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University of Minnesota Extension Service
The following
is from Larry Jacobson and Kevin Janni, agricultural engineers
with the University of Minnesota's Extension Service:
Recent
weather conditions in southern and western Minnesota may result
in excessive roof loads on agricultural buildings due to the
accumulation of snow and ice. The November ice storms that
hit the area left one to three inches of ice on some roofs.
The recent large snowfalls have added to the total weight
that trusses and rafters must support. Because of this ice
layer, snow that has recently fallen on the roof is not sliding
off like it normally does in the winter. Since it's likely
that the ice layer will not melt until spring, each additional
snowfall this winter will intensify the problem.
How much
weight can most roofs take? Snow loads for agricultural buildings
in southern and western Minnesota are generally around 20
pounds per square foot. This does not include the weight of
the wood members (dead load) that make up the truss or rafter,
a ceiling if one is added to the lower cord of a truss, a
wind load, or any equipment hung from the trusses or rafters.
Many roofs for livestock barns and machine sheds are designed
for a "total" (sometimes confused with snow load) load of
25 to 30 pounds per square foot. This assumes that the building
was properly designed by a registered Professional Engineer
(PE), and constructed by a qualified builder according to
the design.
So how
much does snow weigh? It depends. Start by asking how much
water, or ice in the solid form, weighs. A one-inch layer
of water or ice weighs approximately five pounds per square
foot. So a roof designed for a 20-pounds-per-square-foot snow
load could theoretically hold up four inches of ice. How much
snow is that? Meteorologists often estimate that about 12
inches of snow is equivalent to one inch of water. Using that
"rule of thumb," a roof should hold up to four feet of snow.
Wet and packed snow weigh more per inch of depth, meaning
that a roof may only be able to hold up to three feet of snow.
This
level of loading, 20 pounds per square foot, is not intended
to last all winter; there is a fatigue factor. A roof may
be able to support this snow load for several days or a few
weeks, probably no more than 30 days.
What
is a "safe" amount of snow to have on your roof over an extended
period of time? An educated guess would be about half of the
designed load (20 pounds per square foot), or about two feet
of snow or one inch of ice and one foot of snow.
Several
factors affect the amount of snow that can build up on a roof.
They include:
- Roof
pitch--snow cannot slide off flatter roofs (3/12 pitch or
less) easily.
- Drifting--wind
blowing snow around other buildings and trees can create
huge snow drifts and uneven snow loads.
- "Lean-tos,"
or roofs on other lower buildings, may "receive" snow or
ice sliding off another roof above.
- Shingled
roofs don't shed snow as easily as metal roofs.
- There
may be roof valleys or roof areas that collect a lot of
snow.
So what
do you do if you have too much snow on your roof? The simple
answer is to get it off as soon as possible. Generally there
is some time between a large snowfall event and possible structural
failure. Unfortunately, one good way to remove snow from a
roof is to physically get up on the roof and push the snow
off with a shovel and/or broom. There obviously is the safety
concern of falling off when working on a snow-covered and
icy roof. It's important to use ladders, safety ropes and
take necessary precautions. Snow rakes also can be used to
remove snow. When using a snow rake, use extreme caution when
working near overhead electrical power lines. Also, avoid
excessive scraping on the roof or trying to chip off ice.
These practices can damage the roof and lead to a leaky roof.
There
are other, more "innovative" methods of removing snow and
ice from roofs. One involves warming the inside of the building
sufficiently with large heaters to melt the ice layer, and
then hoping the snow and ice slide off. Obviously, a lot of
heat is necessary for even a moderately-sized building, and
it must be an open trussed structure (no flat ceiling), plus
have an uninsulated metal roof. Caution is necessary to prevent
large chunks of ice and snow that slide off the roof from
falling on people, animals or equipment.
For flat
ceiling buildings, putting heaters in the attic is generally
not recommended. That's because of the fire danger, the difficulty
of being able to provide enough heat to get the job done,
and the possibility of creating ice dams along the building's
eaves.
Hopefully
the remaining winter will provide some opportunity for the
existing ice and snow on roofs to melt or slide off. However,
if we continue to receive average or above normal snowfalls
you may want to monitor the snow load situation on your agricultural
buildings and take appropriate action. Check high risks areas,
and if you need to remove snow, be extremely careful.
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Disclaimer
and Reproduction Information: Information in NASD does not
represent NIOSH policy. Information included in NASD appears
by permission of the author and/or copyright holder. More
NASD Review: 04/2002
Web,V2MN,V4MN,Z6,Z7
NAGR5413
Sources:
Larry Jacobson, (612) 625-9733; Kevin Janni, (612) 625-3108
Editor:
Joseph Kurtz, EDS, (612) 625-3168, pkurtz@mes.umn.edu
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