An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining. As water backs up behind the dam, it can leak through the roof and cause damage to walls, ceilings, insulation, and other areas.
Ice dams are formed by an interaction between snow cover, outside temperatures, and heat loss through the roof. Specifically, there must be snow on the roof, warm portions of the upper roof (warmer than 32° F), and cold portions of the lower roof (at freezing or below). Melted snow from the warmer areas will refreeze when it flows down to the colder portions, forming an ice dam.
Although the primary contributor to snow melting is heat loss from the building’s interior, solar radiation can also provide sufficient heat to melt snow on a roof. For example, in southern Canada, enough sunlight can be transmitted through 6 inches (150 mm) of snow cover on a clear and sunny day to cause melting at the roof’s surface even when the outside temperature is 14° F (-10° C), with an attic temperature of 23° F (-5° C).
Gutters do not cause ice dams to form, contrary to popular belief. Gutters do, however, help concentrate ice from the dam in a vulnerable area, where parts of the house can peel away under the weight of the ice and come crashing to the ground.
Ice dams are problematic because they force water to leak from the roof into the building envelope. This may lead to:
In summary, ice dams are caused by inadequate attic insulation, but homeowners can take certain preventative measures to ensure that they are rare. If you have any questions about ice dams on your home, feel free to reach out to one of our Certified Professional Inspectors.
This article originally appeared on nachi.org and is authored by Nick Gromicko. It is used with permission.