• Map Creation and Description
• Local Elevation Adjustment
• Table of Ground Snow Load Minimums and Local Elevation Adjustment
• Hilltop Ground Snow Load Estimate
• Areas Requiring a Site-Specific Case Study
• Minimum Roof Design Snow Load

Map Creation and Description

The snow load values displayed by the online map and returned by the lookup tool are 50-year mean recurrence interval (MRI) ground snow loads. Individual snow reporting stations are also displayed on the map, and are listed in alphabetical order in Appendix B of Snow Load Analysis for Oregon. The map was developed with the aide of PRISM (Parameter-Elevation Regressions on Independent Slopes Model) software, which was originally developed in 1991 by Dr. Christopher Daly of Oregon State University.

In determining the ground snow loads occurring within the state of Oregon, PRISM divides the state into a grid work of 2.5-arc-minute square cells, approximately 4 kilometers in length, or slightly larger than 2-1/2 mile increments. The snow load for each of the grid cells is determined by calculating a regression curve and a load value for each cell. The calculation includes many topographical and meteorological relationships between the grid cell and the nearby snow reporting stations. PRISM accounts for the effects of elevation, rain shadows, coastal proximity, terrain configuration, temperature inversions, and cold-air pooling on precipitation and temperature. More information on PRISM can be obtained from http://prism.oregonstate.edu.

In a significant departure from previous snow load mapping work, the independent variable used in the regression curve calculation was the average annual snowfall, rather than elevation. To achieve this, a two-step process was used to create the map. The first step was to use PRISM to create a data set of average annual snowfall grid points with elevation as the independent variable in the regression curve for each cell. The second step was to use the resulting grid of average annual snowfall values as the independent variable in the calculation of the 50-year snow loads. The result of the second step was the map of 50-year snow loads. More information on this can be found in An Updated Snow Load Map and Internet Map Server for Oregon.

In order to improve the resolution of the map created by PRISM, the 4 km grid cells were divided into 25 cells of 800 meters, or about a 1/2 mile square. The 800 meter grid cell snow load values were calculated by adjusting the snow load values of the 4km cells for the average elevation of each individual 800 meter cell. This resulted in a map that better approximates the variations in in snow load due to the local terrain.

Local Elevation Adjustment

In steep terrain, the site elevation may differ significantly from the average 800 meter cell elevation and the modeled grid elevation. It is important to adjust the snow load for the site if the site elevation is higher than the modeled elevation.

The site design ground snow load can be adjusted by adding the load shown in the table below to the grid cell snow load. For example, if the lookup tool returns a ground snow load value of 160 psf. and a modeled elevation of 2500 ft. for a site in the Cascade Mountains , but the actual elevation of the site is 2700 ft. From the table, the snow load increases at a rate of 7 psf. for every 100 feet of elevation gain. The locally adjusted design ground snow load value for the site would be 160psf + (2700ft- 2500ft)*(7psf/100ft) = 174 psf.

Table of Ground Snow Load Minimums and Local Elevation Adjustment

Hilltop Ground Snow Load Estimate

Because the modeling underlying the mapped ground snow load values is based on average elevation across 4 km (2-1/2 mile) grid cells, terrain features such as hills or valleys that are on the order of 4 km across tend to not show up. For example, a hill with a top elevation of 1000 ft. may be contained within a cell with an average elevation of 600 ft. The mapped ground snow load for the example cell is 24 psf. which corresponds to the 600 ft. modeled elevation. If the site is high on the hill, a minimum design ground snow load should be calculated from the table based on the site elevation to ensure the snow load is not underestimated in this case. If the example site was in Washington County, the rate of snow load increase is 4 psf. per 100 ft. Therefore the minimum design ground snow load for a site at 1000 ft. elevation at the top of the hill would be 1000ft*(4psf/100ft) = 40 psf. Note that checking for the hilltop snow load is different from the local elevation adjustment. The modeled elevation is not a factor in the equation. Only the site elevation and rate of snow load increase.

Areas Requiring a Site-Specific Case Study

Where the site has a mapped or minimum design ground snow load greater than the values listed below, a site-specific case study is required. Reliable snow data was not available during the creation of this map for loads greater than the listed maximum loads. Higher values on the map are based on extrapolation, and are not recommended for design. See Snow Load Analysis for Oregon, Part I, for further information on site-specific case studies.

Minimum Roof Design Snow Load

A minimum design roof snow load of 20 psf x I has been established for all structures in the state of Oregon. This minimum loading shall be applicable to the balanced load case only. The minimum load shall not be reduced for slope or any other conversion factor and is only modified where applicable by a rain-on-snow surcharge. Furthermore, the balanced load case shall not be less than the minimum loading regardless of the value determined by other methods.

• pf min = 20 psf x I (20 lbs/ft2 times Importance factor)

A 5 psf. rain-on-snow surcharge shall be added to the minimum roof snow load if either of the following conditions exists:

1. All roofs with a slope Q < 4.76 ° (1 on 12).
2. Roofs of any slope that constrain runoff. This applies to any roof with a drainage system impeded by parapets or other physical obstructions including those with scuppers, or to a roof with an internal drainage system located on the surface of the roof. Structures with a continuous gutter at the low point eave or comparable system shall not be considered as having constrained runoff.

Exception: The 5 psf rain-on-snow surcharge need not apply to roofs, of any slope, where ALL of the following conditions exist:

1. The roof drainage is NOT constrained.
2. The mapped ground snow load pg < 15 psf.
3. The structure lies west of the Coast Range crest or east of the Cascade Range crest.

At the most northern point of the Coast Range crest, the dividing line shall extend directly eastward and continue north along the county line between Clatsop and Columbia counties.