The answer to 'why is my underlayment water stains' is that the underlayment may be experiencing water intrusion, often due to a roof leak or other problem. Underlayment is the protective barrier installed beneath the roof shingles, and water stains indicate that moisture is getting through the roofing system.
Common causes include improper installation, damaged or worn shingles, flashing issues, and ice dams. Symptoms can include water stains, mold, or even rotting of the underlayment and roof deck. Solutions may involve repairing or replacing the underlayment, addressing the root cause of the water intrusion, and ensuring proper roof maintenance going forward. In many cases, it's best to have a professional roofing contractor inspect and diagnose the issue to determine the appropriate solution.