An Ice Dam Analyzed

I am a partner in a small mechanical engineering firm in Michigan’s Upper Peninsula. We provide preconstruction design services and investigate design failures in existing buildings. In the case shown here, I was called in the fall of 2007 to investigate some large ice dams that had formed on the snow-covered roof of a new home in Toivola, Mich., the winter before. Construction of the house was completed in the fall of 2005, and the owner moved in soon after. By January 2006, substantial ice buildups were occurring along most of the roof edges, causing severe leaks inside as water from melted snow backed up behind the ice and made its way under the asphalt roofing shingles. The flooding was so bad the local building official asked the owner’s family to move out until repairs were completed or the leaks subsided. The homeowner, thinking he needed a new roof, sued the builder, who called me to get to the bottom of the problem.

I learned from the builder that the original plans for the house had called for a conventionally vented roof, with the usual ridge and soffit vents. According to the plans, there were to be insulation baffles between the 2x10 rafters to provide a vent space, with fiberglass insulation, a vapor retarder, and a cathedral ceiling below. However,...