What Is A Snow Load And How Important Is It?

Proper snow guard layouts are vitally important to ensure a successful installation.  Without a professional layout, customers run the risk of under-designing their system and possible system failure.  Spacingtool.com was created to help architects, homeowners and contractors determine proper guard spacing. Project specific dimensions and local “Ground Snow Loads” are calculated along with the testing strength of the various snow guards to produce a proper layout for snow guards.  This free design service also helps you determine the correct guard recommended for the specific project and panel.

You might be thinking, “Dimensions are easy to get, but what is snow load?” Snow load is the downward force on a building’s roof by the weight of accumulated snow and ice. The roof, or even the entire structure, can fail if the snow load exceeds the structural load capacity of the roof system.  It doesn’t take a blizzard to cause problems.  Drifting snow can also cause unbalanced loading that could cause significant structural damage to the roof system.

Snow loads are influenced by elevation, general weather and moisture patterns, slope direction, exposure, roof configuration, and wind direction and severity.  Overestimation of snow loads can unnecessarily increase the cost of construction. Underestimation of snow loads can result in premature failure, high maintenance costs, resource damage, and, in some cases, safety issues.

A snow event is not necessarily a single large snow storm. A snow event can be a series of storms that result in additional snow loads on a building. No two snow events are identical, and the resulting snow loads on nearby buildings from one snow event may be different. One foot of snow on the ground does not necessarily equal 1 foot of snow on a roof. Further, differing snow load conditions are a function of the variables associated with an individual building. The characteristics of snow can differ significantly from snow event to snow event.

States and/or local jurisdictions may amend or supplement the IBC (international building code) or adopt their own code. The building code identifies the ground snow load, which building designers use as the starting point to calculate the uniform design snow load on a building roof.

Ground snow load is defined as the weight of snow on the ground surface (IBC, 2012). Ground snow load values are established using data collected by the National Weather Service. Maps of ground snow loads in IBC and in ASCE 7 indicate a 2 percent probability of the indicated load being equaled or exceeded in any given year. Ground snow loads do not discount that actual snow loads may exceed them, only that the risk of snow-load-induced failure is reduced to an acceptably low level.

Roof snow load is defined as the weight of snow on the roof surface used in design of the building structure (IBC, 2012). It is determined based on multiple factors, including:

  • Ground snow load value n Importance, occupancy, and use of the building
  • Wind exposure of roof
  • Roof slope
  • Roof shape
  • Roof obstructions
  • Thermal condition of the building

When determining our layout for snow guards, we base our calculations on the “Ground Snow Load” of the project, as opposed to the “Roof Snow Load”.  This allows a safety factor to account for unusual weather events, wind drifting, and other unforeseen factors.

When trying to determine the snow load for your area, this information can typically be obtained by contacting your local building officials. Websites like http://snowload.atcouncil.org/ are also an amazing tool in determining these loads, as seen in the example below. We simply entered the address information for our warehouse facility in Lemoyne, PA and the tool did the rest.

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If you have questions about snow loads or any of our products, please feel free to give us a call at 1-800-766-5291 and one of our friendly techs will be happy to help you.

 

 

 

 

Sources

  1. http://www.fs.fed.us/t-d/snow_load/
  2. http://www.fema.gov/media-library-data/7d8c55d1c4f815edf3d7e7d1c120383f/FEMA957_Snowload_508.pdf
  3. http://www.nationwide.com/snow-load-barn-collapse.jsp
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