Roofs carry snow load Time:2017/01/19 14:52:00 Hit:378
Roofs carry snow load
As the sun causes the steady drip, drip, drip of melting snow on rooftops of houses and businesses, the weight of the snow gradually decreases.
Snow can be very heavy. If present in big enough amounts, like the 30 inch snow fall in late March 2009, can cause a roof to collapse as it did to the former Meigs insurance offices in the 300 block of south main, said Brad Blankenship, Pratt city inspector.
While snow is heavy, house roofs are built to withstand a lot of pressure, 30 pounds per square foot according to International Building Code, Blankenship said.
Truss roofs, like the Walmart roof, are built to withstand 40 pounds per square foot.
A roof is a very strong structure. For example, a 10-foot by 10-foot square roof has 100 square feet. At 30 pounds per square foot, that works out to be 3,000 pounds on that 10 x 10 foot roof. That is a ton and a half or about the same weight as a car.
Many of the homes in Pratt are 70, 80, 90 and some are 100 years old and they have no problems with supporting the weight of the snow.
"That's quite a bit of weight on a 10 by 10 roof," Blankenship said. "That just shows you how well houses are built and they don't even have the bracing we have now."
With all the snow from the two storms on the roof, the house holds up a lot weight and does it very effectively because it is designed that way, Blankenship said.
House roofs in this part of the country are built with a pitch so water will automatically drain off and not pool. Because the roof is at a pitch, it also provides natural drainage when the snow melts and lets the snow slide off the roof. The higher the pitch, the less the snow weighs on the roof.
Most homes have a built-in pitch to spread out the snow's weight, most businesses have flat roofs that have to withstand greater loads so they are routinely built to withstand 40 pounds per square foot.
A flat roof isn't really flat, it has a shallow pitch to allow for drainage. For every 12 inches of length on a roof it must have four inches of rise. On a 10 feet roof section, that would amount to 40 inches of rise.
While the pitch on a flat roof is designed to carry a bigger load, some failures are going to happen. In 2009, the Meigs offices could not support the weight of the 30 inches of snow plus the additional snow that accumulated because the buildings on both side were taller and allowed more snow to collect on the roof.
Metal roofs can present a problem. The longer snow stays on a metal roof the more pressure it puts on the roof and it tends to bend. It can take stress but not for extended periods of time. Flat metal is not as strong as bent metal. So bent metal is used for roofing purposes.
The awning in front of Fabulous Finds is now supported with a pair of poles. The metal rods holding the awning have been there for a long time.
Metal gets fatigued the longer it is in place and the poles holding the awning in place has been there for a long time.
Another issue for melting snow is ice. As it melts it forms icicles that hang off roofs, gutters and awnings. It can also collect on anything underneath it as it did on the east side of the Texas Energies building in the 100 block of East Fourth.
While the ice formed on the lines, it was next to the building so the weight was not pulling done on the entire line and the building was carrying some of the weight, Blankenship said.
While the ice can fall and cause injury if someone if under it when it falls, it usually doesn't cause problems unless it is thick on trees or power lines.
Ice on long stretches of power lines or trees can bring lines and tree branches down.
For some time the city has been trimming trees branches away from power lines, the main reason lines come down during an ice storm.
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