Lee’s Summit Roofing: Algae Growth on Roofs

When you look at roofs in your neighborhood, have you ever noticed some that have black roof stains?  Have you ever noticed a pattern on those stains?  Have you ever wondered why those stains appear or what causes them? 

The cause of these black stains is algae.  Here in Lees Summit, we have a moisture rich environment that contributes to the growth of this algae.  The patterns you see on individual roofs relate to moisture (such as dew) and areas with less sunlight, including north-facing portions of the roof or areas with heavy tree coverage.  This algae also travels through the air, so if you notice a neighbor’s roof appears to have this discoloration, it can travel through the air to adjacent houses, and it just might come your way.  This algae is mostly an aesthetic nuisance, and should not affect the life of your roof.  It can be cleaned with a mixture of tri-sodium phosphate (TSP, available at local hardware stores), bleach, and water.  This should be considered annual maintenance as this algae growth will be a recurring problem.
 
You may also notice a pattern where there are no black stains below skylights, plumbing vents, or other areas that have flashed roof penetrations.  This is because zinc in the metal flashing is a natural algae deterrent.  As rainwater flows over the flashing, it dissolves minute amounts of zinc from the metal and provides a small portion of your roof that doesn’t support algae growth.  When you install a new roof, your roofing contractor can provide methods to prevent this algae growth from ever developing.  A roofing contractor familiar with the Lee’s Summit, Missouri area will be the most knowledgeable about how much algae growth is usual for roofs in this area, as well as what some other climate impacts on your roof will be.

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