Drifted Snow and Snow Load: A Deeper Understanding

By: David J. Atkins, PE
Executive Vice President | NTD

The dog days of summer have ended, children are back in school, and before you know it, snow will be falling, but who really wants to think about snow after recently dealing with an earthquake and two tropical systems here in the northeast!

Have you previously observed unusual deflections in the roof structure of your building during a snowstorm? Or has a new piece of rooftop equipment been recently installed? If you answered yes, snow loads, particular drifted snow loads, may be an issue. This is a potential warning of a serious structural problem that requires a structural engineer. Owners need to be aware of the conditions of their buildings, not only to protect their property, but more importantly, to protect the safety of their employees.

During the design process, a structural engineer will determine the anticipated loads on the structure. This will include dead loads, or permanent items, such as lights, ductwork, utilities, and the structure weight itself. In addition, live loads must also be considered, and are required per the building codes and dependent upon the use of the building. Each roof structure must support any suspended item while maintaining a minimum uniform roof load. For most buildings, snow loads replace this minimum design load requirement. Interestingly, building codes have not always required the design professional to consider roof snow loads. The codes in place prior to the 1960s did not reference snow loads on structures. The design standards of that time suggested a minimum uniform distributed load acting on the roof during the structural analysis. In my opinion, the design professionals at that time considered snow loads based on experience and limited historical data without documented guidelines. This experience, as well as additional research and official collections of snowfall data, helped develop the design concepts used today for snow loads, especially drifted snow calculations.

It was not until the building codes of the 1980s that design considerations for roof snow loads and drifted snow criteria were officially outlined. In 1988, national building codes referenced structural guidelines established by the American Society of Civil Engineers (ASCE). This required the design professional to consider the terrain around the building site, roof slopes, adjacent structure heights, roof equipment greater than 15'-0” in width, roof projections, parapet heights, sliding snow effects, and even the thermal conditions inside the building. Depending on the situation, calculated drifted snow loads could exceed 100 psf. This was quite a change from the design requirements in the 1960s.

This does not mean that all buildings constructed prior to drifted snow load code requirements will fail. Realistically, multiple snow events and drifted snow, if the situation exists, has occurred many times during the life cycle of older buildings. If the older structure has not indicated signs of unusual deflections or other failures during past snow events, than the roof most likely has adequate structural capacity. These buildings may have been designed with an increased live load to support suspended equipment or utilities. Another possibility is that the interior space may generate high temperatures, keeping the roof surface warm and allowing the snow to melt. This keeps the snow from approaching critical depths that may cause failures. There are several methods used by structural engineers to review the actual loading and operating conditions of a building that could suggest why a failure has not occurred.

Owners need to be aware of the conditions of their buildings, not only to protect their property, but more importantly, to protect the safety of their employees. A structural engineer should evaluate projects that increase the potential for drifted snow on the roof or reduce the available live load capacity of the structure. This analysis may either conclude that the structural system should be reinforced or that the existing structure is adequate based on the current codes. Either way, this will avoid the panic-stricken phone calls that the “roof is falling” during the next snow event. Instead, everyone can enjoy their hot chocolate, sit back, and watch the snowfall.

Questions? Contact Dave Atkins at 717.434.1505 or datkins@nutecgroup.com