How Ice Dams Form in Ontario (ROOFNOW™ 2025 Guide)

Ice dams are one of the most damaging winter roofing problems for Ontario homeowners. They form when snow melts on the upper part of the roof, flows downward, and refreezes at the colder eaves. This buildup of ice prevents additional meltwater from draining properly, forcing water backward under shingles and into the home.

What Is an Ice Dam?

An ice dam is a thick ridge of ice that develops along the edge of a roof. It forms when warm indoor air escapes into the attic, heating the roof deck from below and melting the snow above it. As the meltwater runs down the roof, it reaches the colder overhang where temperatures are below freezing. The water refreezes, creating a growing barrier of ice.

Ice dams occur most frequently in Ontario because of our freeze-thaw cycles, heavy snowfalls, and inconsistent insulation levels in older homes. They are not caused by roofing materials — asphalt and metal roofs can both experience ice dams if attic temperatures are uneven.

Why Ice Dams Form in Ontario

• Warm indoor air leaking into the attic
• Insufficient or blocked attic ventilation
• Poor insulation allowing heat to escape upward
• Heavy, wet snow on the roof surface
• Rapid freeze-thaw temperature fluctuations

When the attic warms the upper part of the roof but the eaves remain freezing cold, the meltwater is forced to refreeze. Over time, this creates a dam that traps additional water behind it. The pooled water can penetrate under underlayment, shingles, and into the wood roof deck.

How Ice Dams Damage Ontario Homes

Ice dams create multiple structural and moisture-related issues:

• Water leaking into ceilings and walls
• Mold growth inside the attic insulation
• Saturated and damaged drywall
• Rotting roof decking
• Weakened or sagging rafters
• Shingle blistering, cracking, and premature failure

Even new shingles cannot stop water from backing up underneath them. Once moisture infiltrates the roof deck, it accelerates aging and can lead to long-term structural deterioration.

Solutions That Actually Work

• Increase attic insulation to reduce heat escaping upward
• Install continuous soffit and ridge vents for proper airflow
• Seal attic bypasses around lights, fans, and ductwork
• Reduce heavy snow loads using safe removal methods
• Ensure ice and water underlayment is installed in high-risk areas

The key to preventing ice dams is maintaining a cold, consistent attic temperature. Balanced ventilation and proper insulation stop warm indoor air from leaking upward and creating the melt-refreeze cycle that forms ice dams.

Learn more at www.roofnow.ca

Expert Notes (ROOFNOW™)

Ice dams are common in Ontario but entirely preventable with proper attic airflow and insulation. Metal and asphalt roofs alike depend on balanced ventilation to avoid winter moisture issues. When homeowners address heat loss, airflow, and insulation together, the risk of ice damming drops significantly. Understanding this winter behaviour is essential for long-term roof durability.

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ROOFNOW™ provides Ontario homeowners with technical, engineering-based roofing knowledge covering attic airflow, soffit performance, winter moisture behaviour, and long-term roof durability. Explore more at the ROOFNOW™ Knowledge Center, www.roofnowontario.com, or visit the ROOFNOW™ main website at www.roofnow.ca.

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