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 must be a heat source warming the roof elsewhere. This heat primarily comes from the house. In rare instances, increased heat from the sun may cause these temperature differences.
It is primarily heat flowing from the house that causes the nonuniform temperatures of the roof surface leading to ice dams.
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.
- Heat also transfers 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 transfers 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 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.
- 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.
Convection and radiation 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.
Heat loss by air leakage
There is another type of convection that transfers heat to the attic space and warms the roof. In the diagram, the winding arrow beginning inside the house and going through the penetration in the ceiling, from the light fixture 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 knee wall spaces.
Long- and short-term ice dam control
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.
Immediate and long-term actions will increase the snow load that your roof has to carry because the snow will no longer melt.
If your house is built to current codes, there should not be a structural problem. Roofs, like the rest of the home, should be designed to withstand expected snow loads.
In Minnesota, plans showing design details to meet expected snow loads are usually required to receive a building permit. The plans for your home may be on file at your local building inspection office.
To help you understand the plans, or if you cannot find plans for your home, you may want to contact an architectural engineering firm. A professional engineer should be able to evaluate the structure of your home and answer your questions about the strength of your roof.
Natural roof ventilation
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.
Mechanical attic ventilation is NOT recommended
Mechanical attic ventilation is not a recommended solution to ice dams in Minnesota. It can create other attic moisture problems and may cause undesirable negative pressure in the home.
- New construction practices should follow or exceed the state code requirements for ceiling and roof insulation levels to prevent ice dams.
- New construction should have a continuous, 100% effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space.
- Recessed lights, skylights, complex roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.
Hire an energy professional
Weatherization 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 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.
Reviewed in 2019