Short answer
An ice dam forms when heat escaping from the house melts snow on the upper roof, and the water refreezes at the colder edge. The ridge of ice traps more meltwater, which backs up under the shingles and leaks inside. Good attic insulation and ventilation, clean eavestroughs and clearing heavy snow prevent most ice dams.
Ice dams are one of the most common causes of winter leaks in Toronto. You’ll often see them as a thick ridge of ice along the edge of the roof, usually with icicles hanging from the eavestroughs.
How an ice dam forms
- Snow builds up on the roof.
- Heat escaping from the house warms the upper part of the roof deck and melts the snow from underneath.
- The meltwater runs down to the eaves, which hang out past the warm walls and stay below freezing.
- The water refreezes at the edge and in the eavestroughs, building a ridge of ice.
- More meltwater pools behind the ridge, works its way under the shingles and leaks into the attic, walls or ceilings.
The damage they cause
- Water stains and leaks on ceilings and walls, often far from where the water got in
- Wet attic insulation, which then insulates even worse
- Eavestroughs pulled loose or bent by the weight of ice
- Warped fascia and soffits
- Rot in the roof deck if it happens year after year
How to prevent them
- Keep heat in the house. Air leaks and thin insulation in the attic are the root cause. Sealing gaps and adding insulation keeps the roof deck cold.
- Ventilate the attic. Soffit and roof vents let cold outside air flow under the roof deck so it stays close to outdoor temperature.
- Keep eavestroughs and downspouts clear. Clogged troughs hold water that freezes. Clean them every six months, or at each change of season in areas with heavy snow.
- Clear heavy snow. Removing the snow load from the lower roof takes away the water that feeds an ice dam.
If you already have one
Don’t chip at the ice with a hammer or shovel, and don’t climb an icy roof. It’s dangerous, and it’s easy to break shingles and gutters. We clear heavy snow and ice with equipment and techniques that protect the roof, then look at why the dam formed so it doesn’t come back.