An ice dam is a ridge of ice that builds up along the edge of a roof. The ice creates a dam that backs water up and under the roof shingles. Once the water is deep enough, it penetrates the roofing system and creates water damage inside the home.
Beneath the ice dam, undetectable damage is occurring in the attic and wall cavities. The wood framing is wet and maybe rotting, insulation is soaked making it inefficient. Mildew and mold can grow in hidden spots causing odors and other problem inside the home.
Soaked framing and insulation takes a long time to dry out and will continue to contribute to wall damage and interior moisture problems. Uncorrected, the water can cause serious structural damage.
Once the ice dam is high enough to overcome the pitch of the roof, water seeps under asphalt shingles. The alternate freezing and thawing that occurs under these conditions can increase the magnitude of roof leaks. Once the water penetrates the shingles, it flows under the siding and eaves and leaks through the framing into your home.
To prevent ice dams, eliminate heat buildup in the attic space below the roof deck by:
Add insulation to improve the thermal envelope and slow heat transfer to the attic. Surfaces between the attic and the heated living space should be insulated to R-38 or more.
Stopping air “bypass”
Seal any opening that allows heated air to “bypass” the insulation and rise into the attic. The warm air in your home will always try to rise and will push through any small opening. The attic door should be weather-stripped even better than an exterior door. Exhaust fans should not empty into the attic and the area between the frame of the fan and the ceiling should be tightly sealed.
Increasing attic ventilation
This will cool the space and remove unwanted moisture. The temperature in the attic space should be nearly the same as the outside temperature. Avoid blocking any vents with insulation when fixing attic insulation. The best ventilation system is balanced between low-intake from the overhangs and high-exhaust vents on the attic roof.
If ice dams have already developed you may be able to wait out the problem and work on insulation and ventilation in the spring if the water damage is not excessive.