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Roofs & Snow Loads: What You Need To Know During Winter Storms

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From DC.gov:

Residents should call 311 in the event of an emergency building collapse. Also, call 911 for assistance with any medical needs related to the event.

Winter brings a heightened awareness of ice and snow problems. Ice and snow in our face and under foot cannot be ignored, but ice and snow on roofs is sometimes forgotten unless there is a leak or worse. With the large and multiple storms we have experienced recently, excessive ice and snow loads can overload a building's structural members and sometimes even cause a roof collapse.

Excessive snow loads are usually the result of wind creating large snow drifts. Snow drifts can create concentrated loads on roofs that are well in excess of the loads imposed by uniformly distributed snow. Winter rain storms and ice buildup can further increase roof loads.

Drifted snow conditions are common on pitched roofs, curved roofs, in the valley between adjacent parallel pitched roofs, on the lower levels of multilevel roofs and on roof areas adjacent to projections. Drifted snow conditions also occur on roofs with parapets, rooftop mechanical equipment, solar collectors, and other obstructions which provide an area sheltered from the wind.

Large roofs are more prone to snow drifting because there is a larger volume of snow available for drift formation. Another potentially hazardous condition occurs where snow can slide off sloped roofs onto lower roofs.

Warning Signs

Fortunately, few roof failures occur without warning signs. In one school a laminated wood beam broke with such force that security personnel reported hearing what sounded like a gun shot. Warning signs of structural roof problems include roof leaks, cracks in walls and ceilings, and excessive sagging of structural roof elements or ceilings.

Hung ceilings often hide the roof structure but will sag if the roof has excessively deflected. These warning signs should be looked for during the annual structural inspection. Signs of structural roof problems are usually more visible during periods of heavy snow loads. School personnel should include roof inspections during such periods as part of the annual visual structural inspection process.

The roofs of older buildings which have been re-insulated for energy conservation must also be reevaluated to make sure they 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 when there are multiple storms. Replacing a non-ballasted roof with a ballasted roof also may reduce the snow load carrying capacity.

Solving Excessive Snow Load Issues

Two means of solving excessive snow load problems are to reinforce roof areas to handle large potential snow accumulation, or to remove the snow to maintain snow loads at acceptable limits. Know your limits and keep to them. If your buildings are showing any signs of structural overloading, an architect or structural engineer should be consulted. Repeated overloading of roofs can significantly weaken the roof structure over time.

Your best insurance against structural damage from excessive snow and ice loads is to keep a watchful eye on your roofs this winter. Do not forget to check roofs for plugged drains and ice accumulation under the snow.

Note: Information provided by Ryan-Biggs Associates PC via New York State Department of Education

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