Snow Load

The weight of accumulated snow on a building's roof, calculated according to building code methods based on geographic location, roof shape, and exposure, used to design the roof structure to prevent collapse.

What is Snow Load?

Snow load is the downward force exerted by accumulated snow on a building's roof structure. Building codes require that roof structures in areas subject to snowfall be designed to support the weight of snow accumulation without failure. Snow loads are determined based on geographic location, elevation, roof geometry, wind exposure, and thermal conditions.

How Snow Loads Are Determined

The design snow load is calculated starting with the ground snow load for the building location (published in ASCE 7 snow load maps or local building department data), then adjusted for roof slope, exposure, thermal conditions, and the building's importance factor. Flat roofs accumulate more snow than steeply sloped roofs. Areas where wind can drift snow — such as lower roofs adjacent to taller building sections — may have significantly higher localized snow loads.

Snow Load Considerations

Snow load is a critical design factor in northern states, mountainous regions, and areas with heavy snowfall. Ground snow loads range from zero in southern regions to over 300 pounds per square foot in extreme mountain locations. The difference between designing for 20 psf and 100 psf of snow load can dramatically affect the size and cost of roof structural members.

Rain-on-Snow and Drift Loads

Two often-underestimated snow load conditions are rain-on-snow events (where rain saturates an existing snow pack, dramatically increasing its weight) and snow drifting (where wind blows snow from higher to lower roof areas, creating localized accumulations that can exceed the uniform snow load by 2-3 times). Both conditions must be accounted for in structural design.