Ice Dams: Causes, Prevention, and Solutions for Damaging Roof Ice Buildup

Ice damming—when snow melts on a warm area of roof and flows down until refreezing on a colder area—can ruin roofing, sheathing, and non-weather-resistant materials below. Keep this from happening to your home with this guide to preventing and eliminating ice dams.

| January/February 1990

  • Development of Ice Dam
    Fig. 1: The Birth of an ice dam. 
  • 121-106-03
    Ice buildup along the edges of a roof is more often caused by air leaking from the house than by inadequate insulation levels.
  • Map of recommended insulation levels
    Fig. 2: Recommended insulation levels for houses, by region.

  • Development of Ice Dam
  • 121-106-03
  • Map of recommended insulation levels

Glistening icicles dangling from the eaves may be among the beauties of winter, but they can also be a sign of trouble for your home. Icicle formation is often a symptom of ice damming, a serious condition that can ruin the roofing, the sheathing below, the rafters and joists that support the roof, the insulation, the paint on the exterior walls, and even the interior finish of your home.

Ice damming occurs when snow melts on a warm area of roof and flows downslope until it refreezes on a colder area near the eaves. Subsequent meltwater then pools above the icy obstruction and does its dirty work. Because most residential roofing systems are designed to shed water, not to seal it out, pooled water can leak up under shingles or around panel-roof seams, penetrating the sheathing and damaging non-weather-resistant materials below.

Major ice damming is seldom the result of natural, daily freeze-thaw cycles. It is almost always caused when heat lost from the house melts snow on the roof. Warmth leaks from the interior, passing through a poorly ventilated attic (or cathedral ceiling) to the roof (Fig. 1). At the same time, portions of the roof that extend beyond the living area—overhangs, soffits, porch roofs—remain cold and thus refreeze the melt descending from above. It's the difference in surface temperature that causes the problem. Roofs that are all cold or all warm do not get ice dams.

Find and Fix Ice Dam Causes

Now that we understand why ice dams happen, let's look at ways to solve them. There are three common methods of attack: Keep the entire roof cold by reducing heat loss from the house; keep the entire roof cold by venting heat loss before it can warm the roof; or keep the entire roof warm by heating the periphery. From an energy standpoint, the first approach obviously is the most attractive. (Roof ventilation has other arguable merits. In fact, it may be required by building code. But it is not a good sole solution to ice damming.) Therefore, we need to determine why so much heat is escaping from the house—and the most effective way to prevent it.

Insufficient or deteriorated attic insulation can lead to ice damming. If your house lacks up insulation in the roof, or if the R-value is below the guidelines shown in Fig. 2, by all means add insulation. You should also inspect existing insulation to ensure that it fits snugly between the ceiling joists and that it does not touch the roof rafters.

However, according to Gary Nelson, president of Minneapolis Blower Door Company and an experienced researcher in home weatherization, major dams are more often caused by air leakage from the house into the attic or cathedral ceiling than by poor insulation. Air currents carry heat to the underside of the roof much more readily than does radiation through the air or conduction through contiguous materials. Thus a house with a very well insulated but leaky ceiling may still have problems with ice dams.

6/6/2007 8:44:50 PM

What is your feelings on an Aspen Roof System for a shingle roof with a 5-12 pitch. The homes are in a cold region. This system adds a sublayer on sheathing above the wood decking creating a 2" air pocket keeping the top surface colder thus preventing ice damming. Any thoughts?

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