The answer to 'why is my underlayment leaking' is that there are a few common causes, including improper installation, damage from weather or foot traffic, or age-related deterioration. Symptoms of an underlayment leak can include water stains, mold growth, or damp spots on your ceiling or walls.
To address an underlayment leak, you'll first need to identify the source of the problem. This may require inspecting your roof, checking for any holes or gaps in the underlayment, and looking for signs of damage. Depending on the extent of the issue, you may be able to make minor repairs yourself, such as patching small holes. However, if the underlayment is significantly damaged or the leak is widespread, it's best to call in a professional roofing contractor to assess the situation and provide the appropriate solution.