Those Darn Ice Dams


Published by Richard Scott, AIA

You’re dreaming by the fire, as the ice dams conspire. Water drips on your head, and looking up with dread, you are faced I’m afraid, with a roof poorly made - so much for that winter wonderland.  

Ice dams are a well-known and hated condition, often encountered in cold-country ski resorts and hotels. Forming at the colder edges of roofs (especially at overhangs), they act as dams to trap liquid meltwater traveling down from further up the roof slope (Figure 1). This water flows from melting snow and ice originating at the relatively warmer upslope sections of the roof over the heated occupied spaces. As the meltwater flows over the roof edges, it forms icicles that damage roof elements and gutters (Figure 2) and, in many cases, grow to lethal lengths that endanger people below (Figure 3).


Figure 1: An ice dam near the roof edge is holding back meltwater which can make its way under the roof shingles and into the building.


Figure 2: Ice dams and icicles can cause damage to roof construction. They can be particularly difficult to repair and prevent in older historic structures.

Figure 3: Lethal icicles form at the edge of a ski resort roof.


Once the ponded water behind the edge ice dam gets high enough, it travels below the roof shingles and past waterproofing underneath the shingles, finding its way through sheathing gaps and fastener holes until gravity eventually delivers it to ceilings, guests, and furnishings. To add to the building owner’s misery, the actual entry point is not always where the water shows up, causing a maddening and sometimes fruitless search for that elusive entry gap or hole.

Another ice problem that can be even more insidious is the formation of ice within the cold spaces of a building, such as within soffits and attics (Figure 4). Because the ice is hidden, it usually goes undetected as it melts, causing damage within the structure that often isn’t detected until it becomes severe. While ice formation in these spaces can be caused by roof leaks, it can also be caused by warm interior air exfiltrating from the occupied interiors to the colder interstitial spaces (Figure 5). The moisture in the warm air condenses on the colder surfaces, flash freezes, and then builds up as icicles and ice.


Figure 4: Ice formation within a roof soffit

Figure 5: Ice formation within a cold roof soffit due to exfiltration of warm, humid interior air


Prevent the Buildup of Ice Dams

There are several ways to reduce the likelihood of ice dams on top of the roof’s surface. One solution is to install a cold roof system. A cold roof, as the name implies, is a roof surface that is kept cold enough to discourage snowmelt. The roof surface and sheathing are isolated from the interior spaces by insulation as well as by a vented space between the insulation and the underside of the roof. This vented space allows cold outside air to flow along the underside of the sheathing to keep it below freezing temperatures. While this may sound like a typical attic construction, important differences exist in the design, engineering, and installation of continuous insulation and the ventilation system. For example, the ventilation system is more than just installing vents at the soffits and ridge; a proper draw of cold air from the eaves to the ridge is required.  

Snow retention should be designed into a cold roof to add insulation value, prevent uneven buildup, and allow the snow to melt gradually and naturally (Figures 6 and 7). The roof structure also needs to be designed to properly handle the weight of the snow.


Figure 6: Snow fences are helping to retain roof snow, but ice dams are still forming at the roof edges and gutters.

Figure 7: Roof snow retention failure


Another way to prevent ice dams is to install an ice melt system at the roof edges. It’s important that the ice melt systems chosen for extreme locations are robust enough for the amount of snow and ice that occurs in the region; most are not. Liberty Building Forensics Group (Liberty) was called upon to assist clients with ice dam problems at two ski resorts in California and New England. At the California resort, the original electric heat cables installed on the roof and gutters were only powerful enough to melt the snow and ice up to a couple of inches around the cables. Workers were constantly being sent up to the roof edges of the six-story buildings to chop the ice - not a good scenario for either the shingles or for the safety of the workers (Figure 8). The solution we recommended was to retrofit the roof with a proprietary system that included a heated metal panel surface along the roof edge. This configuration provided a better, more even distribution of heat than the heat cables had, and also presented a slick surface that discouraged any sticking of snow and ice. As newer buildings began being constructed at the ski resort, contractors started to use cold roofs along with ice melt systems to solve the ice dam problems.

Regardless of the type of ice dam control measures that are installed, it’s always recommended to add a waterproofing membrane, such as self-adhered sheet membrane (SASM, aka “peel-and-stick”) along the edges of the roof below the shingles and underlayment. This membrane should extend up the slope of the roof past the expected point where water may pond behind ice dams. In cold-country climates, a high-performance underlayment should also be considered (Figure 9).

Figure 8: Workers are chopping ice dams that formed on a ski resort roof six stories up. In addition to safety concerns, this approach can potentially damage the shingles and the underperforming heat cable system.

Figure 9: This is a cold roof under construction with a snow fence, SASM near edges, and high-performance underlayment. The roof will then be shingled. Special metal flashing was designed to minimize water intrusion at the snow fence supports where they are bolted to the structure.

Prevent Ice Formation in the Interstitial Spaces

The solution to preventing ice formation within cold interstitial spaces is to prevent exfiltration of interior air into these spaces. Two approaches are generally required to accomplish this. The first is the installation of a proper air barrier. The typical vapor retarder installed in most buildings is not sufficient in this case. Instead, the air barrier system must meet specific industry standards and must be as close to a perfect barrier as possible, with minimal openings.

Additionally, in cold climates, the building needs to be kept under slight negative pressure relative to the outside air. The intent with this approach is to draw cold, dry outdoor air into the cold spaces rather than pushing warm, humid inside air into them. Negative pressure control is a function of the design, installation, and operation of the HVAC systems.

It is critical to have both a good air barrier and negative pressure to achieve the appropriate conditions needed to prevent ice formation. When advising clients (especially those engaging in new construction), we look at both their building envelope and their HVAC system, as well as at the interaction between the two, to ensure that the whole building meets its pressurization requirements. Depending on climate and other factors, it is often possible to retrofit existing buildings to control ice problems caused by poor air barriers within the HVAC system. Liberty once helped a client solve a condensation and ice condition in a Midwest medical facility by regaining proper control of the HVAC system (Figure 10).


Figure 10: Ice and frost formation is present in the roof cavity of a Midwest medical facility due to exfiltration of warm, humid indoor air.



Ice dams and other ice issues are difficult to control, especially in extreme cold-country buildings such as ski resorts. That being said, by properly planning, installing, and operating the building envelope, HVAC, and/or ice melt systems, the upfront costs more than pay for themselves through eliminating lost room revenue and reducing damage to the structure and finishes.