Ice Dams and why roofs leak in freezing weather

By TIM MULROY

It sounds backwards, doesn’t it? How can your roof leak when it’s well below freezing outside?

Yet some of the worst roof leaks happen during deep winter cold snaps.

The culprit is usually something called an ice dam — and if you lived through the winter of 1999 in Toronto, you know exactly how serious that can be. That was the storm when the military was called in to help clear snow. Insurance claims in Southwestern Ontario topped $50 million — and a large portion of that damage was related to ice dams.

So what’s going on?

What Exactly Is an Ice Dam?

An ice dam is a thick ridge of ice that forms along the lower edge of your roof — usually at the eaves or in the gutters. You’ll sometimes see them in roof valleys or along complicated rooflines too. They look dramatic. But the real damage is what you can’t see.

How Do Ice Dams Form?

Ice dams usually form when three things happen at the same time: There’s a good amount of snow sitting on your roof. The outdoor temperature is below freezing. Your attic is warmer than it should be.

Here’s the chain reaction: Heat escapes from your house into the attic. That warmth melts the snow sitting on your roof. The melted water runs down the roof — until it hits the cold overhang at the edge. There, it refreezes.

Over time, that refrozen water builds up into a wall of ice. That wall blocks the next round of melting snow from draining off the roof.

And water that can’t drain? It backs up. Eventually, it finds its way under shingles and into your home. That’s your winter leak.

The Real Issue Isn’t the Ice

This is important: Ice dams are a symptom — not the root problem. The real issue is heat escaping into the attic.

Building science comes down to three things: controlling heat, air, and moisture.

If your attic stays cold, your roof stays cold. If your roof stays cold, snow doesn’t melt. If snow doesn’t melt, ice dams don’t form. That’s the goal.

“I Added Insulation — Isn’t That Enough?”

Adding insulation is a great first step. Attics are usually easy to access, and topping up insulation can help.

But insulation alone often isn’t the full solution. Heat doesn’t just rise — it moves toward cold areas in any direction. In winter, your house acts like a chimney. Warm air enters at the lower levels and escapes at the top.

That escaping warm air carries a lot of heat into the attic.

Common air leak areas include: Pot lights, bathroom fans, plumbing stacks, chimney gaps, attic hatches, electrical penetrations, ductwork, gaps along framing.

Air sealing — systematically finding and sealing those gaps — is often just as important as adding insulation. Professional air sealing companies use blower door tests to depressurize your house and pinpoint exactly where air is leaking.

What Can a Roofer Do?

Roofers can install ice and water shield — a waterproof membrane installed along the lower edge of the roof and in valleys. It’s good protection. But here’s the truth: it treats the symptom, not the cause.

If your attic is warm, ice dams can still form. The membrane simply gives you a second line of defence. Proper installation should extend the membrane well past the inside of the exterior wall and include protection around valleys and roof penetrations.

What About Ventilation?

Many people assume ventilation solves everything. It helps — but it has limits.

Ventilation depends heavily on wind and proper airflow from soffits to roof vents. Snow-covered vents, nearby buildings, and trees can reduce effectiveness.

In winter, ventilation mainly removes moisture from the attic to prevent condensation and mould.

In summer, it helps reduce extreme attic heat (which can reach 160°F), protecting shingles and lowering cooling costs. But ventilation alone won’t fix a major heat loss problem.

Are Heating Cables Worth the Cost?

Heating cables can reduce ice buildup, and newer systems have sensors that automatically turn them on and off. They can work — but they require monitoring and are more of a management tool than a cure.

What Do You Do If You Already Have a Leak?

If an ice dam is actively causing water to enter the home, the only immediate solution is to remove the ice so water can drain.

There’s no delicate way to do that. It’s often a chip-and-clear process, and there’s some risk involved. Which is why prevention is far better than emergency removal.

The Bottom Line

If your home experiences repeated winter leaks, it’s worth looking beyond the roof itself. Think attic insulation. Think air sealing. Think heat control.

Ice dams aren’t random. They’re a sign that your house is losing heat. And once you control the heat, you control the problem.

– Tim Mulroy is the President of Toronto Roofing Industries Ltd.