December 16, 2021
At flood workshops and conferences, I often hear people say that everyone should have flood insurance because any home can flood. I have often thought, “Well, almost everyone can flood. But my childhood home cannot flood, because my parents live on top of a ridge in the mountains of Pennsylvania. If they flood, everyone has flooded.”
You can imagine my surprise when I called home last winter to find that indeed their house was flooding. The inundation was not caused by heavy rain or a swollen river, but rather from a winter weather phenomenon called an ice dam.
According to the University of Minnesota’s Cooperative Extension, an ice dam is “a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof.”
For ice dams to form, several factors need to take place.
The first factor is the presence of a building in a cold climate with sub-freezing temperatures and prolonged snow cover on roof tops. When a portion of a roof warms up and the temperature exceeds the freezing threshold, snow begins to melt and flow downward. An ice dam forms when another portion of the roof, typically near the roof edge or on the eave, remains cold and retains a sub-freezing temperature, where the water re-freezes and forms an ice dam.
After the ice dam forms, additional meltwater begins to pool on the roof above the dam, and can even be forced to flow backwards, up the slope of the roof, enabling the water to be forced under the shingles, or otherwise leak into the home and damage the ceiling, walls, insulation, or other areas.
In the case of the flood at my childhood home, dammed water was forced to flow upslope, working its way under shingles and getting into the walls of the house. The water then ran down the walls for two stories to the basement, where it pooled on the floor.
The good news about ice dams is that they take time to build up, so they do not typically form in parts of the country that observe sub-freezing weather for short duration, like the southern states. Mike Kampnich, a roofing expert based in Watertown, New York, along the Canadian border, clarifies, “ice dams cannot form in a week or two, they need at least three weeks for the ice to build up.”
Climatology on the average number of days with sub-freezing temperatures can be hard to find, but the Midwestern Regional Climate Center’s Vegetation Impact Program provides an interactive mapping platform to visualize these data. The data were last updated in June 2021, as of the time this article was published, but presumably these data will be updated again soon, as the winter season 2021–2022 is just getting under way, and other data layers on the system are up-to-date.
Readers should note that a national map of consecutive sub-freezing temperatures does not tell us everything we need to know about ice dam formation risk. If a location observes a high temperature of 33 degrees Fahrenheit, the clock is reset for this calculation, even though little snow and ice has melted if the weather was cloudy at that time. Also, snow cover is a necessary factor for forming ice dams—a dry location with little or no snow cover will not have problems with ice dams, even if they observe a long stretch of sub-freezing temperatures.
The geography of the processes for ice dam formation canal so vary regionally, depending on the amount of sunshine that is usually observed during the winter months. In the cloudy Great Lakes region, for example, most of the differential in roof temperature will come from inadequate insulation, which enables portions of a roof to thaw. However, in the Intermountain West, stronger winter sunshine often melts snow on one part of the roof, while areas in the shade remain below freezing. This temperature differential can lead to ice dam formation.
For areas that observe extreme cold in the winter, ice dams may be more likely to form in March or April, when snowpack remains on rooftops, but outside temperatures are closer to freezing, providing greater potential for freezing and thawing. Ryan Tipton, a roofing expert with A&A Roofing Company in Fairbanks, Alaska, noted that most ice dam problems found in Interior Alaska occur in March or April. During this time, average high temperatures are in the mid-20s to mid-30s, a considerable warm up from January, when the average high temperature is 0 degrees Fahrenheit. We would expect to find a similar pattern for portions of Canada and Russia, where extreme cold sets in for several months each winter.
Homeowners should consider three factors for preventing ice dams: better construction practices, products that reduce ice dam risk on roofs, and efforts to manually remove snow from roofs. In case ice dams still form, several products also minimize the impacts of the flooding caused by an ice dam. Let’s investigate each of these topics.
Much of the success for improved roof design that minimizes ice dam formation relates to finding ways to reduce the amount of snow on roofs and the amount of time snow covers roofs. Construction practices related to roof slope and roofing material are important for these factors.
