The answer to 'why is my underlayment ice dams' is that ice dams are a common problem caused by improper insulation and ventilation in the roof. Ice dams occur when snow on the roof melts due to heat escaping from the home, then refreezes at the roof's edge, creating a barrier that prevents water from draining properly. This can lead to water damage and leaks in the underlayment and interior of the home.
Some common causes of ice dams include insufficient attic insulation, poor roof ventilation, and air leaks in the home's envelope. Symptoms include icicles hanging from the roof, water stains on the ceiling, and damage to the underlayment and roofing materials. Solutions may involve improving insulation, sealing air leaks, and ensuring proper roof ventilation.