Ice Dams and Lake-Effect Snow

Where the snow comes from

Lake-effect snow happens when cold air, usually from the northwest, crosses the open water of a Great Lake. The water is warmer than the air, so the air picks up heat and moisture, rises and drops that moisture as snow on the downwind shore. The National Weather Service describes how these bands can be narrow and intense, burying one area while another nearby stays nearly clear (National Weather Service, lake-effect snow safety). Syracuse lies southeast of Lake Ontario, squarely in the path of many of those bands.

NOAA's long-term climate normals put Syracuse's typical seasonal snowfall well above 100 inches (NOAA NCEI U.S. Climate Normals). The National Weather Service Binghamton office, which forecasts for Onondaga County, issues lake-effect snow warnings when bands are expected to set up over the area (National Weather Service Binghamton (forecast office for Onondaga County)).

How snow turns into an ice dam

Snow on its own is mostly a weight question. The trouble starts when it melts unevenly. Heat from the house leaks into the attic through ceiling light fixtures, attic hatches, pipe and wiring holes and thin insulation. That heat warms the roof deck, and the snow resting on it melts from below. The meltwater runs down the roof until it reaches the overhang, which sits outside the heated house and stays cold. There it freezes.

Each day adds a little more ice. The ridge grows, and the next melt has nowhere to go. It pools behind the ice and creeps up under the shingles. Shingles shed water running downhill well but are not built to hold back standing water, so it reaches the decking and, eventually, the walls and ceilings below.

Which roofs are most at risk

• Older homes with little attic insulation or many gaps into the attic.
• Cape Cods and story-and-a-half houses, where living space runs up into the roof and there is little room for insulation and airflow.
• Low-slope sections and valleys where snow and water collect.
• Roofs with blocked soffit vents or no ridge exhaust.

During a dam

Safety comes first. Icy roofs are dangerous, and hacking at ice with hammers or shovels breaks shingles. A long-handled roof rake used from the ground can pull snow off the lower few feet of roof, which reduces the meltwater feeding the dam. Inside, move belongings away from leaks and catch drips. The EPA's mold guidance recommends drying wet materials within a day or two where possible (U.S. EPA, mold and moisture).

Prevention that lasts

The U.S. Department of Energy's Energy Saver pages describe the goal as a cold roof deck: air seal the attic floor, add insulation, and ventilate from soffit to ridge so any heat that does escape is carried away (U.S. Department of Energy, Energy Saver). On the roof itself, a self-adhering ice and water barrier at the eaves and in valleys is the backstop when a dam does form. A replacement is the easiest time to extend that barrier and fix the vents. See ice dams and snow damage, roof replacement and what shapes ice dam work.

Related pages: ice dams & snow damage, roof leak detection & repair, roof inspections, and what shapes an estimate.

Sources: National Weather Service, lake-effect snow safety; National Weather Service Binghamton (forecast office for Onondaga County); NOAA NCEI U.S. Climate Normals; U.S. Department of Energy, Energy Saver; U.S. EPA, mold and moisture.