An ice dam forms when heat escaping from the attic melts the underside of the snow layer on the roof. That meltwater runs down to the unheated eaves and refreezes into a ridge of ice right where the roof overhangs the wall, and water then backs up behind that ridge and works its way under the shingles instead of running off the roof the way it is supposed to.
Why it is an attic problem, not a roof problem
A roof that is uniformly cold all the way to the eaves does not build ice dams, because the snow sitting on it stays frozen and simply blows off or melts evenly whenever the sun hits it. Dams form specifically where warm air is leaking up from the living space into the attic and heating the roof deck unevenly from underneath, usually around poorly sealed attic hatches, recessed pot lights, bathroom exhaust fans that vent into the attic instead of straight outside, and gaps around plumbing stacks that were never properly sealed when the house was built. The pattern is almost always the same: warm patches on the upper roof melt snow, that water runs down over the still-cold lower roof and eaves, and freezes exactly where the heat stops reaching it.
What the water damage actually looks like
Once meltwater backs up behind an ice dam, it can travel sideways and upward under the shingles far enough to reach the roof deck itself, and from there it drips into the attic insulation and eventually down into ceiling drywall below. A brown stain that appears on an upstairs ceiling partway through winter, especially near an exterior wall or a corner, is very often ice damming rather than a leaking roof in the way most people assume the first time they see it. Peeling paint on an exterior soffit, or icicles that seem unusually thick and heavy compared to a neighbour's roof, are both early warning signs worth acting on before the thaw finds its way inside.
Short-term relief once a dam has formed
None of this fixes the cause, but it buys time and limits water damage during a cold snap.
- Use a roof rake from the ground to pull the first metre or so of snow off the lower roof slope, which reduces how much meltwater is available to back up behind the dam.
- Work along the eaves rather than pulling from directly underneath, and stop if the rake will not reach without you stepping onto snow or ice.
- Fill an old sock or stocking with calcium chloride ice melt and lay it across the dam, perpendicular to the eave, to cut a channel through the ice.
- Repeat after any fresh snowfall, since this manages the symptom for that specific storm rather than fixing the underlying cause.
The actual fix is upstream
Stopping ice dams from forming at all means addressing the attic, not the roof surface itself.
- Seal air leaks between the living space and the attic, particularly around the attic hatch, pot lights, and plumbing and wiring penetrations.
- Add attic insulation to reduce how much heat escapes upward through the ceiling in the first place.
- Confirm bathroom and kitchen exhaust fans vent all the way outside through the roof or wall, not into the attic space itself.
- Clear gutters before winter, since a gutter already packed with leaves gives meltwater nowhere to go even before it starts to freeze.
If ice dams keep forming in the same spot every winter, our roofing and gutter repair service in Kitchener, Waterloo and Cambridge can check the flashing and eaves for the damage a repeat dam usually leaves behind.