The answer to 'why is my underlayment water stains' is that it is likely due to a leak in your roof system. Underlayment, which is the protective layer installed beneath the shingles, can become discolored or stained when water seeps through the roof. This is a common symptom of a larger issue, such as damaged or missing shingles, improperly installed flashing, or clogged gutters.
Water stains on the underlayment indicate that moisture is getting into your roof system, which can lead to more serious problems like wood rot, mold growth, and structural damage if left unchecked. It's important to address the root cause of the leak to prevent further damage and ensure the longevity of your roof.