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Brown County. When the surface of a roof changes temperature due to varying degrees of insulation, ice dams form. Source: WHS - State Historic Preservation Office.

If you own or live in a historic house or building with a sloped roof in Wisconsin, ice damming can be a major problem. Ice damming occurs when the temperature at your roof eave is significantly different than the temperature at the peak of your roof. Ice damming can cause roof leaks, rotting walls and damaged eaves.

How Ice Damming Happens

An ice dam can develop when snow melts on your roof from the warm afternoon sun and the water runs down the roof. The melted snow freezes again if the eave is colder than the upper portion of your roof creating an ice dam. The ice dam traps the water cascading down the roof and forces it back up and underneath the roof shingles, where it can leak into your wall cavities.

Prevent Ice Damming on Your Roof

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Dane County. Installing a continuous ridge vent all the way to the roof's edge creates a straight ridge line which is what the house had historically. Source: WHS - State Historic Preservation Office.

You can prevent ice damming on your roof in two key ways:

• Provide adequate ventilation in your attic. With proper ventilation from the eaves to the roof peak, the temperature of your roof will stay more constant and prevent most ice damming.  A good way to improve the airflow in your attic is to add more soffit and roof ventilation. Follow this rule: provide one square foot of net free ventilation for every 150 square foot of attic floor space, split evenly between the soffit and roof. Standard square roof vents provide about one-half square foot of net free ventilation.
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  A continuous soffit vent can be retrofitted in a wood soffit to improve your roof's ventilation. Source: WHS - State Historic Preservation Office

  The best
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  When your attic is not insulated correctly, ice dams can form which can cause damage to your ceiling, walls, siding and roof. Source: WHS - State Historic Preservation Office

  vent option is to install a low-profile, continuous ridge vent with a continuous soffit vent that runs around the entire perimeter of the eaves.
• Ensure your attic is properly insulated. If warm air rises to the peak of your roof, the eave will be much colder and will freeze the melted water running down the roof.

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Maintaining a cold roof by proper ventilation brought in low at the soffits and escaping at the roof ridge is the best way to prevent ice dams. Source: WHS - State Historic Preservation Office

One additional thing you can do is install styrofoam ventilation chutes between your roof rafters and down to the eaves in your attic. This practice will prevent loose insulation from clogging up the vents in your soffits.

For extreme cases, you can hire a roofer to install electric heating cables and an electrician to wire them. These cables can help melt ice at the eave.