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We are in the throes of winter, and many roofs in the northeast are covered with snow. When does the amount of snow that has settled on a roof become hazardous to its structural integrity and performance? Sooner than you may think.
Recent news reports (Click image below to see roof collapse)
 | | Video courtesy of Boston.com |
indicate a large number of roof collapses as a result of the unusually high amount of snowfall this year. Ongoing winter storms, characterized by high winds and heavy snow fall, result in irregular snow accumulations across buildings and landscapes. Structural problems arise for homes and businesses when snow settles on a relatively small portion of roof that is not equipped to handle the additional weight. On Cape Cod, before 2007, the Building Code required a minimum uniform design load for snow of 25 psf for roof designs, which is equivalent to 15 inches of snow fall. Windy storm patterns bring snow drifts that can easily pile up several times that amount, resulting in a tremendous overload on the roof.
 Properly built roof structures are designed to withstand a certain amount of snow load. Irregular shaped roofs or abrupt changes in the roof elevation are susceptible to heavier loading due to accumulations from drifting or sliding snow. In geographical areas vulnerable to severe winter storms, building codes require the structural design engineer to take irregular roof shapes into consideration when designing roof structures. Most engineers will either specify heavier joist members or closer spacing of typical joist members in the high snow load influence area to address this situation.
Older buildings, built before the advent of modern codes, can be at risk due to changes in the roof configuration affected by newer additions or by alterations to adjacent structures. When adjacent to a taller building, low, flat roofs, by nature of their geometry, are at greater risk to snow drift accumulations compared to conventionally pitched roofs. Some newer buildings with additions are vulnerable because the addition may have a roof structure at a different pitch than the original structure. This can create a situation for blowing snow from one roof to slide or to settle on a lower adjacent roof, thereby creating an overload. An ounce of prevention is worth a psf of cure during winter season. Building owners and property managers can take steps to reduce the likelihood of a roof structure being compromised due to snow overload conditions. 1. Maintenance - inspect the roof and make necessary repairs. 2. Clean roof drains, gutters, and downspouts. 3. Inspect flashing and roof parapets (a parapet is a wall-like barrier that extends above a roof) for damage from previous snow overloads. 4. Install sliding snow stops on high roofs to prevent snow from dropping onto lower roofs. 5. Check for any excessive deflection of roof framing members or unexplained cracks in the building. 6. Safely and cautiously remove accumulated snow from the roof. All roofs are not created equal; short of a full review of the existing building plans and a comprehensive evaluation of the existing roof structure, it is difficult to determine whether a roof may fail when faced with a snow overload condition. When in doubt, engage the services of a design professional to evaluate the existing structure. In most cases, a properly designed and constructed roof should not fail under heavy snow load conditions. Most collapses are due to either a design or construction defect. A general assessment of the existing structure and basic review of the design plans can help alleviate any concerns about the integrity of the existing building. 
At minimum, a consistent structural inspection and maintenance program during the winter months, and particularly after blizzard conditions with unusually high accumulations of snowfall, is good preventative medicine that can bring peace of mind during the long winter months. Get answers from the experts We're here for you. Let Coastal Engineering's expert design team help with proper roof structure design, so that roof collapse problems may be avoided. Contact John Bologna, P.E., for more information regarding our engineering design for roof structures, our building technology capabilities, and our construction oversight services. Send e-mail to John Bologna | 
