The answer to 'why is my gutter mildew' is that mildew can build up in gutters due to a combination of moisture, debris, and lack of cleaning. Gutters that are not properly maintained can become a breeding ground for mildew, which thrives in damp, dark environments. Mildew can cause discoloration, odors, and even structural damage if left unchecked.
Common causes of gutter mildew include leaves, twigs, and other organic matter that accumulate over time, as well as poor drainage and improper gutter installation. Symptoms of gutter mildew may include a musty smell, visible black or green growth, and staining on the exterior of the gutters.