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The icicles hanging from eaves and gutters may be a faint memory now. But winter is coming, and along with it one of your worst enemies–ice dams. Ice dams are continuous chunks of ice that form along the margins of your roof. While frozen, they are no more trouble than the icicles that hang down. But during the warmer parts of a day, water melting off the roof pools behind the ice then seeps back up under the shingles. Sometimes water can work its way 5 or even 10 ft. back up under the shingles. Eventually, it drips through the roof into the soffits (the outside overhangs), walls, and worst of all, into your ceilings. You will first see rust spots on drywall fasteners, then perhaps peeling paint, drywall, and stains around windows and doors. Insurance companies pay millions of dollars to thousands of homeowners annually to repair the damage. But it is never enough to cover the time and aggravation of getting everything fixed.


There is a complex interaction among the amount of heat loss from a house, snow cover, and outside temperatures that leads to ice dam formation. For ice dams to form there must be snow on the roof, and, at the same time, higher portions of the roofs outside surface must be above 32° F while lower surfaces are below 32°F. For a portion of the roof to be below 32°F, outside temperatures must also be below 32°F. When we say temperatures above or below 32°F, we are talking about average temperature over sustained periods of time. Since heated air rises the air in an attic will be warmer the closer to the peak or ridge. If the heated air which makes it up into the attic is not ventilated quickly enough, it will melt the underside of the snow as well.

The snow on a roof surface that is above 32°F will melt. As water flows down the roof it reaches the portion of the roof that is below 32°F and freezes. Voila! 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°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. From the attic, it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish. Non-uniform roof surface temperatures lead to ice dams.


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. Heat from the house travels to the roof surface in three ways: conduction, convection, and radiation. Conduction is heat energy travelling through a solid. A good example of this is the handle of 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 through the slanted portion of the ceiling. In many homes, there is little space in regions like this for insulation, so it is important to use insulation’s with high R-value per inch to reduce heat loss by conduction.

The top surface of the insulation is warmer than the other surroundings in the attic. Therefore, the air just above the insulation is heated and rises, carrying heat by convection to the roof. The higher temperatures in the insulation’s top surface (compared to the roof sheathing) transfers heat outward 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: heat loss by air leakage. In many homes, this is the major mode of heat transfer that leads to the formation of 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 of high snowfall.

Other sources of heat in the attic space include chimneys. Frequent use of wood stoves and fireplaces allow heat to be transferred from the chimney into the attic space. Inadequately insulated or leaky ductwork in the attic space will also be a source of heat. The same can be said about knee wall spaces.



In all Ontario 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.


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 through 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.


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

Make the ceiling airtight, so no warm air can flow from the house into the attic space.

Both of these 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.

Natural roof ventilation can help maintain uniform roof temperatures, but if the long-term actions described here are done effectively, then only small amounts of roof ventilation are needed to maintain uniform roof surface temperatures. If heat transfer has been reduced substantially, then snow will build up on the roof and cover natural roof ventilation systems, reducing attic ventilation rates. Natural attic ventilation systems are needed to dry the attic space and remove heat buildup during the summer.

Insulation contractors, who may be listed under Insulation Contractors in the Yellow Pages, are professionals who can deal with the heat transfer problem that creates 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 have an infrared camera that can be used 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, the interior repair should be done together with correcting the heat loss problem that created the ice dam (s) or the damage will occur again.

If you still feel you need our advice, call and ask us to take a look at your home. New roof or old roof under blizzard or like conditions gets the snow off the roof. Shingled roofs are designed to shed water, not to hold water.

Get the snow off your roof in a major snowstorm. The sooner you do this the less ice will have formed, and if done by a professional roofing contractor the less damage to the roof will result.

Ice Dam related articles on the web (Roofers are not to blame!) 


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