The answer to 'how to install underlayment' is to properly install a waterproof or water-resistant membrane beneath your roof shingles. Underlayment protects the roof deck from moisture and serves as an extra layer of protection against leaks. It is an essential component of a durable, long-lasting roof.
To install underlayment, start by ensuring the roof deck is clean and free of debris. Then, unroll the underlayment and align it at the bottom edge of the roof, overlapping each subsequent course by 2-4 inches. Secure the underlayment with roofing nails or staples, being careful not to penetrate the roof deck. Continue overlapping and fastening the underlayment all the way up the roof. Finally, install your roof shingles or other roofing materials over the top.