Ice Dams — SERVPRO of HersheySwatara

Ice Dams

2/21/2014 (Permalink)

With warming temperatures and significant build-up of ice and snow — ice damming is a significant source of water damnage.

Ice dams form when melting snow on a roof refreezes at the edge of a roof. Why does snow melt on your roof when its freezing outside? OK, fair question. Well, it melts because the underside of the roof in the attic above 32 degrees Fahrenheit enough that it warms the outside roof surface to the point that snow melts. Now heres the weird part. Ice dams start or get worse after a heavy snow because of its insulating properties. Since snow is such a good insulator (R-0.5 to R-1 per inch) the outside roof surface is able to warm up easier from the warm attic space, thereby melting the snow faster.

Warm attic spaces occur because of inadequate outside air circulation (ventilation) through the attic (soffit to roof ridge), which is necessary to keep the roof deck cold. Attic warming from poor ventilation is made worse with the introduction of heat from the occupied floor below the attic including sources such as lighting, air leaks, ductwork, etc.

As mentioned earlier, when roof snow is melted by a warm attic space, the water runs between the snow and the warm roof surface. The water then freezes and turns to ice when it gets past the exterior wall and hits a cold unheated roof edge or gutter. The ice dam grows as the snow pack continues to melt, and as water continues to flow down the roof surface. When the water flow hits the ice it creates a larger and larger ice dam.

Ice Dam Damage

If this situation continues, the ice can work its way back up the roof edge, get under shingles, melt and leak into the exterior wall, home or attic. Damage from ice dams may not be readily apparent. As the ice melts and possibly drips into the wall or attic, insulation can be become wet and lose its ability to perform. In some cases if the right temperature and humidity exist, mold may begin to grow in the attic. Often paint will peel or blister weeks or months after the ice dam has melted as moisture from the leak in the wall or ceiling cavities tries to leave and pushes outward.

The most effective long term solution is to reduce or eliminate any sources of heat in the attic and ventilate the attic space of the roof. The underside of the roof deck must be close to the temperature of the exterior side of the roof. Ideally ventilation should be installed using a continuous soffit-and-ridge vent system with baffles at the lower side of the roof. Provide at least a 2-inch space between insulation and sheathing. By providing adequate ventilation as illustrated in this diagram, the temperature of the attic will be lowered thereby lowering the underside roof deck temperature.

It is also critical to eliminate all sources of heat in the attic space. Make sure to eliminate sources of heat such as:

  • uninsulated recessed ceiling can lights installed in the floor below,
  • poor attic floor insulation (Use this DOE insulation calculator to see what insulation levels you should have for your home based upon your zip code location),
  • uninsulated folding attic stair openings,
  • heating ducts,
  • furnace or water heating equipment in the attic,
  • bathroom vent fans that improperly vent to the attic, and
  • other similar problems.


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