The answer to 'how to clean underlayment' is to regularly inspect and clean the underlayment beneath your roof. Underlayment is the water-resistant barrier installed between the roof deck and roofing material, and it needs to be kept clean and free of debris to function properly.
To clean the underlayment, start by safely accessing the roof and using a soft-bristle broom or brush to gently sweep away any leaves, twigs, or other accumulated debris. Avoid using anything abrasive that could damage the underlayment. If there are any areas of the underlayment that are heavily soiled, you can use a mild detergent and water solution to spot clean them, being careful not to saturate the underlayment. Once cleaned, allow the underlayment to fully dry before reinstalling any roofing materials.