Deconstructing Ice Dams

The Rundown

An ice dam is a build-up of ice along the edge of a roof that blocks water and prevents it from draining properly – hence, icicles. Ice dams form when snow on the upper part of the roof melts, runs down the slope, and refreezes when it reaches the eaves. That constant thaw and re-freezing process creates an ice dam. Eventually, water backs up behind the dam and can leak into the building, leading to possible structural damage and costly insurance claims.

Common Causes of Ice Dams

It doesn’t take much for ice dams to form on your roof. They need only three things: Snow, cold weather, and a warm roof. Occasionally, warm, sunny days and cold nights can result in freeze-thaw cycles that lead to ice damming. But most cases we see happen because of heat transfer from the interior of the home to the attic due to air leaks, poor ceiling insulation, and improper ventilation. A condition accurately, if fairly unimaginatively, called “warm roof.”

Here’s how it works: If you think about the way your home is built, the highest part of the roof (the ridgeline) is above the living space, and the outer edges of the roof (the eaves) extend past the exterior walls. Warm air rises – like smoke from a chimney – so any heat lost from the inside the home rises up through the attic and warms the areas of the roof near the ridgeline. But the eaves are cut off from the interior, so they stay at the same temperature as the outside air. That temperature imbalance allows snow to melt at the top of the roof and refreeze at the bottom.

Preventing Ice Dams

Ideally, your house would have a perfect air barrier in the ceiling, plenty of insulation, and roof ventilation equally balanced between the ridge and the eaves to prevent ice dams. Unfortunately, reality is not always ideal. Air can leak through holes made in the ceiling for wiring, vents, chimneys, recessed lighting, attic hatches, and bathroom exhaust fans. A little air seeping into your attic is okay if the attic is properly ventilated. Your heating bill might be through the roof (pun), but otherwise, okay. The real problems start when there isn’t enough ventilation at the eaves to balance the ventilation at the ridge. In that situation, warm air is actually drawn in from the living space in order to make up the deficit.

The Damage Done

Some damage caused by ice damming is easy to spot: Water-stained ceilings, dislodged roof shingles, peeling paint, and damaged plaster. But it’s the damage you don’t see that’s going to lead to bigger problems down the road. Water can leak down within the wall frame, and as a result, structural framing can decay; metal fasteners can corrode; and mold and mildew can form – which introduces a whole new host of complications. Roof leaks can also wet attic insulation, setting in motion a vicious cycle: Wet insulation is less effective, which leads to more heat loss, resulting in ice dams, leaks, and more damage to insulation. Wet wood is also an open invitation for wood-destroying insects like termites and carpenter ants. And that’s the real problem with ice damming – it can lead to more problems. Each one compounding the other.