Which Month Ice Dams Form, City by City
The peak month
For each city at risk, the model counts dam days month by month. January is the worst month in 1,401 cities (83%); February is the worst month in 255 cities (15%); December is the worst month in 41 cities (2%). January leads because it combines the deepest snowpack with the longest stretch of days that never thaw.
Where February is worse
Where snow builds slowly through December and January, the pack is deepest in February, and that is when dams peak. The highest of these are Stevens Point, WI, Wisconsin Rapids, WI, Plover, WI. In those towns, the first thaw after a long February cold spell is the classic time for a ceiling stain.
March is when they leak
In 233 cities, March still averages three or more dam days. Late-season dams are the ones that leak most often: the ice ridge built up over the winter is at its largest, and longer, sunnier days send more meltwater behind it. Keep the bottom of the roof raked until the snow is gone, not just through the cold months. See when a roof rake is worth it, and air sealing or heat cable for the fix that cuts those late dams at the source.