Roof Replacement - What are those black streaks on my roof?

At Windows on Washington, we often hear from frustrated homeowners, “What are those ugly black streaks on my roof?”

It is a commonly held belief that mold or fungus is the main culprit behind the unsightly black streaks on their asphalt roofs.  The truth is that they are the result of airborne algae.  The same organism that causes discoloration on the siding on your home’s shaded section is also creating those unattractive streaks on your roof.

Most manufacturers provide for 10 - 15 years of warranty coverage against this discoloration. However, the coverage is dependent on the exclusion of several common environmental and exposure conditions.

Many homeowners begin to see evidence of this algae formation within 5 years of a new roof installation.  So why does this streaking appear to happen more frequently and severely on newer roofs?  

The exact conditions behind the streaking are variable from roof to roof, but many industry experts attribute the streaking to an increase of filler content used in the manufacturing process of shingles and siding (i.e. limestone and other organics).  As petroleum prices increased in recent years, shingle manufacturers looked to control costs by reducing the petroleum content in favor of fillers which may ultimately provide a more hospitable growing environment for the algae.  Consequently, homeowners experience more streaking on newer roofs as compared to their higher asphalt content predecessors.

Similarly, it has been theorized that the limestone filler used in the manufacturing of vinyl siding leaches out over time, which creates a highly favorable growing environment for algae.  If you have ever rubbed your finger across a piece of vinyl siding that is 10 years old, you will see evidence of this condition on the chalky residue on your finger.

Do you have any of these same questions about your roof or siding?

• What does the warranty coverage include and provide for?
• Is there a way to clean my roof or siding to restore its original look?
• Can I pressure wash my roof?
• What product should use to clean my roof or siding?
• Do zinc strips work to eliminate the streaking?
• Are metal roofs subject to the same issues as asphalt?