The answer to 'why is my gutter mildew' is that gutters can develop mildew due to a buildup of moisture, organic debris, and lack of airflow. Mildew thrives in warm, damp environments and can discolor and damage gutters over time.
Common causes of gutter mildew include leaves, twigs, and other organic matter collecting in the gutters, which can trap moisture. Gutters that are not properly sloped or regularly cleaned can also lead to stagnant water that promotes mildew growth. Overflowing or leaking gutters can also contribute to mildew issues.