Using Snow Data in Snow Load Investigations

By: Michael Drinkwater, P.E. Forensic Engineer

When considering the possibility of structural damage to a building from the weight of snow and ice, it is critical to know weather history at the property. Once this value is known it can then be compared to local building codes.

The National Operational Hydraulic Remote Sensing Center (NOHRSC) maintains a network of stations across the country that measure snow depth, moisture content, density, temperature, and several other variables. This data can be accessed in graphical form for specific timeframes at each station.

The data is accessed via the NOHRSC website. [1] You will need to know the approximate distance and direction the loss location is from city center. You should also consider elevation. In many areas an elevation difference of only a few hundred feet can have a significant impact on snow loads.

Once at the NOHRSC website, type in the city of the loss location. The page then provides a list of the stations nearest that city. Click on the station that has close proximity to the loss location and is at a similar elevation. If no station is a good match, you may need to pull data from two or three stations that bracket the loss location geographically and/or by elevation.

At the particular station website, enter the date that corresponds to the time period surrounding the date of loss. For example, if the date of loss is January 20, it is recommended to search for at least a week before and a few days after. This is to be sure you capture the peak loading. If the graph for this period does not show a clear peak (or indicates a peak outside the search dates) you can search for a broader period.

Once you obtain a graph that captures a peak snow load that corresponds to the date of loss, you can focus in on snow depth and snow water equivalent (SWE). Snow depth is intuitive and simply measures how deep the snow was. It is a good value to compare to reports from individuals at the property during the storm. SWE is the depth of water (in inches) of the snow and ice if they were melted. SWE is the value you use to determine the weight of snow and ice.

Both snow depth and SWE are provided in observed and modeled values. The observed values are based on observations by a person. The modeled values are based on data collected without human observation. Both values are useful, but observed values have greater credibility.

Once a peak SWE is obtained, it can be converted to a per square foot weight by multiplying it by the unit weight of water (62.4 pounds per cubic foot). This is the weight the snow and ice imposed on the property and the value to consider when comparing to local building codes.

For example, let’s work through a simple example with the following parameters:

Date of loss: January 20, 2017
Location: Downtown Bend, Oregon

From the NOHRSC website we will select the OR-DS-29-BEND 1.5 WSW,OR station as it is the nearest to downtown Bend. From the station website we pull the graph below using the January 1 to February 20, 2017, timeframe (Figure 1).

Figure 1: NOHRSC Interactive Snow Information Chart (horizontal yellow line added for clarity)

We can see that the peak observed snow depth is 34 inches and the peak SWE is 5.1 inches. Convert 5.1 inches to feet by dividing by 12 and then multiply by 62.4 pcf to obtain a SWE weight of 26.5 psf.

(5.1 in/12 in/ft) x 62.4 pcf = 26.5 psf

The Deschutes County, Oregon website indicates that buildings within the Bend city limits are constructed to support 25 psf. Based on this we can conclude that it is possible the weight of snow and ice caused structural damage to competent buildings in the area. [2] If the SWE weight were significantly less than local building codes, it would indicate it is not probable the weight of snow and ice caused structural damage. If damage is found in such a case it can be caused by an underlying issue such as decay or construction deficiencies.

It is important to remember that the values obtained from the snow stations are based on ground snow loads and do not directly reflect loading on the roof. If the house is in an open, windy area and/or has a steep metal roof, actual snow loads may be far less than NOHRSC values. If the house is in an area that is sheltered by trees that shed snow onto the roof and/or the roof has a low roof pitch, actuals loads may exceed NOHRSC values.

[1] “National Operational Hydrologic Remote Sensing Center – The Ultimate Source For Snow Information”. 2017. Nohrsc.Noaa.Gov. (accessed April 14, 2017).

[2] “Dial – Deschutes County Property Information”. 2017. Dial.Deschutes.Org. April 14, 2017).

About the Expert

Mr. Michael Drinkwater joined Donan in 2016 as a forensic engineer based out of the firm’s Portland, Oregon office. He has 16 years of engineering experience. His areas of expertise are: foundation defects, structural defects, structural engineering, and roofing defects. Mr. Drinkwater’s additional project capabilities include water intrusion, storm damage, impact damage, and roofing damage. Mr. Drinkwater is a licensed professional engineer in Arizona, California, Colorado, Georgia, Idaho, Louisiana, North Carolina, Nevada, Oregon, South Carolina, Texas and Washington. He holds a Bachelor’s degree in Civil Engineering from the Oregon Institute of Technology.

View his full professional profile here.