snow load

Snow load is the downward force exerted by accumulated snow and ice on a building or structure, measured in pounds per square foot (psf) or kilopascals (kPa). Unlike dead load, which is permanent, snow load is a live load that varies seasonally and annually. The actual weight depends on snow density, which ranges from about 5 psf for fresh, light snow to 20 psf or more for wet, compacted snow. Roof design must account for the heaviest snow accumulation likely to occur in that geographic location over the design life of the structure.

Building codes specify minimum snow loads based on historical climate data and ground snow load maps. The ground snow load is the weight of snow on a flat surface at ground level; the roof snow load is derived from this figure but adjusted for factors such as roof slope, exposure, and thermal conditions. A steep roof at 45 degrees may shed snow and require a lower design load than a low-slope or flat roof. Wind exposure also matters: exposed hilltops accumulate less snow than sheltered valleys, and building geometry affects drift patterns, where snow piles up in valleys and against parapets.

Critical failure modes include uniform loading across the entire roof and unbalanced loading, where snow drifts to one side or accumulates unevenly due to obstructions. Roof-mounted equipment, parapets, and skylights all create drift zones where snow concentrates. Rain-on-snow events can increase effective load by 50 percent or more, since water adds weight and reduces drainage. In cold climates, ice dams form at roof edges and prevent meltwater from draining, trapping additional water weight on the roof structure.

Design and safety margins

Engineers design roof framing to carry the design snow load plus a safety factor, typically 1.4 to 1.6 depending on code jurisdiction and load combination method. Flat roofs require steeper design loads than pitched roofs because snow does not naturally slide off. Maintenance during heavy snow events, such as roof raking or mechanical removal, reduces the actual load but is not a reliable substitute for proper structural design. Some jurisdictions now require slope or deflection limits on low-slope roofs to prevent ponding, where water and settled snow accumulate in depressions and worsen structural deflection.

Regional snow load requirements vary dramatically: coastal areas may specify 30 to 40 psf, mountain regions can exceed 100 psf, and arid climates may specify as little as 10 to 15 psf. Historic collapses during extreme winter storms have repeatedly exposed structures designed to outdated or insufficient codes, making snow load one of the few live loads that still causes catastrophic failure in modern buildings. Designers must always verify the applicable code for the project location and elevation, and account for any future changes to the building that might affect snow accumulation patterns.