Considerations of Snowload on Roof Design in Northern New York State.

snow load pic - edit

It’s officially Winter in the Adirondacks, and that means snow! Lots of Snow.

Being structural engineers in the Adirondacks, we have become quite accustomed to dealing with snow. The thing about snow is that it’s HEAVY. We think of snow in terms of snow load, which is the load determined when we design buildings and other structures.  The Tri-Lakes region (Lake Placid, Saranac Lake, and Tupper Lake) has some of the highest design snow loads in New York State.

The white fluffy stuff may not look it, but snow can contribute significantly to the overall load on a building.  Ground snow loads for portions of northern New York exeed 90 pounds per square foot.  For residential and light commercial structures, this can contribute up to 25% of the building’s total load.

As structural engineers, part of our job is to determine how much snow load to apply to a building. In the simplest terms, the more snow an area gets, the more snow load that is considered in our design. But it’s not quite that straightforward. Like most structural analysis, statistics and probability apply to the selection of the design snow load. The load must be acceptably conservative to keep the building’s occupants safe, without being so conservative that it is economically impractical.  Standard analysis uses a 50-year Mean Recurrence Interval to determine the acceptable risk; this analysis means that every year, there is a 2% probability that the ground snow load will be exceeded.

Snow loads on the ground don’t directly translate to snow loads on the roof.  Roof properties such as pitch,  material (shingles hold more snow than a metal roof), insulation, wind exposure, and the building’s importance (a shed versus a school, for example)  all affect how we calculate the roof snow load. We also anticipate and design for snow drifts, and snow from upper roofs sliding and piling up on lower roofs.

The correct calculation and application of snow load is essential to any thoughtful structural design, especially here in the North Country. For assistance in the design of new structures, or an assessment of an existing building, feel free to contact Joseph Garso, PE, at our office.

For those who are interested in an in-depth article on the subject of roof snow loads, follow this link:

http://www.structuremag.org/article.aspx?articleID=1593

(O’Rourke, Ph.D., P.E., John, and Jennifer Wikoff “Snow Related Roof Collapse and Implications for Building Codes.” Structure Magazine January 2013).

Author: Timothy J. Northrup, P.E.

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