Detailed Answer
Ice dams form when heat escaping through your attic melts snow on the upper roof, and the meltwater refreezes at the colder eaves. Prevention targets the root cause: heat loss.
Long-term prevention (fix the cause)
Attic insulation
Bring insulation to R-49 or higher (current code for cold climates). This prevents indoor heat from reaching the roof deck. Cost: $1,500-$3,500 for a typical attic.
Attic air sealing
Seal gaps around light fixtures, plumbing penetrations, attic hatches, ductwork, and wire chases. Even small gaps let warm air rise to the roof. Cost: $500-$1,500.
Attic ventilation
Install balanced intake (soffit vents) and exhaust (ridge vents) to flush any residual warm air. The target is 1 sq ft of net free ventilation per 150 sq ft of attic floor (or 1:300 with balanced intake/exhaust). Cost: $300-$800.
Ice-and-water shield membrane
Install self-adhering membrane extending 3 feet past the interior wall line along all eaves. This does not prevent ice dams but prevents leaks when dams form. Installed during re-roofing: $500-$1,500.
Short-term solutions (manage symptoms)
- Roof raking: Pull snow off eaves with a roof rake after heavy snowfall, keeping the first 3-4 feet of eave clear. Cost: $30-$60 for the tool.
- Calcium chloride socks: Fill pantyhose with calcium chloride and lay perpendicular across the dam to create drainage channels. Works in emergencies.
- Heat cables: Zigzag heating cables along eaves and gutters. Effective but expensive to operate ($50-$150/month) and address symptoms, not causes.
What NOT to do
- Never chop ice dams with an axe or hammer (damages shingles)
- Avoid pressure washing ice (creates more damage)
- Do not use rock salt (corrodes metal and kills plants)