Ice Dams: Recognition and Prevention

How Ice Dams Form

Ice dams are caused by the interaction of three factors:

• Ice and snow melt at 35°F. Liquid water freezes at 32°F. Minor temperature differentials can lead to major problems.
• Layered roof systems such as shakes or shingles do not keep out standing water. They require a continuous, uninterrupted slope to shed water.
• Attics are warmer than the outside air because heat leaks from the heated portion of the structure up into the attic. Heavy snow cover effectively insulates the attic from cold outside air, allowing the temperature to rise even higher.

Eventually, the temperature of the roof under the snowpack,usually near the ridge, rises above the melting point. Meltwater runs downthe roof, refreezing when it reaches the edge of the insulating snow blanket atthe eaves. They are still far below the freezing point since they are unheatedby air from the structure interior. An ice dam forms. As layer after layer ofmeltwater refreezes, the ice dam grows, with liquid water pooling behind itunder the snow. Soon, this water is deep enough to seep between shingles andinto the attic or wall cavities. Ice dams are sometimes one to two feet thick.Secondary ice dams often form around vents and skylights.

Prevention: The Cold Roof

The best protection against ice dams is a properly designed"cold roof." Minimizing heat gain in the attic while maximizing atticventilation with outside air is the best way to reach this goal.

Attics gain heat in two main ways: conduction due toinadequate insulation; and convection caused by warm air leaking through gaps,usually around plumbing, wiring, ducts and vents.

Reduce conductive heat gain by increasing the insulationlevels. Convective heat gain can be minimized by meticulously caulking andsealing even the smallest penetrations through the ceiling, as well ascarefully gasketing around attic entrances. Although some heat gain isinevitable, when the attic ventilation is adequate the temperature will notreach levels that cause extensive melting. A cold roof is easy to achieve withproperly designed new construction. Reaching the same goal in older homes maybe both difficult and expensive.

Other preventive methods have been tried:

Correction

Once an ice dam forms, the only relief is to remove the iceand snow from the roof. This extremely dangerous work should be left toqualified professionals with the right tools and insurance. Improperly done,the roof can be severely damaged.

Damage: Obvious & Hidden

Ice dams cause millions of dollars of damage every year.Obvious, visible damage may include:

• sagging, ice-filled gutters
• delaminated or rotted roof decking
• stained, sagging wall or ceiling drywall
• damaged and/or dislodged shingles or shakes
• wet insulation (both temporary and permanent problems)

This damage is usually repaired when weather or fundspermit.

Unfortunately, obvious damage may be just the tip of theiceberg. The worst effects of ice dams are often hidden, caused by moisturetrapped inside walls or floors. This damage is seldom discovered until monthsafter all the snow has melted. The insured seldom makes the connection betweenthe damage found and their ice dam of the previous winter. Besides the cost ofrestoration, hidden damage can make future ice damming more severe, waste energy,and even create serious health risks for building occupants.

Hidden damage is entirely preventable. With properinstruments, equipment and training, trapped moisture in building materials canbe detected and removed before it causes severe damage. An upcoming issue ofThe Leader will further discuss hidden damage from ice dams and how to dealwith it.