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Copyrightę 1986 by University of Idaho. All rights reserved.
This site and these resources published online with permission from the University of Idaho Library, the Civil Engineering Department, and Ronald Sack.
Ground and Roof Snow Loads for Idaho
by Ronald Sack &
This page presents resources related to the report "Ground and Roof Snow Loads for Idaho," which was released in 1986 by the Department of Civil Engineering at the University of Idaho. To read the report and it's appendices, click on the cover to the left. To use the map, click on the map to the right.
This map has been made possible by several years of nonfunded work. We respectfully request that if you use this map or report for commercial purposes, you purchase a pdf/map (information at top right) or donate to the University of Idaho Library.
DISCLAIMER: Great care has been taken to be accurate in preparing this map, but neither the publishers nor the University of Idaho can accept responsibility for any errors which appear or their consequences. The final design snow loads are the ultimate responsibility of the engineer, architect, local building official, and/or contractor in charge of the project.
Below are the instructions for obtaining loads.
Obtaining Ground Snow Loads
- Select the normalized ground snow load (NGSL) for the site location. Linear interpolation between contour lines is recommended.
- Normalized contours in lb/ft2/ft on the Idaho map are based on a 50-year mean recurrence interval (an annual probability of exceedence of 2 percent).
- Multiply the NGSL by the site elevation (in ft); this gives the ground snow load (pg) in lb/ft2.
- Design loads should be specified or approved by the local building authority for locations where local records and/or experience indicate that the ground snow loads obtained from the map are inadequate.
- Calculate the flat-roof snow load using Eq. (3 .1) or (7.3-1) pf = 0.7CeCtISpg where Ce comes from Table 7-2, Ct from Table 7-3, and IS from Table 1.5-2.
- Calculate the sloped-roof snow load using Eq. (3 .7) or (7.4-1) ps = Cspf where Cs depends upon roof thermal properties and roof geometry; values for Cs are shown in Sections 7.4.1 and 7.4.2.
- Apply any special design considerations such as: partial loading; unbalanced roof snow loads; drifts on lower roofs; roof projections and parapets; sliding snow; rain on snow; ponding; existing roofs; and other topics. See the ASCE Standard ASCE/SEI, Chapter 7.
The methodology for obtaining ground snow loads for Idaho was published in 1986, and the loads are still the most accurate currently available (2012). The Idaho loads were obtained using a more extensive database than the loads shown in Fig 7-1 of American Society of Civil Engineers, Minimum Design Loads for Buildings and Other Structures, "ASCE Standard ASCE/SEI 7-10," Reston, Virginia, 2010. ASCE/SEI 7-10.
Obtaining Roof Snow Loads
The user is advised to ignore the text on the map under this heading because several changes for the methodology of obtaining roof loads from ground loads have been made since 1986 by the American Society of Civil Engineers, Subcommittee on Snow and Rain Loads. Note in the following points, an equation or table number with a "3" prefix refers to the Idaho report and those with a prefix of "7" or "1" refer to the ASCE Standard ASCE/SEI. The user is advised to use the most recent version of the ASCE Standard ASCE/SEI for obtaining roof loads from ground loads; the publication is revised approximately every five years.