What Causes Ice Dams in Ottawa Homes?

Icicles hanging from the edge of a roof are a familiar winter sight in Ottawa. While they may look picturesque, they often signal a hidden problem: ice dams. As an Ottawa home inspector, I point out issues and educate my clients to help them protect their investment in their home. Ice dams are important to understand because they can lead to roof damage, attic moisture issues, and interior leaks during our long, cold winters.

Understanding what causes ice dams—and how to prevent them—can help protect your home.

What Is Ice Damming?

An ice dam forms when snow on a roof melts, flows downward, and then refreezes at the roof overhang (eaves). Over time, this ice builds up and creates a “dam” that traps additional meltwater behind it. With nowhere else to go, the water can be forced under shingles and into the attic, ceilings, or walls.

Contrary to popular belief, snow is not melting because of sunlight. In most cases, it’s melting because heat from inside the house is escaping into the attic and warming the roof surface.

How Ice Dams Form: Step by Step

Ice damming is the result of uneven roof temperatures. Here’s how the process typically unfolds:

1. Heat loss into the attic

   Poor attic insulation or inadequate air sealing allows warm indoor air to rise into the attic.

2. Snow melts on the upper roof

   The warm roof deck melts snow higher up on the roof.

3. Water flows to colder areas

   Melted snow runs down toward the eaves, where there is no heated space below.

4. Refreezing at the eaves

   The water refreezes at the cold overhang, forming ice.

5. Ice dam grows

   Ice builds up and blocks proper drainage, trapping more water behind it.

6. Water intrusion

   Backed-up water can leak under shingles and enter the attic or interior of the home.

The root cause is the temperature difference across the roof surface.

The Role of Insulation, Air Sealing, and Ventilation

Preventing ice dams requires all three systems working together:

1. Proper Attic Insulation

Adequate insulation slows heat transfer from the living space into the attic. In Ottawa homes—especially older ones—insulation levels are often below current recommendations.

2. Effective Air Sealing

Even with good insulation, warm air can leak through gaps around light fixtures, attic hatches, plumbing penetrations, and wall tops. These air leaks can significantly warm the attic and contribute to ice dam formation.

3. Balanced Roof Ventilation

Roof ventilation helps keep the roof deck cold by allowing outside air to move through the attic. This typically includes:

• Intake ventilation at soffits

• Exhaust ventilation at ridge vents or roof vents

Proper ventilation also helps control moisture. However, if significant warm air is leaking into the attic, ventilation alone may not be enough to prevent condensation or ice dams.

Why Ice Dams Are Common in Ottawa

Ottawa’s climate is ideal for ice dam formation: prolonged cold temperatures, heavy snowfall, and frequent freeze–thaw cycles. Homes with older insulation standards or air leakage issues are especially vulnerable.

As an Ottawa home inspector, I can identify ice dam risk factors during inspections and attic evaluations—sometimes before visible damage occurs.

How an Ottawa Home Inspector Can Help

A professional home inspection can identify:

• Inadequate attic insulation

• Air leakage points into the attic

• Ventilation deficiencies

• Early signs of moisture or roof damage

Addressing these issues proactively can reduce the risk of ice dams, extend the life of your roof, and help prevent costly winter water damage.

If you’re concerned about ice dams or attic performance, consulting a knowledgeable Ottawa home inspector is a smart first step.