What Is an Ice Dam?

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

What Causes Ice Dams?

Nonuniform roof surface temperatures lead to ice dams. Heat loss from a house, snow-covered roofs and outside temperature interact and lead to ice dam formation. For ice dams to form, there must be snow on the roof, and, at the same time, the upper portions of the roof's surface must be above 32 degrees F, while lower surfaces are below 32 degrees F. For a portion of the roof to be below 32 degrees F, outside temperatures must also be below 32 degrees F. (When we say temperatures above or below 32 degrees F, we are talking about average temperatures over sustained periods of time.)

The snow on a roof surface that is above 32 degrees F will melt. As water flows down the roof, it reaches the portion of the roof that is below 32 degrees F and freezes. This creates an 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 are, on the average, below 32 degrees F. 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. In the most common case, the water simply backs up and flows under the shingles. From the attic, it could move 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 lower 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.

Heat from the house travels to the roof surface in three ways: conduction, convection and radiation. Conduction is heat energy traveling through a solid. A good example of this is heating a cast iron frying pan. The heat moves from the bottom of the pan to the handle by conduction.

If you put your hand above the frying pan, heat will reach it by the other two methods. The air right above the frying pan is heated and rises. The rising air carries heat/energy to your hand. This is heat transfer by convection. In addition, heat is transferred from the hot pan to your hand by electromagnetic waves, and this is called radiation. Another example of radiation is to stand outside on a bright, sunny day and feel the heat from the sun. This heat is transferred from the sun to you by radiation.

In a house, heat moves through the ceiling and insulation by conduction. In many homes, there is little space in regions like this for insulation, so it is important to use insulation with a high R-value per inch to reduce heat loss by conduction.

The top surface of the insulation is warmer than the other areas in the attic. Therefore, the air just above the insulation is heated and rises, carrying heat by convection to the roof. The higher temperature in the insulation's top surface, compared to the roof sheathing, also transfers heat to the roof by radiation. These two modes of heat transfer can be reduced by adding insulation. This will make the top surface temperature of the insulation closer to surrounding attic temperatures, directly affecting convection and radiation from this surface.

There is another type of convection that transfers heat to the attic space and warms the roof: warm air leaking from the living space. In many homes, this is how heat transfer happens. This can cause ice dams.

Exhaust systems like those in the kitchen or bathroom that terminate just above the roof may also contribute to snow melting. These exhaust systems may have to be moved or extended in areas where lots of snow may accumulate.

Other sources of heat in the attic space include kneewalls, leaky ductwork and chimneys. Frequent use of wood stoves and fireplaces allow heat to be transferred from the chimney into the attic space and then to the roof.

Dealing With and Preventing Ice Dams

Ice dams can be prevented by controlling heat loss from the home.

Immediate actions:

• Remove snow from the roof. A "roof rake" and push broom can be used to remove snow, but may damage the roofing materials. (Note: Make sure you take safety precautions if you do this yourself. Or hire a professional.)

• In an emergency situation, where water is flowing into the house, make channels through the ice dam to allow the water behind the dam to drain off the roof. Hosing with tap water on a warm day will help.

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.

Long-term actions:

• Increase the ceiling/roof insulation to cut down on heat loss by conduction. State code requires total R-value of 38 above the ceiling for new homes. In narrow spaces, use insulation products with high R-values (6-7 per inch).

• Make the ceiling air tight so no warm air can flow from the house into the attic space.

These long-term actions will increase the snow load that your roof has to carry because it will no longer melt. Can your roof carry the additional load? If it is built to current codes, there should not be a structural problem.

Attic and/or roof ventilation can help keep roof temperatures uniform, but if the long-term actions described here are done well, then only small amounts of ventilation will be needed. If heat transfer has been reduced, then snow may build up on the roof and cover roof ventilation systems anyway. But attic ventilation systems are needed to dry the attic space and remove heat buildup during the summer.

Mechanical attic ventilation is not a recommended solution to ice dams. It can create other attic moisture problems and may cause undesirable negative pressure in the home.

Weatherization contractors, listed under Energy Management and Conservation Consultants or Insulation Contractors in the Yellow Pages, are professionals who can deal with heat transfer problems that create ice dams. A blower door test should be used by the contractor you hire to evaluate the airtightness of your ceiling. In addition, they may use an infrared camera to find places in the ceiling where there is excessive heat loss.

Interior damage should not be repaired until ceilings and walls are dry. In addition, you should correct the heat loss problems that created the ice dams, or the damage will occur again.

Preventing Ice Dams in New Homes

You can prevent ice dams in homes being built by following or exceeding the state code requirements for ceiling/roof insulation levels and having a continuous, 100 percent effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space! Recessed lights, skylights, complicated roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.

Mold, Mildew, and Air Quality

Moisture entering the home from ice dams can lead to the growth of mold and mildew. These can cause respiratory problems. It is important that the growth of mold and mildew be prevented. Dry out portions of the house that are wet or damp. (See steps listed earlier to get rid of the water source.)

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Source: "Ice Dams," by T. Larson, L. Hendricks and P. Huelman, University of Minnesota Extension Service, 1997. Reviewed by Jim Philp, UMaine Cooperative Extension forestry specialist.

For more information on emergency preparedness, contact your county UMaine Extension office.

© 1998
Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment.