What Causes Ice Dams
As heat rises from a home, it melts the accumulated snow on the roof. That melted snow travels down the roof in liquid form until it reaches the eave line and gutter, where it refreezes due to colder temps. This ice ridge continues to expand, blocking the flow of subsequent snow melt.
As water continues to melt higher up the roof, it collects behind the ice dam in the form of a puddle. Because that water sits over the warmer portion of the roof, it doesn’t freeze.
In order for ice dams to form, there needs to be roof snow buildup, home heat loss, and subfreezing temperatures. The more snow, the larger the heat loss, and the longer the subfreezing temperatures remain, the higher the likelihood that ice dams will materialize.
How to Prevent Ice Dams
Homeowners can’t control the weather, but they can do something about heat loss. Proper insulation and ventilation of the attic space is intended to keep the roof surface at or near outdoor temperatures.
Typical steps include insulating the attic floor and installing soffit, gable and/or ridge vents to expel heat. Folding attic stairways and recessed light fixtures also need to be insulated. All penetrations into the attic from the heated living space need to be addressed.
To facilitate sufficient cold air flow from soffit vent to ridge vent, baffles or tubes are installed between the ceiling insulation and the underside of the roof. This might involve opening up the ceiling.