Clean shingles? Replace shingles? Ignore?

[Mike Warder]

Hey everyone, 

This roof is 8 years old and the shingles are quite an eye sore from all angles. My understanding is that the algae in the shingles can also destroy ththe shingles over time. I asked my property manager about cleaning them, and her roofer suggested replacing and quoted $1,100. Is replacing the way to go? If so, is that a reasonable quote?