The answer to 'why is my underlayment hail damage' is that your roof's underlayment has been damaged by hail. Hailstones striking the roof can puncture or tear the underlayment, a critical water-resistant layer installed beneath the shingles. This can lead to leaks and water damage if not addressed properly.
Underlayment damage from hail is a common problem, especially in areas prone to severe storms. The symptoms include visible holes, rips, or thinning of the underlayment material. If left unrepaired, this damage can allow water to seep through to the roof deck and interior of your home, causing further problems. In many cases, the entire underlayment will need to be replaced to restore proper protection and prevent future leaks.