Building code-directed snow loading generally apply to roofs of simple geometry. Roofs with complex shapes and profiles require further in-depth snow distribution study by the design team. This paper has proposed a generalized methodology whereby the designers can proceed to do so.

Analysis of long-term weather data for a given location gives a project designer insight into the variables of wind, temperature, and snowfall over many winter seasons. The designer can then empirically validate the proposed code-driven snow loads and establish design load values above code minimums as may be necessary. By making empirical judgments based on long-term weather data analysis, the designer can plot predicted snow distribution contour lines on a project's proposed roof design and estimate the unbalanced snow mass. Freeze/thaw cycles, sliding, and snow consolidation are then factored in, and composite roof snow densities are estimated. Once that has been done, potential snow loading can be derived and compared with the proposed design loads. Loads exceeding normal design practice can now be taken into account.

Although these methods of determining snow distribution (snow load) on complex roofs are not precise, they don't have to be. In snow country it is better to be conservative and over-design than to have damage or collapse due to that 50-year storm season coming next winter.


1. Western Regional Climate Center Website,

2. Tobiasson, W., Buska, J. and Greatorex, A., "Attic Ventilation Guidelines to Minimize Icings at Eaves," Interface, January 1998, pp. 17-24. Updated version from Issue 21 of Energy and Buildings published in 1994 by Elsevier Science S.A.

3. Mackinlay, I., Flood, R., "Roof Design in Regions of Snow and Cold ", Proceedings of the Fourth International Conference on Snow Engineering. Trondheim, Norway, 19-21 June, 2000. Rotterdam, Balkema: 213 224.

1 2 3 4 5 6

Snow Distribution on Complex Roofs
Presented at the Fifth International Conference on Snow Engineering, Davos Switzerland, July 2004
by Ian Mackinlay FAIA and Richard S. Flood AIA/CSI.

Case study building designed by the architectural office of Bohlin Cywinski Jackson, Seattle, Washington, U.S.A. The photograph in Figure 1B is by Ian Mackinlay. Illustrations for Figures 3, 4A, 4B, 6, and 7 are by Molly Blunden.