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Dealing with and preventing ice dams

Cross-section drawing of house roof with ice dam showing water damage.
Cross section of house roof with an ice dam

What is an ice dam?

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation and other areas.

Effects of ice dams

Moisture entering the home from ice dams can lead to the growth of mold and mildew. These can cause respiratory problems.

  • Prevent the growth of mold and mildew by immediately drying out portions of the house that are wet or damp.
  • Take immediate action to get rid of the water source, clean the home environment and maintain its air quality.

What causes ice dams?

Nonuniform roof surface temperatures lead to ice dams.

Heat loss from a house, snow cover and outside temperatures interact to form ice dams. For ice dams to form there must be snow on the roof and, at the same time, higher portions of the roof's outside surface must be above 32 degrees F (freezing) while lower surfaces are below 32F. 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 32F 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 32F. 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.

What causes different roof surface temperatures?

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.

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Preventing and dealing with ice dams

In all Minnesota communities it is possible to find homes that do not have ice dams. Ice dams can be prevented by controlling the heat loss from the home.

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    Hire an energy professional

    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.

    Warning

    • 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.

    Timothy Larson, Lewis Hendricks, Patrick Huelman, Extension specialist and Richard Stone, Extension educator

    Reviewed in 2019

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