The answer to 'why is my underlayment ice dams' is that ice dams are a common problem that can cause damage to your roof's underlayment. Ice dams form when melting snow refreezes at the edge of your roof, creating a barrier that prevents proper drainage. This can lead to water buildup and leaks that can soak the underlayment, causing it to deteriorate over time.
Some common causes of ice dams include poor attic insulation, inadequate ventilation, and heavy snow accumulation. Symptoms may include visible icicles, water stains on the ceiling, and damage to the underlayment and shingles. To solve the problem, you may need to improve insulation, add ventilation, or remove snow buildup. In severe cases, it's best to call a professional roofer to assess the damage and make necessary repairs.