Had serious ice dam problems for 3 winters on my 1972 Toronto home before actually solving it. Posting what worked and what did not.
What Did NOT Work
Roof rake: helped temporarily but exhausting and you miss spots. Did not address the cause.
Heat cable on eaves: kept the eaves clear but the water still backed up in the middle sections of the roof where no cable ran. Also $40-$60/month in electricity during ice dam season.
Ice melt socks: temporary drainage channel only. Water damage continued.
The Actual Fix: Attic Air Sealing + Insulation
Had a contractor do a thorough attic air sealing job first:
- Foam-sealed all 14 recessed light housings (these are enormous heat leaks)
- Sealed plumbing stack penetrations
- Sealed attic hatch (was a bare plywood door with zero insulation)
- Sealed top plates at partition walls
Then blown cellulose from R-16 existing to R-58.
Total cost: $3,100 including all materials and labour.
Results
Zero ice dams in the two winters since. Before, I had massive dams on three exposures every year.
Why It Works
Ice dams form because heat escaping from below warms the roof deck unevenly. The upper roof is warm (above living space) so snow melts. The eaves are cold (above the soffit, no heat below). Water hits the cold zone and freezes. Air seal + insulate the attic = keep the roof deck uniformly cold = no differential melt = no dam.
Key Metric
If you look at your roof on a cold day and see a clear line where snow has melted off vs not melted, you have confirmed the heat leak pattern. The melt line shows you where the insulated ceiling ends and the cold soffit begins.