It's natural to be worried about the condition of a roof if it looks like the photo above.  You might reasonably think that the shingles are damaged or defective in some way.  But there's good news -- it's likely just algae!

This roof algae, which appears relatively frequently in/around Harrisonburg and Rockingham County grows and thrives by consuming the materials that roof shingles are made of --- for example, calcium carbonate.   The algae does not seem to cause the shingles any harm, but it makes for an unattractive roof --- that makes home buyers wonder whether something is wrong with the roof.

How can you get rid of these stains, you might ask?

• Per eHow, some simple cleaning with bleach will do the trick.
• Or you could install zinc strips at the top of the roof to stop new growth.
• Or you could (at some point) replace your shingles with algae block shingles that include copper granules.  The copper prevents the algae from growing.