Heavy snowstorms can result in roof damage or collapse. What are the risk factors?
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The roof of a copper wire plant collapsed under the weight of heavy snow in Williamstown, New York, on Feb. 18.
When a winter snowstorm is in the forecast, icy, snowy roads aren't the only thing to be concerned about. Heavy snow carries the risk of roof collapse, but all roofs and all types of snow are not the same.
In terms of the overall weight of snow, a wetter snow that has a high water content will be heavier than a dry and fluffy snow, said Alyson Hoegg, supervisor of Forecasting Operations and AccuWeather Senior Meteorologist.
“These wetter snows can add strain to roofs that may cause issues depending on how much snow is on a roof,” Hoegg said.
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Owner Brang Miller shovels snow off the roof of Mammoth Fun Shop and locals try to dig out in between storms. Over 550 inches of snow has fallen at the main lodge as residents try to dig out. (Wally Skalij/Los Angeles Times via Getty Images)
In the northeastern United States, roofs are generally designed to support 30 pounds per square foot, but some roofs are built to support 40, 50 or even 100 pounds per square foot, according to the ASCE Standard Handbook.
The inches of snow accumulation do not directly dictate the weight that is placed on a roof. The weight of snow is actually dependent on the water content.
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One inch of water weighs 5.2 pounds per square foot, and snow can have different distributions of water content, which depends on a number of variables including outside temperature.
When snow weighs less than 10 pounds per square foot, it is manageable, but concerns arise when it approaches 20 pounds per square foot. In the northeastern United States, roofs are generally designed to support 30 pounds per square foot, but some can support up to 100 pounds per square foot.
A car drives by Schmidt's Collision and Glass after the roof collapsed after taking on heavy snow during this week's lake-effect snowstorms on Saturday, Nov. 22, 2014, in Hamburg, New York. (AP Photo/Mike Groll)
Not all snow is the same
Other factors that can cause issues include how long a heavy, wet snow is on a roof, if there is any precipitation that occurs after the heavy, wet snow occurs and how much of that precipitation occurs.
If the heavy and wet snow is on a roof for a long period of time, the structural integrity of the roof may be compromised, but this would have to be a significant amount of weight for an extended period of time for this to occur generally, Hoegg said.
Snow mixed with rain can also bring problems.
If a heavy, wet snow occurs, or even at times a drier snow happens and then rain occurs either later in the storm or a few days later before this snow melts, that rain will partially be absorbed by the snow, leading to additional weight. Hoegg said.
If this water can't run off adequately, it will linger in the unmelted snow. If the rain only occurs for a short period of time, that generally isn't an issue. The bigger issues occur when there is already a significant snow pack in place and we receive rain for a prolonged period of time with temps near and just above freezing.
The snow at that point isn't melting quickly and the water is being held up in the snow. That combination could cause structural issues depending on the overall weight of the snow.
Many of the building codes factor this in with their load management for the roof and internal structure.
>>Better protect your employees and assets this winter with AccuWeather's Snow Warning ServiceTM. Contact AccuWeather today.
Residents attempt to shovel out their driveway after a series of winter storms in the San Bernardino Mountains in Southern California on March 6, 2023 in Crestline, California. Some residents have been stranded in Crestline close to two weeks due to the snowfall while the local grocery store was severely damaged when its roof collapsed under the weight of the snow. (Photo by Mario Tama/Getty Images)
Differences in roof styles
Gable roofs, which have two sloping sides, can become problematic in heavy wind conditions, when snow will blow from one side and settle on the other.
Some sloped roofs, with enough slope and that are short enough, can allow snow to blow from one side then onto the ground.
But sloped roofs are also prone to ice dams, which can cause interior and roof damage.
A-frame roofs, which have very steep slopes, speed up water run-off so that it doesn't have the opportunity to refreeze.
However, these sloped roofs typically aren’t feasible for large industrial buildings, hospitals or any other large building.
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Men shovel snow as a storm sweeps through Times Square, Tuesday, March 14, 2017, in New York. (AP Photo/Mark Lennihan)
While most roofs are not entirely flat, roofs with a level appearance allow snow to blow off the roof easily. However, some of these roofs have external systems on the roof, including air conditioning and parapet units, which encourage snow drifts to settle around the unit.
These roofs are also less prone to ice dam formation, but the draining systems on top of the roof can freeze over trapping melting water on the roof.
Wistar said that there really is no ideal roof type to battle winter weather, and it all really comes down to the type of snow and the amount of wind.
>>Better protect your employees and assets this winter with AccuWeather's Snow Warning ServiceTM. Contact AccuWeather today.
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