February, 2009
News in Brief
Inspection News and Views from the American Society of Home Inspectors

Prevent Roof Collapse


Reprinted with permission from The Institute for Business & Home Safety’s Disaster Safety newsletter at www.disastersafety.org.

A series of roof collapses and reports of extensive interior water damage prompted engineers from the Institute for Business & Home Safety (IBHS) to develop guidelines to help educate home and business owners about the risk factors posed by ice and snow and what steps to take to reduce their risk.

Here are guidelines developed by Tim Reinhold, IBHS chief engineer and senior vice president of research.
The age of the building can be 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:
  • There is usually a larger margin of safety against excess snow loads for heavy roofs than for lightweight roofs.
  • On flat roofs, the step-down area between roof sections is a potential source of roof overload.
  • Most roof designs can handle at least 20 pounds per square foot.
These designs can range from:
  • 10 to 20 pounds per square foot in Mid-Atlantic states
  • 40 and 70 pounds per square foot in New England
Estimating the weight of snow:
  • Fresh snow: 10-12 inches of new snow is equal to one inch of water, or about 5 pounds per square foot of roof space
  • 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 about 5 pounds per square foot of roof space.
  • 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 pounds per square foot of roof space, which is getting close to 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.