The answer to 'why is my flashing algae growth' is that algae growth on roof flashing is a common problem caused by moisture, lack of ventilation, and environmental conditions. Flashing is the metal or plastic material used to seal roof edges, vents, chimneys, and other penetrations, and it can provide an ideal environment for algae to thrive.
Algae growth on flashing is typically green or black in color and can spread over time, creating an unsightly appearance and potentially leading to further roof damage if left untreated. Causes can include poor roof ventilation, excessive shade, and high humidity levels in the local climate. While algae growth is generally not a structural issue, it can lead to premature deterioration of the flashing material and should be addressed promptly.