Ice Dams and Eavestroughs: What Clean Gutters Can and Can’t Prevent

What an ice dam is

Natural Resources Canada (NRCan) describes ice dams as large masses of ice that collect on the lower edge of a roof and in the gutters. They build up when snow on a warmer part of the roof melts, runs down and refreezes over the colder overhang.

Over time the ridge of ice grows. Meltwater collecting behind it can back up under the shingles and leak into the eaves, walls or ceilings below.

Why heat loss, not gutters, is the root cause

NRCan’s guidance is clear that ice dams start in the attic. Uninsulated attics usually do not have ice dams, because heat melts the snow as it lands. A well-sealed, well-insulated attic keeps the roof cold, so ice dams generally do not form either. The trouble comes in between, when some heat leaks into the attic and warms part of the roof.

That is why cleaning or replacing an eavestrough on its own will not stop a recurring ice dam. NRCan’s preferred solution for most houses is to keep house heat out of the attic by air sealing and insulating.

Where clean eavestroughs still help

Gutters are not the cause, but they are part of the winter drainage picture:

• A trough full of wet leaves holds water that freezes solid, adding weight and leaving no room for meltwater.
• Blocked outlets and downspouts cannot carry water away during a thaw, so it spills over the edge and refreezes as icicles or ice on walkways.
• Clear eavestroughs let meltwater drain on milder days, when most winter runoff happens.

Late-fall cleaning, after the leaves have dropped and before steady freezing weather, gives the system the best chance of working through freeze-thaw cycles. It will not fix attic heat loss, but it removes one source of winter ice build-up.

Where ice dams tend to form

NRCan notes that ice dams are more frequent on roofs with many valleys and dormers, or with a large overhang. It suggests looking at the roof after the first heavy frost or light snow in fall and noting where the snow melts first. Common spots include:

• a horizontal melt line on a 1½-storey house, where the short knee wall meets the ceiling
• above a bathroom or kitchen exhaust fan ducted through the roof
• around plumbing vents and skylights
• above a leaky attic access hatch

Established Mississauga neighbourhoods have plenty of additions and replacement homes with complex rooflines, so it is worth taking that look once the first frost arrives.

What not to do

• Do not chip at ice dams with an axe, pick or shovel. NRCan warns this damages the roof surface.
• Do not climb onto a snowy or icy roof. NRCan notes that removing ice dams and snow is not always easy or safe.
• Do not stand under heavy icicles or an overhanging ridge of ice, especially during a thaw.
• Do not leave a downspout outlet buried in snow or ice. The City of Mississauga suggests clearing it or using a heat cable to keep it open.

A pre-winter checklist for Mississauga homes

1. Clean eavestroughs and downspouts once most nearby trees have dropped their leaves.
2. Check that downspout extensions are attached and carry water at least 3 ft from the foundation, as the City recommends.
3. After the first frost or light snow, look for spots where the roof melts first.
4. If ice dams come back every winter, have the attic assessed for air leaks and insulation by an insulation contractor or energy advisor.
5. Keep downspout outlets and walkways clear of snow and ice through the season.

Where Diamond Shine fits

Diamond Shine handles the eavestrough side: hand-clearing debris in late fall so the system can drain through the winter. Attic air sealing and insulation are specialist work, and they are the long-term fix for ice dams.

To book a late-fall cleaning, see our gutter and eavestrough cleaning page. For drainage at ground level, read our guide to blocked downspouts.

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