Ice dams can occur when heavy snow builds up on a roof. Roof overhangs are the coldest part of the roof because they extend out past the walls of your home. For this reason, ice build-up usually occurs around your overhangs and gutters, while snow on the rest of your roof may melt.

Ice dams can also be found at the rooflines of built out windows such as bay, bow or garden windows. Often, people mistake water infiltration from ice dams for leaking windows. There is a simple way to identify the difference; if the window only leaks in the winter, and doesn't leak in the summer rain, the problem can be properly identified as an ice dam.

Ice dams are formed by temperature changes after snow falls. During the warmer hours of the day, some of the snow from your roof melts, and gravity directs the water to flow towards the ground. The water then refreezes when temperatures later drop, forming an underlying ice barrier. This ice barrier is generally located at the bottom of the roofline where it meets the overhang; this is called the freeze line. Once an ice barrier forms at the freeze line, the daily warming cycle is not sufficient to melt through it, and any subsequent melting snow cannot flow away from your home through your gutters. This water then backs up underneath shingles, damaging the attic, ceilings, and walls.

Three things are required for an ice dam to form:
1. Snow
2. Heat to melt the snow
3. Cold to refreeze the melted snow into solid ice

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 traveling through a solid. A good example of this is the heating 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 (Figure 1). In many homes, there is little space in regions like this for insulation, so it is important to use insulations 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. In Figure 1, the winding arrow beginning inside the house and going through the penetration in the ceiling, from the light to the attic space, illustrates 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 snow fall.

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 duct work in the attic space will also be a source of heat. The same can be said about kneewall spaces.

Insulation is a major factor in keeping ice dams at bay. Homes in the northern United States should be equipped with ceiling insulation of at least R-38 (about 12 inches of fiberglass or cellulose). This insulation should be consistently applied to all floor areas in your attic and then sealed so that no living space heat can escape.

Ventilation is another key factor in preventing ice dams. A soffit-to-ridge ventilation system is the most effective ventilation scheme you can use to cool roof sheathing. Power vents, turbines, roof vents and gable louvers do not perform as well and therefore are not recommended.

Air Leakage: Insulation restricts heat loss, but a special effort must be made to block the flow of warm indoor air into the attic or roof area. Small holes allow significant volumes of warm indoor air to pass into attic spaces. In all construction, avoid making penetrations through the ceiling whenever possible. When you can't avoid penetrations or when you need to air-tighten existing holes, use urethane spray-foam (in a can), caulking, packed cellulose, or weather stripping to seal all ceiling leaks like:

1. Wire penetrations
2. Plumbing penetrations
3. Ceiling light fixtures
4. Attic hatches
5. Chimneys
6. Bathroom exhaust fans
7. Intersections of interior partitions with the ceiling

Be sure to check all insulation and ventilation in your attic before the start of winter. Fill any holes in the insulation to make it airtight, and make sure all vents are clear of debris to allow air to flow through.

You can remove snow buildup as it accumulates around your roofline by raking the snow off. Buy an extension attachment for your yard rake that will allow you to reach your roofline from the ground level. Use caution when doing this or any other outdoor winter project to prevent accidental injury from slipping. Never climb ladders to accomplish roof snow removal; ladders are always dangerous, but in winter, the risk of slipping increases dramatically.

Calcium chloride can be applied at your roofline, and in your gutters to help melt snow and ice. It is available at most local hardware stores. It works best if you put the product in an old pair of stockings and place them in your gutters before any major snow falls.

Contrary to popular belief, gutters do not cause ice dams. However, gutters can allow ice and water to concentrate at vulnerable roof-eaves areas, particularly if the gutters are not clear of debris. As gutters fill with ice, they often bend and rip away from the house bringing down fascia, fasteners, and downspouts. (Note: Marlboro Seamless Gutters does offer gutter covers, heated gutter solutions, gutter cleaning and snow removal services to help reduce risks of ice dams or damage to gutter systems.)

If you notice that snow is melting in specific areas of your roof, you may want to have a licensed contractor check the insulation in your attic space. Your attic space should be the same temperature as the outside air to prevent ice damming.

Ice dams can form with as little as 1-2 inches of snow accumulated on a roof, followed by several days of cycling temperature changes.

Please note, Marlboro Seamless Gutters does offer a full compliment of snow removal services both for your roof and for your property. Please email snow@seamlessgutter.com or call us at 508-481-8400 for more information or for your custom quote.