The answer to 'why is my roof decking ice dams' is that ice dams are a common problem caused by poor insulation and ventilation in the attic. When warm air from the home escapes into the attic, it can melt snow on the roof. The melted snow then refreezes at the roof's edge, forming an ice dam that prevents proper drainage and leads to water backing up under the shingles and into the roof decking.
Ice dams can cause significant damage to the roof, including rotting the decking, leaks, and even collapse. They are more common in homes with inadequate attic insulation, improper ventilation, or poor roof design. Homeowners should address the root causes rather than just removing the ice dam, as it will likely reoccur.