Without a properly engineered ventilation system on the roof, ice-damming can threaten a building’s health. An ice dam is a ridge of ice forming at the eaves of a roof, cricket, or valley that prevents melting snow and water from draining off. The water backing up behind this obstruction can refreeze, creating an ice dam. These dams can be the cause of roof failures; once significant amounts of moisture enter the building, there is a high probability mould and other negative impacts can occur.