Days of severe winter weather and heavy snowfall have resulted in roof collapses on commercial buildings in Massachusetts, New Hampshire and Washington State. Emergency managers in these states are warning facility managers to pay attention to the accumulation of ice and snow on their buildings. The Institute for Business and Home Safety is offering the following information and engineering guidelines that can help avoid what in many cases are preventable events.
The age of the building is a major factor in the snow load risk
Newer building codes provide much better guidance for estimating snow loads, particularly the increased loads near changes in roof elevations where snow drifts and snow falling from the upper roof can build up on the lower roof near the step.
Older roofs can also suffer from corrosion of members and connections which can reduce its ability to resist high snow loads.
Buildings with light weight roofs, such as metal buildings or built up roofs on bar joists generally provide less protection from overload than heavy roofs.
The safety margins used by engineers are based on a combination of the weight of the roof and the snow loads. Consequently, there is usually a larger margin of safety against excess snow loads for heavy roofs than for light weight roofs.
For flat roofs, the step-down area between roof sections is particularly susceptible to roof overload because of the tendency for ice and snow collection, especially during periods of windy weather.
Roof top equipment and roof projections, such as mechanical equipment, which is over two-feet tall causes snow accumulation due to drift, creating the need for higher snow load consideration in these areas.
An even more serious condition can be created when a taller building or a taller addition is built adjacent to an existing building.
Unless the existing building is strengthened in the area next to the new taller building or addition, snow accumulation on the lower roof near the step could produce much higher loads than those considered by the original designer for the existing building.
The design plan is the best source for determining how much snow load a building can handle.
These designs can range from a snow load of 10 to 20 lbs per square foot in Mid-Atlantic states; between 40 and 70 lbs per square foot in New England.
IBHS offers these general guidelines to help estimate the weight of snow:
Fresh snow: 10-12 inches of new snow is equal to one inch of water, or about 5 lbs per square foot of roof space, so you could have up to 4 feet of new snow before you need to worry.
Packed snow: 3-5 inches of old snow is equal to one inch of water, or about 5 lbs per square foot of roof space, so anything more than 2 feet of old snow could be dangerous.
The total accumulated weight of two feet of old snow and two feet of new snow could be as high as 60 lbs per square foot of roof space, which is getting toward the design limits of even the best designed roof.
If there’s ice, it’s much heavier, with one inch equaling about a foot of fresh snow.
For safe removal that won’t endanger you or damage your roof, consult a roofing contractor.
For more information visit the IBHS Web site: www.DisasterSafety.org.