Ice Dams: Why Northern Illinois Roofs Leak in February

How an ice dam actually forms

Heat escaping from a heated attic warms the upper roof deck enough to melt the snow sitting on it, even when outdoor air stays below freezing. That meltwater runs down the roof until it reaches the eaves — the unheated overhang beyond the exterior wall — where it refreezes, since the eaves aren't warmed by attic heat the way the roof over the living space is. Each melt-refreeze cycle adds another layer of ice at the eave, building a dam that gets thicker as the winter goes on.

Why this hits in February specifically

Ice dams need repeated freeze-thaw cycling, not just cold. Early winter snow often melts off before enough of it accumulates to matter. By February, northern Illinois has usually built up a season's worth of snowpack, and the accumulated freeze-thaw cycling has had time to compound — which is why the worst ice dam damage tends to show up in the back half of winter rather than at the first snowfall.

Why the dam causes a leak, not just an icicle

Once ice builds up at the eave, meltwater from higher on the roof has nowhere to drain — it pools behind the dam and finds its way under the shingles instead of over them, since shingles are designed to shed water running downhill, not water sitting still or pushing uphill under pressure. That water can travel under the shingle layer and into the attic or ceiling before it ever shows up as a visible drip inside the house.

Ice itself adds damage beyond the leak

Ice is roughly three times heavier than snow per cubic foot, so a substantial ice dam adds real structural load to the eave. And where water gets into small cracks in shingles or flashing, it expands by roughly 9% as it freezes — widening those cracks a little more with every cycle, independent of whatever water damage the dam itself causes.

What actually prevents ice dams

Removing ice after it forms treats the symptom. The lasting fix is reducing how much heat reaches the roof deck in the first place:

• Attic insulation — enough of it, and no gaps, so less heat escapes upward from the living space
• Air sealing — gaps around recessed lights, plumbing stacks, and attic hatches leak warm, moist air into the attic far more than people expect
• Balanced ventilation — soffit intake and ridge exhaust working together keeps the attic closer to outdoor temperature, so the roof deck doesn't get warm enough to melt snow unevenly

Ice-and-water shield at the eaves and valleys, installed during a roof replacement, is the backup layer that keeps a dam from causing an active leak even when conditions still produce one — see Roof Replacement for how that's installed.

Signs worth checking now

Consistent icicles at the same section of eave every winter, or a ceiling stain that only appears in late winter, both point toward an ice dam pattern rather than a one-time event. Worth an inspection before the next hard freeze-thaw stretch.