Snow-Induced Roof Failures and Prevention Methods

Guest Author:  Alyssa Stangl | BAE/MAE | Pennsylvania State University 2013

Every year, winter brings everything from light dustings of snow to intense snow storms – storms that are notorious for causing roof failures. As a result of this, snow loading is an important design factor for structures. Extreme snow loads have always been a cause for concern for designers and owners alike, and they are often responsible for roof failures and other structural problems, causing millions of dollars in damages and operations interruptions as well as public safety concerns.  It is the engineer’s responsibility to design a structure to withstand a wide variety of snow loadings and events.

Common Snow Failure Load Conditions

A few types of snow loadings are known to be common causes of roof failures.  These include snow drifts, unbalanced snow loads, rain-on-snow loads, and sliding snow. Engineers are required by the International Building Code (IBC) and ASCE 7 to calculate and design for these different types of roof snow loads.  Roof geometry and heavy snow falls can exacerbate these effects by creating higher than anticipated loads on a small area of a roof, as opposed to a uniform load over the entire roof, creating an unbalanced condition. These loadings are often difficult to predict but guidelines are available for addressing these situations. 

Snow drift is a common collapse culprit.  In general, drift loads need three elements to form: a source of drifting snow, wind capable of lifting and blowing snow, and a geometric irregularity.  The most common geometric irregularities are roof steps or changes in roof elevation, parapet walls, and gable roofs.  These conditions can occur at the same location at the same time creating a “combined drift” condition.  Another common culprit is sliding snow; this occurs when snow slides from an upper roof onto a lower roof.  This is of particular importance on roofs that are slippery and unobstructed or do not have snow guards.  If the snow falls from a substantial height, the issue of dynamic loads may be imposed on the lower roof as well.

In some cases, the snow load alone does not cause the roof to fail.  There often can be other contributing factors including inadequate design, heavy snow accumulation, unoccupied building status, roof contour and obstructions, limited building heat loss, lack of emergency procedures, and poor workmanship. 

Prevention

 Snow induced roof failures can be devastating; however, there are preventative measures that can be taken to reduce the risk of failure.  Some steps that can be incorporated include: monitor and inspect the roof for damage regularly and know the maximum “safe” snow depth for the roof in question.  When snow accumulates, monitor the snow depth and remove snow accumulations that reach 50% of the safe maximum depth if at all possible.  It is also important to keep snow, ice and debris such as leaves cleared from storm drains.  Furthermore, there are many warning signs to look for during or after a snow event that may indicate possible failure.  These include sagging ceiling tiles or sprinkler heads, popping or creaking noises, sagging roof members, doors or windows that cannot be opened or closed, cracks in walls, roof leaks, or cracked or split members. 

 Snow Removal

After a snow event, the method of snow removal is very important.  The key is to avoid creating an unbalanced loading condition by removing snow in an incorrect pattern.  As an example, RiskTopics: Snow Loading Roof Collapse prepared by Zurich Corporation provides a thorough step by step guide for the snow removal process for one type of steel framed building. 

It is important to note that the correct snow removal process and sequence depends on the type of roof system and framing components in addition to any unbalanced loading conditions that may exist.  As such, it is recommend that a structural engineer be consulted for any major snow removal situation.  

Some of the main points of the Zurich article are highlighted below:

  1. Remove drifted snow first.
  2. Remove snow from the middle bays of the roof structure.
  3. Remove snow evenly from both sides of the roof so that the live load remains as balanced as possible.
  4. Do not remove snow from one roof and place it on another.
  5. Be careful when removing snow or ice to not cause damage to the roof membrane. 
  6. Avoid walking on areas of the roof and compacting snow on adjacent roof segments.

Looking for More Information on Snow Failures?

For a more comprehensive discussion on this topic including additional recommendations and references,  link to the full article Snow-Induced Roof Failures and Prevention Methods available on the Failures Wiki.

Photo Credit:  Snow in Boston MA during Nemo Storm, Elizabeth Parfitt

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3 Responses to “Snow-Induced Roof Failures and Prevention Methods”

  1. Chris Collings
    July 21, 2014 at 11:59 am #

    Safe Roof Systems builds automated roof deflection alarms systems that identify joist deflection caused by accumulating snow, ice, or ponding water. Our alarm systems provide notification in real time – directing attention to the roof areas that require action. saving time, reducing risk for personnel and saving facility managers a lot of money – every year.

    please visit our web page – http://www.saferoofsystems.com for further details.

    Best regards,
    Chris Collings
    Safe Roof Systems Inc
    http://www.saferoofsystems.com

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