Ice Dams Start
In Your Attic

A roof that is warm at the top and cold at the eaves. University of Minnesota Extension puts it precisely: for an ice dam to form there must be snow on the roof, and the higher parts of the roof surface must be above 32 degrees while the lower parts are below. Snow melts on the warm upper roof, runs down, and refreezes at the cold eave. The heat warming the upper roof came out of your house.

Because your attic is warmer. The Minnesota Extension notes that in many homes air leakage from the house into the attic is the major way heat reaches the roof. Top plates, recessed lights, the attic hatch, and bath fans venting into the attic all deliver warm house air straight to the underside of the roof deck. Your neighbor’s ceiling is tighter, or the attic is insulated to depth, or both.

They melt channels through the dam so water has a path off the roof. They do not stop the dam forming, they use electricity all winter, and they do nothing about the heat leaving your house. We treat them as a stopgap for a roof that cannot be fixed from inside, not as a solution.

The same fix the Extension recommends: make the ceiling airtight first so warm house air cannot reach the attic, then add insulation to cut the heat that conducts through, and keep the attic ventilated so what heat does get up there leaves. A roof that stays uniformly cold cannot grow a dam.

Pulling snow off the lower roof removes the raw material, and it can prevent damage in a bad storm. It is also cold, risky work every snowfall. It is what you do while you wait to fix the attic, not instead of fixing it.

Yes, and the sun is part of why. A warm afternoon on a snow-covered roof melts the upper roof, and the eaves are still in shade and below freezing by late afternoon. Add a leaky ceiling and the pattern repeats every clear day after a storm. Homes in Fort Collins, Longmont, Boulder, and up in Estes Park see it every winter.

Water backed up behind the dam finds its way under the shingles. The Extension describes it flowing into the attic, where it can run into exterior walls or through the ceiling insulation. That means wet insulation, stained ceilings, peeling paint, and in time, mold and rotted sheathing.

A blower door test depressurizes the house and a thermal camera shows every warm plume rising into the attic, in minutes. On a snowy day the roof itself is the map: bare streaks over a snow-covered roof mark the leaks underneath. We photograph both for you.

Those are the hardest cases because there is no open attic to seal from. Knee walls, slopes, and the flat ceiling section each need their own air barrier and insulation, and sometimes spray foam at the roof deck is the only way to make the roof uniformly cold. We scope it before we promise anything.

Usually. Air sealing and an attic top-up are a one- or two-day job in most homes, and fall is the right time: the attic is cool enough to work in and the fix is in place before the first storm.