stained shingles on my house were installed in 1963 and except for the western side, they look like new but they are not installed 3 feet underground either. The wood guy said that sealing the wook seals in the moisture and rots from the inside out. The beams seem pretty dry to me but what do I know.