Ice Dam

An ice dam starts with an attic that is warmer than it should be. Heat leaking up from the living space warms the roof deck from underneath, melting the bottom layer of snow sitting on the upper and middle parts of the roof even while outdoor temperatures stay well below freezing.

That meltwater runs downhill under the snowpack until it reaches the eave, the part of the roof that overhangs the exterior wall and sits over unheated space rather than the warm attic. Without that attic heat underneath it, the eave stays at the outdoor temperature, and the water refreezes there into a ridge of ice.

Once that ridge forms, it acts exactly like a dam. More meltwater arrives from above, but instead of continuing down and off the roof, it pools behind the ice, with nowhere to go but up and under the shingles, since shingles are built to shed water running downhill, not to hold back water pressing against them from below.

That backed-up water can work its way past even sound shingles and underlayment, since the whole system depends on gravity carrying water downhill rather than water sitting still or pushing upward against a barrier.

The root cause is almost always uneven roof temperature rather than the snow or cold itself, which is why the fix is rarely just removing the ice after the fact. Proper attic insulation and ventilation keep the entire roof deck closer to outdoor temperature, so snow melts evenly, or not at all, instead of melting high and refreezing at the eave.

Ice and water shield installed along the eave during a roof replacement is a direct defense against ice dam leaks even when the underlying attic temperature issue has not been fully solved, giving that vulnerable edge its own waterproof barrier beneath the shingles.