Ice damming occurs when snow on a roof thaws and once at the eave re-freezes, do to colder temperatures at the roof overhang. This can cause large icicles, a hazard in themselves, that damage gutters and the roofing material. With some types of roofing, ice damming can cause leaks, both into the wall cavity and the interior of the building. When an ice dam is formed and builds a substantial thickness, water collects on the back side of the dam and eventually flows below shingles up the slope of the roof. Without a high-quality roofing underlayment, water intrusion can occur to the interior of the building.
Prevention: ice damming is caused by heat loss from the interior of the building. The lost heat melts any snow on the roof. This condition can be reduced in three ways. Increase the amount of insulation in the attic, or below the roof surface. Ensure continuous air and vapour barriers (vapour barriers on the interior side of the insulation). Finally, provide increased and continuous attic or cavity ventilation (in strapping layers), distributed as equally as possible between the soffit and the ridge (for sloped roofs). By venting the space below the roofing, either within an attic space or between strapping layers, any escaping warm air from conditioned spaces will be cooled by outside air, reducing the chance for melting snow. With shingle applications, high-quality membrane underlayments can be used to prevent leaks, even if ice damming occurs in susceptible areas.
The best option is to prevent the issue and not battle the symptoms associated with ice damming.
Let us know if you have a roof in need of replacement or repair, or if attic insulation or ventilation upgrades are required. Plan ahead for next year and stop damning your roofs.
The opinions expressed herein are those of Allan Avis Architects Inc.. This Bulletin is circulated for the purpose of bringing architectural information to the attention of the reader. The data, commentary, opinions and conclusions, if any, are not intended to provide the reader with conclusive technical advice and the reader should not act only on the information contained in this Bulletin without seeking specific professional, engineering or architectural advice. Neither Allan Avis Architects Inc. nor any of its officers, directors, members or employees assume any responsibility for any of the information contained herein or the consequences of any interpretation, which the reader may take from such information.