Streaks or Discoloration on Roof Approximately five years after installation, roofs can begin to show signs of dark streaks. The dark streaks usually appear on the north-facing slope of the roof and are mistaken for dirt or granule loss. These streaks are commonly caused by algae growth that is prominent in areas of the roof that receive less sunlight. Moisture & Algae Roof slopes that are shaded or receive little sunlight will hold moisture for a longer period of time which can result in algae growth. The discoloration or streaking is an aesthetic concern and does not affect the life of the roof system or its waterproofing characteristics. Zinc strips can be added to a ridge of a roof to prevent algae from forming. Various solutions can be used to remove algae growth, although this is a temporary solution, as it will grow back. Use Caution When Cleaning It is important to use extreme care when attempting to remove discoloration as many harsh treatments or power washing can harm your roof system and result in granule loss.