Technical bulletins
February 2019

The Dangers of Ice Damming

By Carla Ladner P.Eng.,

With sub-zero winter temperatures and months of snowfall, Canadian winter weather can result in an influx of property damage claims. Specifically, in the right conditions, ice damming can be a common concern.  

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. Ice dams occur when snow on a roof melts and runs down the roof surface until it reaches a cold surface, usually a metal eaves trough or the roof overhang. The snowmelt will refreeze upon contact with the cold surface and then block future drainage. This results in the additional pooling of further snowmelt on the roof which will either freeze, causing a larger ice dam, or work its way behind the shingles, allowing moisture ingress. This process can occur repeatedly throughout the winter and can result in the accumulation of a significant quantity of ice on a given roof. Rapid temperature fluctuations (repeated freezing and warming) can exacerbate the ice damming process.

If unmonitored and unaddressed, ice damming can cause significant damage to a residential structure. This damage includes dampening, and possibly decaying, the plywood or oriented strand board (OSB) roof deck, moisture ingress into the attic space thereby affecting the insulation, and water damage to the ceiling and walls. Over time, if repeated ice damming and subsequent moisture ingress occurs, damage to the roof structure, including decay (rotting) of the structural framing of the roof and decreased fastener strength can occur. In extreme circumstances, roof failure can occur as a result of additional ice loading on a roof and/or water related deterioration of a structure.    

To prevent ice damming, it is imperative that the attic space be properly sealed with a vapour barrier and appropriate insulation to prevent warm and humid air from the conditioned living space from penetrating the attic space. It is essential that adequate ventilation of the attic space be provided and maintained to ensure that during colder temperatures, the attic space is maintained at a cold temperature, ensuring that the roof surface also remains cold. A cold roof surface will prevent snowmelt during cold exterior temperatures (which leads to ice dams)  and ensure that snow only melts when it will adequately drain to the eavestroughs and be directed away from the roof. Adequate roof ventilation is typically accomplished by using a combination of soffit ventilation at the base of the roof with gable end vents, ridge vents along the entire ridge of a roof, box vents and/or whirly birds.

Recently, a few provinces have made changes to their building code to prohibit the use of soffit vents along the eaves facing nearby houses when homes are constructed close together. These changes were instituted to reduce the spread of fire; however, it means additional venting is required, typically in the gable end and/or at the ridge. This has led to challenges in adequately ventilating residential attics as some contractors have struggled to understand new venting requirements.

Significant ventilation and insulation challenges typically arise in flat or vaulted ceilings. Rigid insulation or an increase in attic depth is often required to prevent ice damming. Attention to detailing when installing these ceilings is required and a contractor with experience in appropriate insulation and/or ventilation of these types of spaces is necessary.

In addition to insulation and ventilation issues, any influx of heat into the attic space should be avoided. Breaches in the insulation and air barrier from wall openings extending into the attic space, mechanical penetrations or uninsulated mechanical ducts can also result in a warm attic space, which would heat the roof surface.

A common misconception about ice damming is that it directly affects the shingles. While it is possible that ice works its way beneath the shingles and may cause water infiltrations or minor damage adjacent to the eaves, ice damming does not typically cause significant damage to the overall roof shingles. More often, the heat from the attic space can cause premature deterioration of the shingles or other roofing materials, the shingles are damaged from typical wear and tear, or they are damaged by the attempts to remove the snow and ice from the roof.

While removal of snow from a roof can help to prevent ice damming, extreme care must be taken when accessing a roof in the winter and it is often best to leave the snow removal to a professional. Also, improper snow removal techniques could damage the roofing materials (shingles). Removal of ice frozen to a roof surface will almost certainly damage the shingles and is not recommended.

Experts at CEP-Sintra can assist in determining specific causes of ice damming and in recommending repairs from associated damage.

If you have any questions or would like to learn more about this topic, please contact our Civil Engineering team at 877 686-0240 or info@cep-sintra.ca