Increasing roof slope reduces ice dam formation because snow more easily slides off steeper roofs. Melt water also runs off steep slopes faster, giving the water less time to refreeze and more force to push through and over ice dams. By contrast, flatter roof angles hold more snow and do not drain melt water as efficiently, increasing the risk of ice dam formation and impacts.
My father explained that the ice dam at our family home only formed on the flattest roof, while the steep slopes of the other roofs prevented ice dams from forming. The photos below show the slopes of multiple roofs at his home, with an arrow depicting the roof that supported the ice dam.
The construction material of roofs also impacts the formation of ice dams, as metal roofs minimize their formation because they enable snow to slide off them easier than shingled roofs. Shingles increase the surface friction of the roof, essentially “grabbing” the snow and holding it, which slows the rate the snow will slide off the roof.
I researched this article from Osceola, New York, on the TugHill Plateau, in the heart of the Great Lakes Snowbelt. The National Weather Service annual snowfall map shows this region as a bullseye, exceeding 200 inches of snow accumulation annually, but many locals insist annual snowfall on “The Hill” exceeds 300 inches. Whatever the exact number, one thing is certain—this region observes the highest snowfall totals east of the Rocky Mountains inNorth America.
We can learn a lot from observing building practices in areas that face the greatest extremes, so I drove around Osceola the day after a 5-inch snowfall in December 2021, to see how local roof design worked to minimize snow loads on roofs, which would decrease the formation of ice dams during the winter. I noticed that most buildings had steep, metal roofs, which were making quick work of this relatively small snow load for this region.
The pictures below tell some interesting stories, as they were all taken within 10 minutes of each other. The weather had been cloudy since the snow fell, so roof design was the primary factor in the amount of snow cover for each roof. Note that some roofs were already free of snow, like the former church building, which now doubles as the town hall and library. In other cases, snow had begun a slow-motion slide toward the ground. Note the picture of the A-frame cabin and garage, in which the snow had already slid off the A-frame, but was still in the process of sliding off the garage.
Minimizing the surface area of a roof can also reduce ice dam formation, because longer roofs have more surface area on which water can refreeze. A staff member with Calel’s Roofing in Leadville, Colorado, commented, “Roofs that extend above two stories are worse than shorter roofs, because they have more surface area for refreezing.”
Reducing the surface area of roof eaves can also minimize the formation of ice dams, because eaves are the coldest part of the roof. A balance must be found with such designs, however, as eaves need to extend some distance from the walls to transport water away during heavy rainfall in the warmer months.
If homeowners are moving into a home that has already been constructed, they can still minimize ice dam formation by improving insulation in the attic. Ryan Tipton clarified, “stopping heat loss to the roof is the most important way to reduce ice dam formation.”
The University of Minnesota Cooperative Extension materials on the topic suggest contractors conducting a blower door test, in an effort to evaluate how airtight a ceiling is. They also suggest using an infrared camera to detect areas of excessive heat loss.
Mr. Tipton also mentioned that newer construction sometimes uses an energy heel truss, in which the truss is built higher, enabling thicker insulation to be placed near the outer wall. Unfortunately, in some older buildings, the truss is not deep enough, so the homeowner cannot insulate the attic to the edge, because room needs to be given for ventilation and air flow near the edge.
If roofs are not steep enough to slide snow, or are constructed with snow-grabbing shingles, several products can be placed on roofs to minimize the formation of ice dams. Heated coils and tape are such products that are placed near the roof edge to radiate heat and melt snow and ice. They can be plugged in or switched on from inside the home.
Roofing contractors installed heating coils at my family home, near the edge of the flatter roof that developed the ice dam during the winter of 2020–2021. A wire runs from the coil inside the house, where my father simply plugs it in to warm up the wires and melt snow on the roof. The schematic below, taken from the product manual of Frost King Roof and Cutter De-Icing Kit, depicts an optimal zig-zag pattern in which the coil could minimize ice dam formation, or open up channels for water to flow if ice dams form.
