The answer to 'how to repair underlayment' is to inspect and replace any damaged or worn underlayment on your roof. Underlayment is the protective layer installed beneath your roof shingles that helps prevent leaks and water damage. Over time, it can become worn, torn, or degraded, requiring repair or replacement.
To repair underlayment, first identify the affected areas by doing a thorough visual inspection of your roof. Look for any cracks, holes, or loose seams in the underlayment. Once you've located the problem spots, use a utility knife to carefully cut away the damaged underlayment. Then, apply a new layer of underlayment, overlapping the edges by at least 6 inches to ensure a proper seal. Secure the new underlayment in place with roofing nails or staples. If the damage is extensive, it may be best to replace the entire underlayment rather than attempting repairs.