Preventing Ice Dams on Homes

Understanding how ice dams form

When heat from the interior of a house with a sloped roof escapes into the attic space, it warms the underside of the roof. Meanwhile, the roof eave outside the heated space remains a colder temperature.

As snow accumulates on the rooftop, it melts over the warmer portion of the attic and the melt water runs down the roof. When it encounters the cold edge of the roof it refreezes. The refrozen water along the roof edge creates an “ice dam” and consequently, the melted snow running down the roof begins to back up underneath the roof covering. This water will soak the roof sheathing and leak into the attic unless there is a barrier above the sheathing. Sealing the roof deck is an effective way to prevent the water from entering your home and causing damage.

Preventing ice dams

A two-step approach is the most effective way to reduce the size of ice dams. First, keep the attic floor well insulated to minimize the amount of heat from within the house that rises into the attic. Second, keep the attic well ventilated so that the cold air outside can circulate through it and reduce the temperature of the roof system. The colder the attic, the less thawing and refreezing on the roof. These two measures are the best ways to keep ice dams from increasing in size.

Step One: Insulating the attic

The attic floor should be airtight, have sufficient insulation, and keep the transfer of heat from the downstairs to the attic at a minimum. Even a well-insulated attic floor may have a number of openings that can permit warm air from below to seep up into the attic. For instance, these items may cut through the attic floor:

  • exhaust pipes and plumbing vents
  • fireplace and heating system c hi m n e y s
  • light fixtures

Seal all openings around these penetrations, but be careful not to block attic vent s with insulation. The at tic vent s, as explained below, must be kept clear so that they can do their job. Additionally, pull-down stairs or a set of regular stairs leading up to the attic from the lower level can be avenues for rising heat. Weatherstripping around the edges of the attic access door and insulation on the attic side of the door should minimize the passage of heat to the attic.  Any heat-generating equipment in the attic should be relocated.

Step Two:  Ventilating the attic

There are several ways to ventilate your attic. You can do it with eave vents, soffit vents, a ridge vent, a gable vent, or some combination of these. Most modern residential roofs combine a ridge vent with soffit or eave vents. To the extent that household heat penetrates the attic, it should be able to rise and escape through, for instance, a ridge vent, while soffit or eave vents pull in cold air to replace it. Local building codes generally require a minimum level of ventilation.

Proper ventilation of the attic to let cold in, together with air sealing and insulation on the attic floor to help keep household heat out of the attic, work to minimize the likelihood of ice dams.

Removing ice dams

IBHS does not recommend chipping or breaking ice dams due to the damage that can be inflicted on the roof. If you are not physically capable of going onto the roof or are unable to easily reach the roof, consult a roofing professional.

For low slope roofs or flat roofs:

  • Removing the snow will remove the source of a potential ice dam.

  • Use a heavy duty push broom with stiff bristles to brush off the snow on low slope or flat roofs.

  • A shovel or snow blower should not be used since they may tear up the roof cover system.

For steep slope roofs:

  • Removing the snow will remove the source of a potential ice dam.

  • A roof rake may be used for most single story buildings while remaining on the ground to pull snow down the roof slope.

  • Do not pull snow back against the slope or sideways since the snow may get underneath the cover and can break shingles.