Soft washing is a low pressure cleaning process commonly used to safely treat and remove organic matter: mildew, bacteria, algae (Gloecapsa magma), fungus, moss, lichens in addition to dirt, and grime.  The detergent is applied through a specially designed spray apparatus that is low pressure and will not damage the shingles.  Shingles should never be cleaned with a pressure washer.  Dirty looking shingles are becoming more and more common.  However, that dirty look may not be dirt.  In most cases, it's an airborne algae that feeds on the limestone which are in the shingles, greatly affecting the lifespan of the shingles.

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Once algae starts to take over, that area is also more vulnerable to the growth of lichen and moss.  These are more harmful as they actually root to the shingles and when they die, or re removed incorrectly, the shingles will sustain damage. 

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Soft washing with the right equipment and detergents can remove algae streaks instantly.  A good word picture, for this, is shampooing.  It is important to apply the detergent to all slopes of the roof even if the algae only appears in a specific area.  The algae grow in colonies and just because you don't see it doesn't mean it's not there.

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Therefore, the whole roof should be treated for best results.  If your roof has black streaks, it's likely homes in close proximity to yours will also have black streaks.  This will make your roof susceptible to the return of algae (because it is airborne).  This is only one of the reasons why we recommend treating shingles with Soy-lution after cleaning them.