The answer to 'why is my underlayment algae growth' is that algae growth on your roof underlayment is typically caused by moisture and poor ventilation. Excess moisture can allow algae and mold to thrive, leading to unsightly discoloration and potential damage to your roofing materials.
Some common causes of underlayment algae growth include improper attic ventilation, leaks in the roofing system, or high humidity in your local climate. Symptoms may include green, black, or reddish-brown staining on the underlayment, as well as a musty odor. To address this issue, you'll need to identify and resolve the underlying moisture problem, then clean and treat the affected area to prevent future growth.