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Snow on the Roof

The Basics

What this means for building owners is that snow is a concern in most areas, yet unpredictable from year to year. And it is not just the removal of snow that is of concern, but the load it places on our buildings and potential for collapse or other safety concerns. This issue of Engineering Advisor is devoted to what you should know and can do to prevent such problems – before it is too late.

The Risks

The risks to a roof tend to depend on whether it is a flat roof or a sloped roof.

A flat roof may be subject to structural distortion or even failure and collapse. The effect can be gradual and compounding. Distortion may create areas for ponding, which in turn create areas where loads are increased, thereby creating more distortion, etc. The eventual result could be a collapse but such activity can also lead to leaks as joints are stressed and fail.

A sloped roof will generally not have the load problems that a flat roof will, although collapses do occur. More common problems include leaks (and damage caused by leaks) as well as harm to vehicles and pedestrians from falling snow and ice.

In both cases, the structure of the roof may contribute or minimize problems. A flat roof with low parapets and little surrounding it may shed snow just by the wind blowing across it. Conversely, snow may be concentrated by the wind in certain areas such as walls or roof valleys and it is in these areas of concentration where the weight exceeds the design load for the structure. Both roof types may also have opportunities for drains and equipment to be blocked by accumulating snow, rendering them non-functioning.

Why Worry?

If the codes already address snow loads and the related risks, why should a building owner be concerned? There are several reasons why you may want to pay attention to the risk potential and perhaps take steps to avoid that concern.

First, the codes have been changing. Design snow loads are increasing. Regardless of whether you subscribe to current theories about climate change, the code authorities have determined over the years that previous requirements were simply inadequate. So if your building is older, it may not have been designed to withstand current snow loads.

Which begs the question as to whether it was designed properly in the first place. Code enforcement has also gotten more rigorous. But it is worth hauling out the plans (if you have them) to be sure that the building was designed per the code requirements.

And if the building was designed properly, it is still worth inspecting the building to ensure that it was built according to the plans. We have encountered many situations where field modifications have altered the intent of the design.

If pre-manufactured components were used, the next question to ask is, were they manufactured properly and/or were they damaged during shipment. We have seen situations where improper welds on properly designed trusses actually led to failures under certain conditions.

Any building system must be maintained in order to continue functioning properly. If snow is routinely removed from the roof, was that performed to minimize the damage to the roof? Have ongoing leaks caused damage or weakened structural members?

Finally, improvements can also impact the effect of snow. For example, energy upgrades mean less heat loss which may mean less snow melting and thus more snow accumulating on the roof.

Preventing Snow-Related Problems

The first step in preventing problems related to snow accumulation is a simple audit. This can be performed by internal staff or an engineering consultant who will confirm that the roof was designed and built to current codes, is free of defects, and has been properly maintained.

The second step is to have an Operations and Maintenance plan in effect long before the snow falls. There are two primary components to the plan.

The first component involves monitoring. What should the building maintenance staff look for to be alert to potential problems. Monitoring the weather, taking measurements of snow accumulation, and monitoring distortion or deflection are key elements of the plan. The latter can even be automated using photoelectric or laser technology.

The second component involves snow removal. When accumulation reaches the point where it represents a hazard, serious thought should be given to its removal. But that is not as easy as it sounds. Placing personnel on the roof, let alone equipment, may put the roof beyond its carrying capacity, thereby endangering those involved. Thus, you need to consider how the snow is to be moved, especially when there are parapet walls around the roof. And to where? There should be a designated space where an accumulation will not cause a problem. And finally, how should it be removed? Many techniques result in more damage to the surface, thereby creating new problems down the road.

© 2013 Criterium Engineers


The Engineering Advisor is intended to enhance your knowledge of technical issues relating to buildings.  For additional information on any subject, please feel free to call us.  Our commitment is to provide you with timely, accurate information.