The answer to 'why is my roof decking ice dams' is that ice dams are a common problem caused by improper insulation and ventilation in your roof. Ice dams form when melting snow refreezes at the edge of your roof, creating a barrier that prevents further drainage. This can lead to water backing up under your shingles and into your roof decking, causing damage.
Some common causes of ice dams include insufficient attic insulation, poor attic ventilation, and uneven roof surface temperatures. Symptoms include icicles along the roof edge, water stains on the interior ceilings and walls, and damage to the roof decking and shingles. Solutions may include improving insulation, adding ventilation, and removing the ice dam before it causes further damage.