The black streaking and patches that can be seen on a roof are the result of an algae that continues to grow and spread over time.
This algae feeds on the material that is in the shingles and present on the roof tiles. It grows and spreads most quickly in areas on the roof that are shaded and/or are likely to retain moisture. The algae growth often starts on the north side of a roof or under a tree canopy.
Cleaning solutions that kill off the algae fully will result in roofs staying clean longer than other methods. This is what occurs with cleaning solutions that contain bleach and other algaecides. Roofs cleaned using low pressure chemical cleaning should remain clean for 2-3 years before needing to be re-cleaned. However, as noted above, this is very dependent upon the environmental conditions. Roof areas that do not get much direct sunlight or remain in damp conditions likely will have spots that reappear earlier. It is best to retreat these areas as soon as possible rather than wait for it to spread.
If you are concerned with the ongoing care and maintenance of your roof you can contact us to learn more about Mallard Systems exclusive Maintenance Program or click here.
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