A weekend blizzard piled tons of snow on rooftops in downtown Pratt, Kan., causing roofs on some older commercial buildings to collapse and water to seep into ceilings.
Several downtown businesses were evacuated and two were declared unsafe for occupancy Monday after melting snow caused a partial roof collapse and a complete ceiling collapse, according to local news reports. The collapse prompted firefighters to inspect all downtown businesses as well as public buildings, including schools, to look for more potential roof failures.
The potential exists for more collapses and interior water damage as the snow melts. The Institute for Business & Home Safety (IBHS) has developed the following guidelines to help determine when a roof may be at risk of collapse:
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.
- Your older wood structures are more likely to have issues with snow loads.
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 light weight roofs.
- Flat roofs, the step-down area between roof sections is a potential source of roof overload.
- Most roof designs can handle at least 20 lbs per square foot. [20#'s are typical in the Midwest.]
These designs can range from:
10 to 20 lbs per square foot in Mid-Atlantic states
40 and 70 lbs 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 lbs 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 lbs 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 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.
Hope this helps.
a•r•c
Gerry & Folks
