Deconstructing Ice Dams

Every winter, our cities and towns are transformed into picturesque winter wonderlands, with snow-covered  roofs and icicles hanging from eavestroughs, sparkling in the sun. But underneath the quaint and charming  façade is a hidden danger. Those icicles are a warning sign; a harbinger of doom; or at the very least, a  symptom of ice damming and an indicator of potential water damage.

THE RUNDOWN

An ice dam is a build-up of ice along the edge of a roof that blocks water and prevents it from draining properly – hence, icicles. Ice dams form when snow on the upper part of the roof melts, runs down the slope, and refreezes when it reaches the eaves. That constant  thaw and re-freezing process creates an ice dam. Eventually, water backs up behind the dam and can leak into the building, leading to possible structural damage and costly insurance claims. 

COMMON CAUSES

It doesn’t take much for ice dams to form. They need only three things: Snow, cold weather, and a warm roof. Occasionally, warm, sunny days and cold nights can result in freeze-thaw cycles that lead to ice damming. But most cases we see happen because of heat transfer from the interior of the home to the attic due to air leaks, poor ceiling insulation, and improper ventilation. A condition accurately, if fairly unimaginatively, called “warm roof.” Here’s how it works:

If you think about the way your home is built, the highest part of the roof (the ridgeline) is above the living space, and the outer edges of the roof (the eaves) extend past the exterior walls. Warm air rises – like smoke from a chimney – so any heat lost from the inside the home rises up through the attic and warms the areas of the roof near the ridgeline. But the eaves are cut off from the interior, so they stay at the same temperature as the outside air. That temperature imbalance allows snow to melt at the top of the roof and refreeze at the bottom.

Ideally, your house would have a perfect air barrier in the ceiling, plenty of insulation, and roof ventilation equally balanced between the ridge and the eaves. Unfortunately, reality is not always ideal. Air can leak through holes made in the ceiling for wiring, vents, chimneys, recessed lighting, attic hatches, and bathroom exhaust fans. A little air seeping into your attic is okay if the attic is properly ventilated. Your heating bill might be through the roof (pun), but otherwise, okay. The real problems start when there isn’t enough ventilation at the eaves to balance the ventilation at the ridge. In that situation, warm air is actually drawn in from the living space in order to make up the deficit. 

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ADAM LOHONYAI, Forensic Engineer, MEng, EIT

Adam specializes in structural forensic investigation and holds a Master of Engineering degree from the University of Alberta. He is currently a member in training of the Association of Professional Engineers and Geoscientists of Alberta (APEGA). Adam’s background includes work as a structural engineer providing consulting services for new building construction, personal fall arrest systems, roll-over protection systems, structural assessments, repairs, and failure investigations. He also has experience teaching engineering labs and researching building envelopes, structural health monitoring, and masonry walls at the University of Alberta.