Deep snow can mean serious trouble, and rain on top of snow can significantly add to the weight of snow trapped on top of a roof. Even a partial roof collapse can cause extensive damage to the structure and interior contents of a property. Melting snow is just as bad and can cause flooding through leaks in a roof structure.
The age of a building is a major factor in its snow load risk. Newer building codes provide much better guidance for estimating maximum snow loads, while older buildings or buildings with light roofs such as metal buildings have a lower tolerance for built up snow on their roofs.
The best source for determining how much snow a building's roof can handle is the design plan. These designs can range from carrying a load of 10 to 20 lbs per square foot in the Mid-Atlantic, to between 40 and 70 lbs per square foot in New England.
The Institute for Business and Home Safety offers these general guidelines to help estimate the weight of snow a building can tolerate:
- Fresh snow: 10-12 inches of new snow is equal to about 5 lbs per square inch 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 about 5 lbs per square foot of roof space, so any amount over 2 feet of packed snow could be dangerous.
- Ice: Ice is 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 for 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 and into the attic or down the inside walls. Commercial buildings with flat roofs are particularly vulnerable to water leaks if ice dams keep water from flowing into roof drains.
Severe weather can't be avoided, but following our guidelines will help your property avoid damage that may accompany it.
UPDATE: Perfect reason to be cautious of how much snow accumulates on your roof!
"Two Rescued After Roof Collapse in Lynn" - WHDH-TV