Ice Dams and Eavestroughs: Why Toronto Gutters Freeze and What Actually Stops It

Every Toronto winter has a stretch where it snows, warms up to plus three for an afternoon, and drops to minus twelve that night. Do that a few times and you get a ridge of ice along the edge of the roof, icicles hanging off the eavestrough, and sometimes a brown stain spreading across a bedroom ceiling.

That ridge is an ice dam. People blame the eavestrough for it, and the eavestrough does play a part, but it's rarely the whole story.

How an ice dam forms

1. Snow sits on the roof.
2. Heat escaping from the house warms the roof deck from underneath, and the snow on the upper part of the roof starts to melt even when it's below freezing outside.
3. The meltwater runs down the roof until it reaches the edge, which hangs out past the warm walls and is as cold as the air.
4. It freezes there. Next time, the new meltwater hits that ice and freezes on top of it.
5. The ridge grows, meltwater pools behind it, and the pooled water backs up under the shingles and into the house.

The key thing: an ice dam needs a warm roof above a cold edge. That's usually a house problem, not just a gutter problem.

Where the eavestrough comes in

A trough that's clogged with leaves from the fall, or sloped wrong so water sits in it, fills with water that freezes solid. Now the ice has something to build on. The meltwater that should have run down the downspout backs up onto the roof edge instead, and the dam grows faster.

A full trough of ice is also heavy. It is common to find hangers pulled out of the fascia and whole sections sagging in the spring, because a run of eavestrough was never designed to hold a solid bar of ice.

Downspouts freeze too. The bottom elbow and the extension on the ground are the usual choke points. Once a downspout is blocked, the whole run behind it backs up.

What actually helps

Clean the troughs late in the fall. After the leaves are down, not before. In a neighbourhood full of mature maples and oaks, which is most of midtown and the east end, doing it once in early October isn't enough.

Check the slope. Troughs should fall gently toward each downspout. A trough that holds water in the middle after a rain will hold ice in the middle in January. Re-hanging a sagging run is a small repair, usually in the $300 to $900 range.

Give the downspouts a clear path. Make sure the elbow and the extension on the ground aren't sitting in a low spot where water collects and freezes. If you're in Toronto and still have downspouts going into the sewer, they have to be disconnected, and our downspout disconnection guide covers where the water should go.

Fix the heat loss. This is the real cure for recurring dams. It means air sealing around attic hatches, pot lights, bathroom fans and plumbing stacks, adding insulation, and making sure the soffit and roof vents let cold air wash under the roof deck. A cold roof doesn't melt snow from underneath, so there's nothing to refreeze.

Ice and water shield at the eaves. When a roof is replaced, a self-adhering membrane goes down along the lower edge under the shingles. If water does back up, the membrane keeps it out of the house. It's standard on a proper re-roof and one more reason not to cut corners there.

What not to do

• Don't chip at the ice with a hammer or an axe. You'll break shingles and puncture the trough.
• Don't climb a ladder onto an icy roof edge. Every winter someone gets hurt doing this.
• Don't throw rock salt on the roof. It eats aluminum, kills the plants below and stains everything.
• Heat cables can help on a problem spot, but they're a patch over the heat loss, not a fix. They also use power all winter.

If water is actively coming inside, a roofing company with a steamer can cut channels through the dam without damaging the roof. That's the safe way to remove one.

Do gutter guards help?

They help with the clogging half of the problem. Good mesh guards keep leaves out so the troughs drain in the fall, which means less standing water to freeze. They don't stop ice from forming if the roof above is warm, and snow can sit on top of them. They're worth it on a lot with big trees. They run about $1,200 to $2,500 installed on a typical house.

When it's time to replace

If your troughs sag every spring, drip at the seams, or are pulling away from the fascia, a new seamless run is often the smarter spend. For a typical detached home, full seamless aluminum replacement runs about $2,000 to $4,800, and most houses are done in a day. If you're going to replace them, it's also worth reading our post on 5-inch vs 6-inch eavestroughs.

We've been installing eavestroughs around Toronto since 1979. You can get a number for your house on the eavestrough estimate, or measure your roof from satellite with Find my Gutters.