What actually causes ice dams on an Ontario roof, and what a clean eavestrough can and cannot do

Short answer: an ice dam is caused by heat escaping through your roof. A blocked eavestrough does not start one, but it makes one bigger, heavier and harder to get rid of.

We clean gutters for a living, so it would suit us to tell you a fall cleanout prevents ice dams. It does not, not on its own. Houses with no eavestroughs at all still get them. Here is what is really going on up there, and where the trough fits in.

The three things an ice dam needs

Every ice dam needs the same three things at once.

1. Snow on the roof. A few centimetres will not do it. A thick layer will, because snow is a good insulator and it traps heat against the shingles.
2. Heat coming up from the house. Warm air leaks into the attic and warms the underside of the roof. The bottom of the snow melts, even when it is well below zero outside.
3. A cold edge. The last part of the roof hangs out past the wall of the house. There is no warm attic under it, so it stays as cold as the air.

Put those together and the meltwater runs down under the snow until it reaches the cold overhang, where it freezes. The next trickle freezes on top of that. After a week of this you have a ridge of ice along the eave, and the water arriving behind it has nowhere to go. It pools, and pooled water gets under shingles, because shingles are built to shed water running downhill and nothing else.

That is when it shows up inside: a stain where the ceiling meets an outside wall, usually in February.

So where does the heat come from?

Mostly from holes, not from thin insulation. Warm air rises and finds every gap in the ceiling below the attic:

• pot lights
• the attic hatch
• bathroom fans that vent into the attic instead of outside
• the gaps around plumbing stacks and wiring
• the top of a chimney chase

Seal those and add insulation over top, and the roof stays cold. A cold roof keeps its snow frozen until the weather melts it all at once, which is what you want.

Ventilation is the other half. Air should come in at the soffits and leave near the peak, which carries away whatever heat does get through. Soffit vents buried under insulation are a common find in older attics.

None of that is our trade. It is work for an insulation contractor or a roofer, and if you get dams every winter, that is the call to make.

What the eavestrough has to do with it

The trough sits right on the cold edge, which is exactly where the ice forms. What is in it going into winter changes how the season goes.

A trough packed with leaves holds water like a sponge. It freezes into a solid block at the first hard frost, weeks before any real snow. So the ridge of ice is already there, level with the roof edge, before the first melt arrives. The dam starts sooner and builds higher. The downspouts are plugged too, so on a mild January afternoon the melt cannot leave, and it all freezes again that night.

A clear trough with open downspouts gives that mild-afternoon water a way off the roof. On a house with a small heat leak, that can be the difference between icicles and a leak. On a house with a big heat leak, the trough will still ice up, and the dam will still form above it. It will just be smaller and it will go away faster.

That is the honest limit. A clean eavestrough is not a guarantee against ice dams. A blocked one makes a bad attic much worse. It also puts the weight of all that ice on the hangers, which is how troughs get pulled off the fascia in March.

Where in Simcoe County they are worst

Collingwood and Blue Mountain. The Georgian Bay snowbelt buries roofs here in a way Barrie does not see, so the first of the three ingredients is never missing. Chalets add a problem of their own: nobody is there midweek to notice a dam forming. We aim to have Collingwood gutters cleared by early November.

Horseshoe Valley, Sugarbush and Craighurst. The high ground in Oro-Medonte holds snow longer than the shoreline at Shanty Bay and swings through more freeze and thaw. Every swing adds a layer.

Older storey-and-a-half houses. The kind you see in Orillia's North Ward and in Allandale in Barrie, with bedrooms tucked under a sloped ceiling. There is very little room between that ceiling and the roof for insulation or moving air, so the roof runs warm. These are the houses with the big icicles.

Valleys and dormers on any house. The cover photo is the corner where a dormer meets the main slope of a metal roof, just after we cleared it. Corners like that collect the deepest snow in winter and the most debris in fall, and they feed straight into the trough below.

How to check your own house from the driveway

Wait for a morning a day or two after a good snowfall, and look at your roof next to the neighbours' roofs.

• Bare patches high on the roof with snow still along the eaves means heat is getting out. That is the pattern that builds a dam.
• Even snow from peak to edge is a cold roof. Good.
• Thick icicles along one stretch only points to a leak under that stretch, often a bathroom fan or a row of pot lights.
• Ice on the siding or coming out of the soffit means water is already getting behind the trough.

If you already have one

Three things not to do:

• Do not chip at it with a hammer, an axe or a shovel. You will break shingles and dent the trough long before you move the ice.
• Do not throw rock salt on it. It eats at aluminium and it kills whatever is planted underneath.
• Do not go up on the roof. A snowy roof over ice is the worst footing there is.

What does help is a roof rake, used from the ground. Pull the snow off the bottom metre or so of the roof after each big snowfall. Less snow there means less meltwater reaching the cold edge. If water is already coming in, call a roofer who steams ice off, and book the attic work for spring.

What to do this month

The attic is the cure, and that is someone else's job. Ours is the cheap half, and it has a deadline. The troughs need to be empty before the first hard freeze, which for most of the county means late October to mid-November. If you have gutter guards, they need checking too, because ice sits on top of a cover as well as under it. We went through that in the guards post.

A single-storey home is $150 to $200. A standard two-storey is $200 to $300, and large or complicated rooflines are $300 to $500. That covers troughs cleared by hand, every downspout flushed until it runs, and the debris bagged and taken away. We are based in Horseshoe Valley, we carry $2 million in liability insurance, and there is no rural travel charge anywhere in Simcoe County.

The gutter cleaning page has the details, or you can book online.