If your Anchorage house grows icicles and ice dams while your neighbor’s stays clear through the same storm, the difference is the roof, not the winter: more ceiling air leaks, a valley or dormer concentrating meltwater, a cathedral ceiling, or a sunny southern exposure can put a dam on your house while the identical weather leaves the house next door alone. Ice dams are a property of the specific roof they form on.
The one-sentence physics
Ice dams aren’t caused by cold — they’re caused by a roof that’s warm in the wrong place. Heat escaping into your attic melts the snowpack from below; the meltwater runs downhill under the snow and refreezes at the cold eave, where there’s no heated room below it. Every melt cycle adds a layer. (The full mechanism, with diagrams, is on our ice dam education page.)
Which means the question isn’t “how cold was it?” — it’s “whose roof is delivering more heat to the snow, and whose roofline concentrates the water?”
The four factors that separate your house from your neighbor’s
1. What’s leaking heat into the attic. Warm air escapes upward through recessed lights, attic hatches, plumbing and wiring penetrations, and the tops of interior walls. Air leaks matter even more than insulation depth — moving air carries heat past insulation instead of through it. Two houses with identical insulation scores can leak very differently.
2. The roofline itself. Valleys, dormers, and multi-level roofs concentrate meltwater — a valley collects snow from two roof planes and funnels all of its meltwater through one path. A simple gable sheds; a complex Anchorage roofline collects.
3. Cathedral ceilings. Less room between your ceiling and your roof means less insulation and less airflow — the roof deck runs warmer, and there’s often no practical way to fix it without reconstruction.
4. Exposure. Spring sun can warm dark shingles far above air temperature — which is why even a well-built roof can grow ice on its south side while the north side stays clean. Sun-driven melt can’t be insulated away.
”So my roof was built wrong?”
Not necessarily. Ice dams are a heat-and-water problem, not a shingle problem — even correctly installed roofing sheds flowing water but can’t hold back standing water. And under a deep enough snowpack, even an airtight, heavily insulated roof deck creeps above freezing; building scientists have documented thick eave ice on roofs insulated far beyond code. The snow itself is an insulating blanket holding your roof’s heat in.
What to do with this
Because damming is a property of the roof, it’s predictable: if your house grew a dam last winter, it will likely grow one this winter — Anchorage’s own municipal guidance says as much. The fix starts with knowing your roof’s specific mechanism, which is exactly what a free on-site assessment maps: where your roof melts, where it refreezes, and what actually needs doing. Sometimes the answer is attic work, sometimes it’s heat cable keeping the drainage path open, often it’s a measured combination — and if the honest answer is “rake your roof and seal your attic hatch,” that’s what we’ll say.