Posted on December 20, 2019 by Heritage
Winter is upon us and many have already seen snowfall this season. This weather doesn’t just impact road conditions and thermostat temperatures. Buildings and their roofs are also affected by snow and ice accumulation. For those living in snow-prone areas, understanding how snow loads and ice dams impact your structure is essential. This knowledge can help you ensure its integrity and longevity, as well as the safety of those in and around the building.
When snow and ice accumulate, the downward force that weight creates on the building’s roof is the snow load. If built in a snow- or ice-prone area, even well-designed and constructed buildings in good condition can fail if the snow load exceeds the weight it was designed to manage.
Knowing the limits of your building helps you prevent snow-induced structural failure, expensive damage and repair costs, as well as injury to people in and around the structure. This requires an understanding of the snow load your roof can support and monitoring snowfall and accumulation during winter weather events to ensure loads don’t surpass acceptable limits.
Most roofs can withstand 20 pounds per square foot of snow. However, there are many factors to consider when properly calculating your building’s snow load. To calculate snow load, multiply the depth of snow in feet by the weight of a cubic foot of snow. If the snow weighs 10 pounds per cubic foot and there are 1.5 feet on the roof, each square foot of the roof is getting approximately 15 pounds of pressure. But how do you know how much snow weighs?
Density/ Snow Load Value:
the weight of each cubic foot of snow varies based on its density. Two feet of light snow places a lighter load on a roof than packed snow of the same depth. For example, wet, heavy snow can weigh six or more times as much as lighter, dryer snow. This means that even if the same amount of snow has fallen, its weight can vary. See below for estimations on the density of varying accumulation types for use when calculating snow loads:
– Light Snow: 5-20 lbs./ sq. ft.
– Packed Snow: 20-40 lbs./ sq. ft.
– Packed Snow with Ice: 40-57 lbs./ sq. ft.
– Ice: 57 lbs./ sq. ft.
It’s worth noting that—apart from some exceptions in Southwest Alaska and the Pacific Northwest—snow in the western United States is typically lighter and less dense than snow that falls on the East Coast, which is usually more dense and wet.
The rate at which snow accumulates greatly impacts overall snow load. If a roof takes on six inches of snow one day and is higher temperatures melt the snow the next day, accumulation is less of a concern. However, if temperatures remain low after an initial dump of snow and more falls on top, you must consider the accumulation from both snowfalls when calculating snow loads. In addition, if warm temperatures and a second freeze follow an initial snowfall, the initial snow can melt and freeze into ice. Because ice weighs more than most snow types, it’s vital to understand the conditions that can cause ice to develop. It’s also important to consider the rate at which snow is falling. Faster snowfall rates require close monitoring of snow loads, and also leave less time for removal before loads surpass acceptable limits.
It’s possible for snow to accumulate at different depths in varying locations on a single roof. The resulting unbalanced snow loads can lead to sliding and drifting that can threaten the roof’s structural system. Uneven distribution can also be a result of the shape and slope of the roof. Keep an eye out for more on this in part two of this blog series!
Wind can easily transport lighter snow from one area of a roof to another. It can also push it up against rooftop equipment and penetrations. This causes uneven accumulation and puts areas where there is additional snow present under greater load pressure. This is particularly likely with roof sections that are in the shadow of higher portions of the building.
Gravity will eventually promote the sliding of snow down the slope of roofs. While this may seem harmless, it can actually be problematic for a couple of reasons:
– As snow slides, its weight becomes unevenly distributed. This puts more pressure in some areas of the roof. In addition, snow from a higher part of the roof can suddenly slide down onto lower sections like porches, sunrooms and canopies. This can immediately double snow loads in these areas and promote collapse from the added weight and impact force of the new snow.
– When the snow does finally fall off the roof, the force generated can uproot rooftop equipment and damage roofing material. Additionally, anyone or anything beneath the eaves can easily be damaged or injured by the weight of the falling snow (and any equipment that come down with it).
Snow guards can help keep snow and ice from sliding down roof slopes and endangering people and property below. Work with the manufacturer of your roofing materials to select a snow guard that’s compatible with your roofing. Having an experienced contractor install snow guards can also help prevent roof damage and ensure they work properly.
Of course, many of these factors can be mitigated or made more predictable through careful building design. In part two of our steel building and snow loads blog series, we review important design factors, snow load minimization methods and common snow-removal tips that can help when planning building projects in snow- and ice-prone areas.
For more information on how steel buildings perform in winter weather, please reach out to your local Heritage Building Systems representative today!