Calculating snow loads per IBC 2021

Why snow loads matter: the Knickerbocker Theater

Laurent opened the session with the Knickerbocker Theater disaster, describing it as one of the deadliest structural collapses in US history. Snow accumulation caused the roof to collapse during a storm over Washington, DC. He also cited the Metrodome in Minneapolis, which he noted had its roof collapse five times in total, four of them due to snow. Laurent used these examples to underscore how seriously snow-related failures can be misunderstood or underestimated, and cited a 2011 count of newspaper articles reporting that just in the northeastern United States, approximately 400 roofs collapsed under snow in that single season.

How ground snow load is established

Ground snow load is the fundamental local input for all snow load calculations. Laurent explained that it is determined from approximately 10,000 weather station locations across the United States, where measurements are taken at regular intervals. The measurement captures not just depth but actual load, since the density of snow varies significantly depending on how much it has been compacted.

Laurent noted that the ASCE 7-16 hazard tool online can return a ground snow load for a given location, but many areas require site-specific studies instead of returning a mapped value. Local jurisdictions frequently establish their own ground snow loads, which are often higher than the ASCE 7 mapped values. He demonstrated this using Mammoth Lakes, California, where the city code specifies 230 PSF for elevations below 8,500 feet and 300 PSF above that elevation, far higher than any national map contour for the region.

From ground to roof: exposure, thermal, and importance factors

Three factors convert the ground snow load to the flat-roof design value. The exposure factor accounts for wind action clearing snow from the roof: surface roughness is the same parameter used for wind load design, and rougher terrain in forested or urban settings means less wind and more snow accumulation. The roof exposure factor considers the specific building: a structure sheltered by tall conifers gets a higher factor than one exposed on all sides.

The thermal factor accounts for how a building's heat affects snow accumulation. A warm, poorly insulated roof melts snow from below, reducing load. A cold or unheated structure may actually be colder than the ground and accumulate more snow over a winter than the ground itself. For the Mammoth Lakes example, Laurent selected a ventilated attic condition, which carries a thermal factor of 1.1, increasing the snow load above the base value. The importance factor reflects occupancy risk; for a standard residential structure in risk category 2, it is 1.0.

Slope reduction and unbalanced loading

Roof slope affects how snow stays on or slides off. Laurent distinguished between roof materials: metal roofs shed snow more effectively than asphalt-shingle roofs. For asphalt shingles, slope reduction only becomes meaningful at very steep pitches. A six-in-twelve slope produces no reduction; even a ten-in-twelve slope reduces the load only modestly. A slippery metal roof at six-in-twelve, however, reduced the Mammoth Lakes design load dramatically, from 159 PSF down to approximately 60 PSF.

For gable and shed roofs with spans less than 20 feet, the unbalanced load case is straightforward: the full ground snow load, multiplied by the importance factor, is applied to one side only. Laurent showed that for the Mammoth Lakes rafter system, the unbalanced case governed: the balanced load was 159 PSF, but the unbalanced case required designing to the full 230 PSF ground snow load on one rafter. When Laurent entered 230 PSF into the rafter calculator, even 14-inch TJI joists failed, and the design required a TJI 560 to pass all limit states.

ASCE 7-22 updates to watch

Laurent covered the main differences between ASCE 7-16 (referenced by IBC 2021) and the upcoming ASCE 7-22. The updated standard eliminates the site-specific study requirement that previously applied to areas shown in blue on the ASCE 7-16 map. Ground snow load values are now based on reliability targets: the LRFD load factor for snow drops from 1.6 to 1.0, and the ASD factor drops from 1.0 to 0.7. The new ground snow load maps show an average increase of approximately 12 percent nationwide, with some regions such as Baltimore, Maryland seeing increases of around 30 percent. A new parameter called winter winds also affects drift load calculations, with increases of up to 25 percent in the Midwest and Northeast and reductions for areas west of the Rockies and in the Southeast.