Chapter 8 - S: Snow Loads
© 2007, T. Bartlett Quimby
Last Revised: 11/13/2007
Snow loads are prevalent in northern and/or mountainous regions all over the world. The snow load provisions of ASCE 7-05 provide guidance for determining the magnitude of those loads based on geographic location and the nature of the structure being considered.
In colder regions, the peak snow load is not the result of a single event. It is the result of accumulation from many storms over the course of a winter season. In between winter storms, the roof systems that support the snow may lose some of the accumulated snow as the result of wind activity, melting from warm temperatures, or melting from building heat.
Roof slope, roof sheathing materials, the thermal characteristics of the structure, and exposure to wind all have an impact on the amount of snow that may be present on a roof over the course of a winter season. ASCE 7-05 accounts for each of these factors.
The basis for ASCE 7-05 snow load computations is the ground snow load, pg. This value is modified to become a flat roof snow load, pf, by multiplying by a constant that accounts for roof snow loss that ground measurements don't see. In addition, the value is modified by coefficients that account for building exposure to wind, the thermal characteristics of the building, and the importance of the structure.
For sloped roofs, the flat roof snow load is modified to account for slope and the roughness characteristics of the roof. Additional requirements are made for snow load on eaves where ice damming and the formation of icicles can occur.
Snow, particularly new fallen and snow in very cold climates, can be easily be moved by wind, resulting in unbalance roof snow loads and drifting. The imbalance may create critical loading cases in some structures. Drifting creates surcharge loadings on lower roofs that are in the wind shadow of higher wind obstructions. It is important to quantify these effects.
Finally, sloped roofs may shed the snow that falls on them. The snow that slides off a higher roof onto another one creates additional loading on the lower roof that must be considered.