Ice dams can be a persistent problem in regions with significant snow accumulations during extended periods of below-freezing temperatures. Ice dams form when roof areas become warm enough to melt snow, typically because of the combination of the insulating effect of heavy snow accumulation and heat loss from conditioned spaces. Water from melted snow runs down the roof under the snow cover and refreezes along roof overhangs where the roof surface temperature is lower. As more water from the melting snow becomes trapped behind the dam of ice, it can back up under shingles and infiltrate the underlayment, leading to leaks.
Melting snow that produces ice dams can be controlled if a roof surface temperature is maintained below freezing. To this end, NRCA recommends minimizing the amount of heat reaching the underside of the roof deck by adding a continuous ceiling air barrier or alternatively sealing interruptions in the ceiling vapor retarder with air-impermeable materials such as closed-cell spray polyurethane foam.
Secondly, a balanced attic ventilation system will help remove stray heat so temperatures throughout a roof deck are maintained close to the outside temperature. In areas with a history of ice dam formation, vent opening area and ventilation space height (in cathedral ceilings) should be appropriately sized to establish uniform rising air flow, which will cool a roof deck to below freezing when outside temperatures are below freezing. Guidelines for designing cathedral ceiling ventilation to prevent ice dam formation have been developed by researchers working at the Cold Regions Research Engineering Laboratory (CRREL) of the U.S. Army Corps of Engineers and are available by the link/document below.
Measures that prevent stray interior heat from reaching a roof deck may not always be sufficient to prevent ice dam formation. Sometimes, ice dams occur even when attics are correctly insulated and ventilated because of solar heat and variation in snow cover thickness. A watertight barrier of self-adhering membrane underlayment is needed and required by municipal codes at eaves as a protection against ice dam leaks.