If snow does not naturally melt or slide off roofs, removing snow from roofs can help minimize the formation of ice dams, as well as potential roof collapse if the snow is heavy and wet. In areas of the United States and Canada that observe prolonged, heavy snowfall, it is not uncommon to see workers shoveling snow from roof tops. However, untrained individuals should use extreme caution from standing on slippery roofs during the wintertime.
In other cases, heavy machinery is used to remove the snow. Leona Chereshnoski showed me the photograph below during our podcast interview at the family farm where she grew up in Osceola. In the picture, a trac-hoe was removing heavy snow from the roof of the Fiddlers’ Hall of Fame and Museum. John Heurin, a fifth-generation local, told me that sometimes old timers would remove snow by using a long rod in a sawing motion, essentially cutting the snow off the rooftop.
Although this article shares perspectives on how to get snow to slide off roofs, readers should be aware that this topic is more complex than it appears on the surface. While snow sliding off roofs is good news for the structural health of the building, it can be bad news for the safety of pedestrians and motorists. In high-traffic areas, falling snow and ice can inflict injury or even death on people below, while damaging vehicles.
This creates a dilemma, whereby snow sliding off roofs is in the best interest of the building, but not necessarily in the best interest of the passerby. Designers and builders need to look at construction holistically in busy, snow-prone areas, perhaps designing a building with multiple small roofs that will not retain excessive snow cover on any one surface.
In high traffic areas, snow guards placed near roof edges above doors and walkways serve to intentionally keep snow on roofs, so sliding snow and ice does not strike people below. The photograph below, taken at Snow Ridge Ski Resort, in Turin, New York, on November 27, 2021, shows angular snow guards placed above the doors. This design will keep snow from sliding off this part of the roof, where people are passing below. Note that the angle of the snow guards will deflect melt water, while still enabling it to flow off the roof. Most of the roof does not have a snow guard, enabling snow to slide off it.
This article has discussed the science behind ice dam formation, as well as some practices and products that homeowners can use to minimize the presence of this hazard on your home. However, if ice dams do form, builders can minimize the impact of flooding that forms behind an ice dam my installing products to seal the roof.
Many of these products relate to a protective layer builders can install under the shingles to prevent water from saturating the roof or walls if it does back up on the roof. Mike Kampnich referenced ice and water guard, which is a rubber membrane that can be rolled out on the rooftop.
Ryan Tipton referred to this product as ice and water shield, a self-adhesive rubber underlayment sheet that is placed under the shingles.He uses sheets that are 39” wide, and places two of them near the roof edge, to satisfy codes in Interior Alaska, that require this rubber shield to extend to at least 24” from the outer wall. Two sheets are necessary because the first sheet starts at the edge of the eave, and will not extend inward to more than 24” from the outer wall on the roof. The idea behind this code is to provide a layer of protection near the roof edge in case an ice dam does form.
In summary, ice dams form when snow cover on roofs melt and re-freeze, with ice forming near the roof edge. Additional melting that occurs after the ice dam has formed will pool water on the upslope side of the dam, enabling water to get under shingles and saturate the roof.
Building practices to minimize ice dam formation included esigning steep, metal roofs and using adequate insulation to minimize heat loss to the roof. Heating tape and coils can melt ice and snow near the roof edge, minimizing ice dam formation, while rubber sheeting under the shingles can protect roofs from flooding, if water is forced under shingles.
Ice dams form when sub-freezing temperatures keep snow on roofs for multiple weeks, most commonly in the United States in the northern states, and the intermountain west, as well as much of Canada. If heavy snow cover persists in these areas, home and business owners often hire workers to remove the snow by shovel or machinery, which reduces the threat of both ice dams and roof collapse.
As we move into the winter of 2021-2022, I am hopeful that the new heating coils installed on the roof edge of my childhood home prevent the formation of any more ice dams back home. We will find out soon, as the first snowstorm of the season is forecast to paint a layer of white on the landscape later this week.