Ice Dams in Ontario: Causes and How to Prevent Them
Those thick icicles hanging from the roof edge look like winter charm. They are actually a warning. Behind them sits an ice dam, and an ice dam can push water up under your shingles and into your home.
Ice dams are one of the most common winter roof problems in Ontario. Here is what causes them, the damage they do, and how to stop them before they leak.
What Causes Ice Dams
An ice dam forms from a simple cycle. Heat escapes from your home into the attic and warms the roof deck. Snow on the warm upper roof melts and runs down toward the cold eaves, which hang past the heated part of the house. At the cold edge the water refreezes into a ridge of ice. That ridge is the dam. More meltwater backs up behind it, and with nowhere to drain, it works its way under the shingles.
The Real Culprit: Attic Heat
Ice dams are not really a weather problem. They are an insulation and ventilation problem. A properly insulated and vented attic keeps the roof deck cold, so the snow does not melt unevenly. Poor insulation and blocked soffit vents are what start the cycle. This is why your soffits matter, covered in our guide on fascia vs soffit.
The Damage Ice Dams Cause
- Water forced under shingles, leaking into the attic and ceilings.
- Rotted roof decking and framing from repeated moisture.
- Soaked insulation that loses its value.
- Mold growth in the attic and walls.
- Damaged eavestroughs and fascia from the weight of ice.
- Stained or sagging ceilings inside.
The leaks ice dams cause are a major winter source of roof damage. See our guide on roof leak repair for what to do if water gets in.
How to Prevent Ice Dams
Improve Attic Insulation and Ventilation
This is the real fix. Good insulation keeps household heat out of the attic, and proper soffit and roof ventilation keeps the deck cold and even. Together they stop the melt and refreeze cycle at its source.
Keep Eavestroughs Clear
Clogged gutters trap water and ice at the edge, which feeds the dam. Clean them before winter. See our guide on how often to clean eavestroughs.
Install Ice and Water Shield
During a roof replacement, a membrane called ice and water shield goes along the eaves under the shingles. It is a waterproof barrier that protects the roof edge even if a dam forms. It is standard on quality roofs in Ontario and worth confirming in any roofing quote. See our guide on roof replacement cost.
What Not to Do
Do not chip at an ice dam with an axe or hammer, which damages the shingles underneath. Do not use rock salt directly on the roof, which can stain and harm the shingles and plants below. If a dam has formed and is leaking, call a roofer who can clear it safely.
Ice Dam FAQ
What causes ice dams on a roof?
Heat escaping into the attic melts snow on the upper roof. The water runs to the cold eaves and refreezes into a ridge of ice, which backs water up under the shingles.
Are ice dams a sign of a bad roof?
Usually they signal poor attic insulation or ventilation rather than a bad roof. Fixing the attic addresses the root cause of the dams.
How do I prevent ice dams?
Improve attic insulation and ventilation, keep eavestroughs clear, and have ice and water shield installed along the eaves during a roof replacement.
Can ice dams cause roof leaks?
Yes. Water trapped behind a dam works under the shingles and leaks into the attic and ceilings, which can rot framing and feed mold.
Should I remove an ice dam myself?
Avoid chipping at it or using rock salt, which damages shingles. If a dam is leaking, call a roofer to clear it safely.
The Bottom Line
Ice dams come from attic heat, not just cold weather. They force water under your shingles and cause leaks, rot, and mold. Prevent them with attic insulation and ventilation, clear eavestroughs, and ice and water shield. Never chip at a dam yourself.
Worried about ice dams this winter?
Peace of Mind Exteriors addresses ice dams, attic ventilation, and roof edge protection across Oakville and the Golden Horseshoe. Call (905) 484-7785 or request a free quote.