Ground snow load in Idaho

The Idaho building code does not take ground snow loads from the national maps or from federal case-study methodology: it requires values from Ground Snow Loads for Idaho, 2015 Edition (Al Hatailah, Godfrey, Nielsen, and Sack, University of Idaho Department of Civil Engineering), with final values set by the local building official, which publishes a ground snow load for every city, town, and unincorporated township in the state. For a Idaho address, that study — confirmed with your local building department — is the number a reviewer will accept, so Nivometra does not generate reports for Idaho sites. The station data below is still the real snow record behind the state’s values, and useful context for understanding them.

These are the stations with the highest 50-year ground snow load estimates in the state. Each value is a statistical estimate derived from that station’s snow record, not a measured load, and not a design value for any particular address.

The spread is the point: Idaho’s stations span 8,860 ft of elevation, and ground snow load tracks elevation and local climate far more than state lines. A statewide figure, or a value borrowed from a station at a different elevation, can be off by a large factor in either direction.

A 50-year ground snow load is the weight of snow on the ground, in pounds per square foot, with a 2% chance of being exceeded in any given year. It is the statistical basis of the ground snow load maps in ASCE 7-16 and earlier building codes, and the form of value their site-specific case studies call for. (ASCE 7-22 moved to strength-level values on a different basis; see what changed in ASCE 7-22.)

The values above are per-station estimates from SR-20-1, the ERDC/CRREL report federal agencies use for site-specific snow load analysis. Each is derived from the snow measurements at that station, so it describes the station’s location, not your address. Between stations, the defensible way to estimate a value is the SR-20-1 case-study method: take the surrounding stations, adjust for elevation, and interpolate.

If you own a building, your design snow load is set by your local building department; the guide on how much snow a roof can hold explains what the number means for an existing roof.

If you are designing one, start from Ground Snow Loads for Idaho, 2015 Edition (Al Hatailah, Godfrey, Nielsen, and Sack, University of Idaho Department of Civil Engineering), with final values set by the local building official — the Idaho building code takes its ground snow loads from that study, and your building department can confirm the value for your town.