A snow load is the burden placed on the ground or on a structure by snow, rime, and ice that has accumulated. Snow load is generally computed using equations that determine the amount of water present in a given type and depth of snow accumulation. The ground snow load is computed first, based on an average sample of flat ground, and the load carried by roofs is then normally derived from that figure. Building codes require structures to be able to comfortably support the heaviest snow load that could be reasonably expected to accumulate in the geographic area where they are situated.
Snow tends to accumulate on the ground and on structures in cool climates during the winter. Accumulations of snow are often measured in terms of depth. This can give a rough idea of the actual weight of snow present, but the actual weight of snow varies widely, with light powdery snow weighing very little, and dense, wet snow weighing a great deal. Snow load calculation is based on the weight of water in the snow in a given area, rather than the depth of the snow.
The weight of water in a particular snowpack can be determined by measuring the depth of the snow, and then applying formulas based on the composition of the snowpack. These formulas are based on experimental evidence collected over many years, and are quite accurate. This method of calculation is preferred because actually melting and weighing a representative sample of snowpack is a difficult process.
Ground snow load refers to the weight of water per square foot that is exerted on the ground by the snow pack. This statistic is not often directly relevant, although heavy snow can damage some landscaping features. Roof snow load is a much more important statistic and is computed from the ground load in a region.
The load placed on a particular roof is influenced by the design of the building and other environmental factors. Sloped roofs, especially steeply-sloped roofs, such as the A-frame styles common in very snowy climates, shed snow very efficiently, and carry a much lower load than flat-roofed structures. Melting may also decrease or redistribute the weight of snow on a roof.
Building codes mandate that structures be able to withstand the greatest snow load that is likely to accumulate. Code requirements thus vary widely from region to region. Homes in Florida are not expected to bear a snow load at all, while homes in areas that experience a great deal of snow, such as Alaska or the Rocky Mountains in California, may be required to be able to carry loads of over 300 pounds per square foot (61 kg per square meter).