13th February 2019 by Michele Pratt
If you find yourself in an area that accumulates snow, how do you know if you are protecting yourself, and your workers, from potential accidents and injuries while working on a roof? The year 2011 saw a surge in numerous building failures in the northeastern part of the United States due to multiple major snow storms.
Building codes are used by structural engineers to determine snow loads on roofs. They utilize the IBC (International Building Code) to assist with this determination. In addition, states and or local government can supplement or make changes to the IBC to develop their own code. As per FEMA, “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% 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.”
Factors considered when determining snow load are: the shape and slope of the roof, wind exposure, and obstructions, thermal condition of the building and its occupancy. You also need to account for the different ranges in snow weight because snow can range from 3 pounds per square foot to 21 pounds per square foot. The difference is determined on the moisture content of how wet and heavy the snow is.
FEMA also indicates the need to look at other variables when determining snow load and safety of its removal such as unbalanced snow load, drifting snow, sliding snow, snow fall, ambient temperature, rain-on-snow load and snow melt between storms. Additional factors such as snow cleats on a sloped roof and efficiency of a drainage system will determine how often it would be needed to provide snow/ice removal to the roof.