Determining Critical Snow Build Up
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The Institute for Business & Home Safety (IBHS) today released information to help determine if snow buildup will be a problem and when removal is critical. "Deep snow is deep trouble," said Dr. Tim Reinhold, IBHS Director of Engineering. "And rain on top of snow can significantly add to the weight. Even a partial roof collapse can cause extensive damage to the interior contents of a home or business. When all that snow comes in, it melts, and can flood the building."
The age of the building is a major factor in the snow load risk, although snow load designs have not changed much in recent years. Light metal buildings will typically have less capacity to handle a high snow load. For flat roofs, the step-down area between roof sections is a potential source of roof overload because of the tendency for ice and snow collection.
The best source for determining how much snow load a building can handle is the design plan. IBHS says most roof designs can handle at least 20 lbs per square foot. These designs can range from 10 to 20 lbs per square foot in Mid-Atlantic states, and 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 5.2 lbs per square foot of roof space, so you could have 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 5.2 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 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.
To help minimize the risks from melting snow, IBHS suggests keeping attics well ventilated to reduce the risk of ice dams forming. A warm attic melts snow on the roof, causing water to run down and refreeze at the roof's edge where it's cooler. An ice buildup blocks water from draining and forces water under the roof covering and into the attic or down the inside walls.
Commercial buildings with flat roofs are particularly vulnerable to water leaks if ice dams clog roof drains. Water can also find its way inside on sloped roofs along the eaves.
To maintain proper drainage, remove snow from window wells and all walls. Clear debris from basement drains. Make sure gutters are clean and stable to ensure proper drainage.
IBHS research shows about $1 of every $5 paid for home and business property losses in recent years has stemmed from damage due to freezing weather, snow and water leaks. For more information, visit the IBHS web site http://www.DisasterSafety.org.
IBHS is an independent, nonprofit, scientific and educational organization wholly supported by the property insurance industry. The organization works to reduce the social and economic effects of natural disasters and other risks to residential and commercial property by conducting research and advocating improved construction, maintenance and preparation practices.