Ice Dams

What causes ice dams, and how can you prevent them and the damage they can inflict?

In our cold, northern winters ice dams are a familiar sight. While dramatic rows of icicles dangling over the eaves of a house may seem an interesting seasonal feature, in reality they can damage your roof, start leaks, and endanger people (and pets) below.

Ice dams form when warm air from the house rises into the attic, warming the roof deck. When the deck temperature rises above freezing, usually near the ridge, it melts the snow where it touches the warm roof. As the snowmelt runs down the roof, it encounters the section that extends beyond house’s exterior wall, the eave, which rests over unheated space. When the eave is below freezing, the snowmelt refreezes.

As the cycle repeats, an ice dam builds up that now traps the running water so that it can no longer reach the edge. If the roof has eavestroughs, the dam usually starts there and works its way up the roof. As the ice obstacle grows, new snowmelt runs over the top and forms icicles. It can also back up under the shingles, causing interior leaks.

Diagram showing how ice dams form on your roof

Keep Eavestroughs Clear of Debris

To help snowmelt run off the roof, the eavestroughs need to be free of leaves and other debris. The slower water runs out of the gutter, the less opportunity it has to clear before it freezes.

Install Sufficient Insulation

Keeping the warm house air from rising to the attic goes a long way towards preventing ice dams. Beefing up the attic insulation will help ease the problem, but forming a complete air barrier in the attic might be impossible, especially if you have vaulted ceilings.

Add Effective Ventilation

When outside air can move up from the eaves to the higher points of the attic, it can help move the warm air out before it heats the roof deck. Soffit vents combined with a ridge vent do a great job of ventilating the attic, both in winter and summer.

Install Ice and Water Shield

Technically, an ice and water shield, installed along the eaves, won’t prevent ice dams. However, because the shield thoroughly sticks to the roof deck, it prevents any water from leaking into the house should any get backed up under loosened shingles. Adding an ice and water shield happens at the time of a re-roof.

Install Heat Cables

The areas of your roof most prone to ice damming could benefit from a run of heat cables. Attached in a zig-zag pattern along the eaves and plugged into a regular electrical outlet, heat cables keep the roof overhang warm enough to prevent ice from forming in the first place. It’s a good idea to run a cable in the gutter and downspouts at the same time.

Rake Snow

Sometimes, it’s not your attic that creates an ice damming problem. The sun can melt the snow, as well. But because the sun sets early in winter, the day’s snowmelt may not have time to run to the ground before the night time temperatures drop below freezing again.

After a heavy snow fall, the long freeze and short thaw cycle can cause an ice dam in the same way that a warm attic would. Raking as much snow off the roof as possible not only removes the ice dam potential, it relieves the weight on the roof, too, which is always a good thing.

Be careful not to damage your shingles while removing the snow! If you’re unsure then consult a professional roofing company in London.

Header photo credit: State Farm via Visual hunt / CC BY

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