Determining the Weight of Snow
The weight of snow can be determined by multiplying the snow depth in feet by 10-15 pounds per cubic foot,
depending on relative wetness. Snow that has partially thawed and refrozen has the density of ice, which is 40-
60 pounds per cubic foot. Wet snow presents more of a problem than powder-like snow that falls when the air
temperature is well below freezing.
The Cornell Cooperative Extension Service publishes this table to measure the weight of snow loads:
| snow depth on roof (ft) | "dry snow" lbs/sq. ft | "wet snow" lbs/sq. ft | |
| 1 | 3 | 21 | |
| 2 | 6.5 | 42 | |
| 3 | 9.5 | 62 | |
| 4 | 12.5 | 83 | |
| 5 | 15.5 | 104 |
At-Risk Structures
Buildings that are particularly susceptible to roof collapse include older buildings with wood bowstring truss
roofs, pre-engineered buildings, and buildings with steel joist roofs. Unoccupied buildings are also particularly
susceptible to damage. Since the buildings are not in use, it is unlikely that they will be inspected and unless
cleared, accessibility for building operators to take emergency measures is limited. Poorly maintained roofs or
roofs with temporary repairs are also at a greater risk. Uneven snow loads also increase the likelihood of a
collapse.
Buildings that are retrofitted with additional insulation for energy conservation must have high load capacity.
Insulation adds dead load and retards heat loss. This, in turn, retards melting and allows for increased snow
accumulation. Before undertaking an energy conservation project, check with the architect for loading capacity.
Prevention
Educational institutions can promote the life of their buildings pre-construction (new or roof replacement) by
having the architect check roof construction with the anticipated snow fall and snow load in the region. Officials
should complete preventive inspections of buildings to identify areas where snow can accumulate, during
structural safety inspections, for example. Safely remove rooftop accumulations in vulnerable areas, if possible.
Only crews with experience and training should be allowed to do this - no exceptions. Have someone with
knowledge of the roof locate and mark the position of skylights prior to the start of the job.
There are certain signs that a collapse may be forthcoming. According to the experts at Cornell Cooperative,
metal structures may not show stress before failure, however, wood structures often do. Signs of failing wood
structures include:
*Creaking or moaning in the building
*Bowing of truss bottom chords or web members
*Bowing of rafters or purlins
*Bowing of headers or column
*Leaks in and around skylights
Preventing Frozen Pipes
Pipes burst as a result of increased pressure in the pipe when there is ice in the line. Once there is a thaw
(typically after heat is resorted), water is released where the crack or bursting occurred.
At what temperature do pipes freeze? This depends on how well the pipes are protected. Pipes that are
properly insulated and are located on the inside of the building are less likely to freeze. However, openings,
cracks or unsealed construction openings in outside walls that allow cold exterior air to flow across and around
the pipes increase the chances of freezing. In southern climates, where insulation is designed for average
regional winter weather, outside temperatures below 20°F may be sufficient to cause freeze-ups.
What can be done during cold weather to prevent freeze-ups? During a cold snap, there are several actions
that can be taken to reduce the likelihood of a freeze-up:
Maintain adequate heat. Minimum temperatures may have to be adjusted during periods of extreme cold
weather. You may want to set the thermostat at the same temperature during the day and night.
Let the water run. Letting a faucet drip (with water from both supply lines) helps reduce pressure in the
pipes if needed.
Checking on vulnerable areas. Check basements, crawl spaces, storage areas, attics, garages that
have bathrooms or utility sinks, and under cabinets facing noninsulated exterior walls. Areas where pipe
have frozen in the past should be monitored. Temporary fixes include safely using portable heaters to
raise the air temperature, adding insulation, and blocking exterior openings. Keeping outside doors
closed and cabinet and closet doors that house plumbing open can help prevent freezing.
Make sure the damper of unit ventilators are working properly.
Check to see if the temperature sensing devices (i.e., freeze stat) are connected and functioning.
If you suspect frozen pipes. Keep the faucet open and shut off the water. Check other pipes in the building to
see if any others are affected. Do not use a blowtorch, kerosene or propane heater, charcoal stove, or other
open flame device to thaw a frozen pipe. If you are going to attempt to thaw out a pipe, use a hair dryer. Raise
the air temperature in the area also.
Resources: Wright Specialty Insurance Company www.wrightspecialty.com