From the Desktop of Nancy Kapp, Realtor

A few years ago I was driving up to my house one day when I noticed black streaks on my roof.  They’d probably been there for a while, but on that particular day I just happened to notice them.  My neighbors have a huge Fourth of July fireworks display every year.  It is very impressive for a neighborhood display and I thought to myself (since I’m right behind the house where they’re shot off) that it’s from the ashes of those annual fireworks displays.  WRONG!


I found out that the black streaks on my roof are a form of algae, probably gloeocapsa magma, also known as blue green algae.  So how did they get there?  Algae spores are airborne, so if there are any in the neighborhood they will spread.

They are most commonly found in warm, humid environments.  That pretty much describes North Carolina.  The black streaks come from the protective layer that the algae forms to protect itself from damaging UV rays — kind of like a suntan.  You’ll also probably notice, if you have the black streaks, that they are only on one side of the roof — usually the north side.

Roof algae is more common today than it was 20 years ago.  That’s because asphalt shingles got their name from what they were made of — mainly asphalt.  Nowadays it’s more cost effective to add fillers, such as fiberglass and crushed limestone. Limestone is a food source for the algae.  Who knew?  I have a feeling this was an unintended consequence.

The good news is that they are not harmful or dangerous.  The bad news is that over time they can cause your roof’s protective UV granules to come off.

Roof algae is easily removed by a simple roof cleaning.  A few words of caution — if you decide to do this yourself, be sure to check your roof warranty or homeowner insurance policies.  And never use a pressure washer, even on the lowest setting.  Using a power washer will definitely void any roof warranties.

As I have a little bit of height anxiety, I’m not climbing on my roof, but for the more adventurous of you, the website at the end of this post has information on how to do it yourself.  There are professional roof cleaning services for those of you who are like me.

Once you have had the roof algae growing on your roof, if you have it cleaned, they are likely to return.  They are airborne and our climate seems to support their growth.  With that said, there are a few things you can do to slow the growth or keep it at a minimum.  The reason the algae grows on the north side is because it doesn’t get as much sun.  If you have tree branches that shade your roof, keeping them trimmed will help.

You can also install copper strips close to the peak of your roof because copper kills the algae.  Whenever it rains, the water will wash down the roof and kill any algae that may be lurking there because the water will have algae-killing molecules from the metal.  I’m not certain which is less expensive — installing the copper strips or occasionally having the roof cleaned, but they are both options.  Finally, should you have to get your roof replaced for whatever reason, they now have copper-infused shingles.


Information and photo taken from:  https://roofrevivers.com/reason-black-streaks-on-your-roof/


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