Nonuniform roof surface temperatures lead to ice dams, according to the University of Minnesota Extension Service.
Heat loss from a house, snow cover and outside temperatures interact to form ice dams. There must be snow on the roof and, at the same time, higher portions of the roof's outside surface must be above freezing, while lower surfaces are below freezing.
These are average temperatures over sustained periods of time. For a portion of the roof to be below freezing, outside temperatures must also be below freezing.
The snow on a roof surface that is above freezing will melt. As water flows down the roof it reaches the portion of the roof that is below 32 degrees and freezes. This causes the ice dam.
The dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that average below 32 degrees. So the water above backs up behind the ice dam and remains a liquid. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish.
Since most ice dams form at the edge of the roof, there is obviously a heat source warming the roof elsewhere. This heat is primarily coming from the house. In rare instances solar heat gain may cause these temperature differences.
Ice dams can be prevented by controlling the heat loss from the home.
Remove snow from the roof. This eliminates one of the ingredients necessary for the formation of an ice dam.
A "roof rake" and push broom can be used to remove snow, but may damage the roofing materials.
In an emergency situation where water is flowing into the house structure, making channels through the ice dam allows the water behind the dam to drain off the roof.
Hosing with tap water on a warm day will do this job.
Work upward from the lower edge of the dam.
The channel will become ineffective within days and is only a temporary solution to ice dam damage.
First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space.
After sealing air leakage paths between the house and attic space, consider increasing the ceiling or roof insulation to cut down on heat loss by conduction.
Weatherization contractors, (also "Energy Management and Conservation Consultants" or "Insulation Contractors") are professionals who can deal with the heat transfer problem that creates ice dams.
The contractor you hire should conduct a blower door test to evaluate how airtight your ceiling is. They also may use an infrared camera to find places in the ceiling where there is excessive heat loss.
Do not repair interior damage until ceilings and walls are dry.
Interior repair should be done together with correcting the heat loss problem that created the ice dam(s) or the damage will occur again.
Anyone on the roof during the winter or performing work on the roof from below risks injury and may cause damage to the roof and house.
It is important to contact professionals to carry out this job.
Whenever a house is tightened up, ventilation systems, exhausting devices and combustion devices must have enough air to operate safely and effectively.