The answer to 'why is my underlayment algae growth' is that algae growth on roofing underlayment is a common issue caused by moisture and exposure to the elements. Underlayment, which is installed beneath the roof shingles, can provide a moist, humid environment that promotes the growth of algae, moss, and other organic matter.
Some common causes of algae growth on underlayment include poor ventilation, inadequate drainage, and excessive shade or tree cover. The algae can discolor the underlayment, retain moisture, and potentially lead to further damage to the roofing system over time. To address this problem, it's important to identify and resolve the underlying causes, which may require professional roofing services.