Snow on the Roof A Potential Costly and Dangerous Condition

Snow on the Roof

 The winter months bring us a more heightened awareness of what a major snowstorm  can do to our commute to school or work and, our voicemails if we are a  property manager. However, with the heavy snow comes a very real potential for structural failure  of a condo association’s roof system that may have tragic results.  

 We have all seen on the news where a roof has experienced a complete or partial  collapse after a heavy snowstorm. The cause, referred to as, “excessive snow load” can result in significant structural and property damage and may result in  injury.  

 Typically, it is only a report of a water leak that brings attention to our  roofs in the winter months. Many times the leak is the result of ice damming in the gutter or melting snow  entering into a vent. However, sometimes the leaks may be from movement of the  structure due to the weight of the snow and ice.  

 On steep sloped roof systems, excessive snow loads are usually the result of  wind creating large snow drifts but they also may be from large accumulations  of wet snow from one or several storms. Excessive snow loads most often occur  in valleys or at a stepped location against a wall. Additional snow load as  well as an impact load with force can also occur when snow or ice slides off a  sloped roof onto a lower roof.  

 These scenarios can create concentrated loads on roofs that are well in excess  of the loads imposed by uniformly distributed snow and that may exceed the  design loads. Winter rain storms after a snow event and resulting ice  accumulation can further increase roof loads and the associated risks. It is  important to realize that that each roof has its own unique characteristics and  design standards for snow loads. Remember, only an expert with experience  calculating snow loads and design of roof structures should design a roof  system or assist with a particular repair.  

 What are the Warning Signs?

 Different types of roofing envelopes yield different signs and evidence of roof  stresses due to excessive snow loads. Warning signs of structural roof problems include but are not limited to sudden  roof leaks, cracks or movement in interior walls and ceilings, excessive  sagging of structural roof elements or ceilings and even unusual sounds  emanating from the attic space. Signs and evidence of a potential roof failure usually occur when the excessive  snow load occurs. However, and significant cracking or walls or ceilings or other concerning  changes in condition should be analyzed regardless of the season. Repeated or long term overloading of a compromised roof can significantly  weaken the roof structure over time by fatigue.  

 Flat roofs have their own challenges during winter snow events. A parapet wall  can cause wind driven snow to excessively accumulate adjacent to the wall. The  roofs of a building which have been reinsulated for energy conservation should  also be reevaluated to make sure the roof can handle additional loads of  accumulated snow. Added insulation may increase the probability that more snow  stays on the roof longer, thereby increasing snow loads especially when there  are multiple storms and increasing the fatigue factor.  

 Replacing a non-ballasted roof with a ballasted roof also may reduce the snow  load carrying capacity of that roof system. Not having the drainage benefits  like that of a pitched roof, the flat roof is only as effective as its roofing  substructure, roofing membranes and drainage design. The chronological events  of cold temperatures at night freezing the snow melt around roof drains and  scuppers create an excessive snow and ice load at these specific locations. Monitoring and maintenance of drains and scuppers should be exercised during  winter months.  

 Differential loading can also be a great concern. This occurs when there is a  large accumulation of snow load on one section of the roof and none on another.  This can result in reaction or movement of the structural members that could  turn problematic.  

 Prevention Begins with Design

 Prevention of excessive snow load problems starts with proper design. Typical  design standards/codes require allowance for a snow load of 30 pounds per  square foot, which equates to approximately 12–48 inches of snow dependent on how dense or wet the snow actually is. More  northern areas have higher requirements. It is best to be more conservative. A cubic foot of snow can weigh from 7 pounds  for snow that is new and dry (fluffy) to 30 pounds for compacted icy snow or  very wet snow. Therefore it is possible that a 12-inch wet snowfall is  approaching the design limits.  

 If your building is older or compromised from construction deficiencies, it may  be necessary to reinforce roof areas to handle large potential snow  accumulation. Alternately, it may be necessary to remove the snow or maintain  snow loads at acceptable limits of your individual roofing design. Snow removal  should be done by knowledgeable, experienced contractors. Special care should  be taken when removing snow from rooftops to prevent damage to the roof’s shingles or membrane. Snow should not be removed all the way down to the roof  covering. Plastic shovels rather than metal should be used or care taken if  using a mechanical remover. In the interest of worker safety, a lift or other  means should be used if the integrity of the roof is a potential concern.  

 It is important to stay current with preventative maintenance throughout the  year to maintain the association roofs’ integrity. It is equally important to monitor rooftop conditions throughout the  winter during a storm to identify ice buildup, snow drifts and blocked drains  especially if there is a concern about snow loads or a historical data  suggesting a design flaw. If there is a concern, have the roofs inspected. If  your buildings are exhibiting any signs of structural overloading as discussed  above, an architect or structural engineer should be consulted. Your best  insurance against structural damage and costly repairs from excessive snow and  ice loads is to keep a watchful eye on your association’s roofs during the winter.       

 Allen Campbell is a project manager with The Falcon Group, Engineering,  Architecture & Energy Consultants, based in Bridgewater, New Jersey.

 

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