Ice dams in Minnesota and Wisconsin winters: causes and prevention

How an ice dam forms

An ice dam starts with heat. Warm air from the living space leaks into the attic and warms the underside of the roof. Snow on the warm part of the roof melts, even when the outside air is well below freezing, and the water runs down toward the edge.

The edge of the roof, the overhang past the outside wall, has no heated space under it, so it stays cold. When the meltwater reaches it, it refreezes. Day after day, the ice builds into a ridge, and the next round of meltwater pools behind it. Shingles are made to shed water running downhill, not to hold back standing water, so that pooled water can back up under them and leak inside.

Signs you have an ice dam problem

• A thick band of ice along the eaves or in the gutters
• Large icicles, especially with water dripping behind them
• Water stains or damp spots on ceilings or walls near outside walls
• Peeling paint or stains on the soffits and fascia
• Frost or damp insulation in the attic near the eaves

What to do if you have one now

The safest step is to remove snow from the lower part of the roof with a long-handled roof rake while standing on the ground. Less snow means less meltwater feeding the dam. Never climb onto an icy roof, and do not chip at the ice with a hammer, shovel or ice pick: it is easy to break shingles and damage gutters that way.

If water is coming inside, move belongings, catch the drips and take photos. Once the weather allows, have the roof and the attic looked at, because a leak from an ice dam can leave wet insulation and damaged roof decking that you cannot see from inside the house.

Preventing ice dams starts in the attic

Because ice dams come from heat escaping into the attic, the long-term fixes are about keeping the roof deck cold and evenly cold. Gutters and heat cables can help manage the symptoms, but they do not fix the cause.

• Air sealing: close the gaps where warm air leaks into the attic, such as around light fixtures, plumbing and wiring openings, chimneys and the attic hatch.
• Insulation: enough attic insulation, spread evenly, keeps the heat of the house below the ceiling.
• Ventilation: soffit vents at the eaves and vents at or near the ridge let cold outside air flow under the roof, which keeps the roof deck closer to the outdoor temperature.
• Bath and kitchen fans: vent them outdoors, not into the attic, where their warm, moist air adds to the problem.

The roof itself

When a roof is replaced, the details at the eaves matter. In cold climates, roofers commonly install a self-adhering ice and water barrier along the eaves and in valleys, under the shingles, so that water backed up by an ice dam has a harder time getting into the house. A replacement is also the time to check the roof deck along the edges for water damage from past winters and to improve roof ventilation.

Material matters less than the attic, but some roofs shed snow more easily than others. A metal roof lets snow slide off, which can reduce the snow load that feeds an ice dam. If you are planning a roof replacement, ask how the eaves, underlayment and ventilation will be handled.

Gutters and ice

Gutters do not cause ice dams, but clogged gutters full of leaves can freeze solid and make the problem worse at the edge of the roof. Clean gutters in the fall, make sure downspouts drain away from the house, and fix gutters that sag or pull away from the fascia. Seamless gutters and gutter guards help keep the system clear; our gutter guide has more.

If ice dams have caused leaks or damage, Omega Exteriors can inspect the roof and the gutters and explain the options. Call (651) 998-8500 or request a free estimate.