WeatherFanatics: Winter Rooftop Risks: What You Need to Know to Prevent a Collapse

Monday, March 10, 2014

Winter Rooftop Risks: What You Need to Know to Prevent a Collapse

By Kristen Rodman, AccuWeather.com Staff Writer

March 10,2014; 6:20PM,EDT

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Although spring is on the horizon, the detrimental impacts of this year's harsh winter still loom as threats for roof collapses continue.
"Usually one single storm by itself is not enough to ever cause a roof collapse," AccuWeather Forensic Meteorologist Steve Wistar said.
However, a winter like this one, with a series of back-to-back snowstorms and numerous cold fronts let multiple snowfalls accumulate without allowing snow to melt in between storms, increases the risk for roof collapses.
This winter following a series of snowstorms in the Northeast, the roof of the WGAL Channel 8 studio in Lancaster, Pa., partially collapsed due to snow and ice on Friday, Feb. 14, 2014, according to Lancaster Online.
"This winter there were some collapses in February because the snow had been accumulating since December, so there was a buildup," Wistar said.
The shape of a rooftop is an important factor when determining the vulnerability of a roof. A pitched or slated roof is less of a risk than a flat roof.
"As snow sits on roofs, especially flat ones, it compacts and becomes more dense," AccuWeather Meteorologist Mark Paquette said.

Part of the roof at Markham Metals Inc. is collapsed due to the accumulation of heavy snow in Wilmington, Mass., Wednesday, Feb. 2, 2011. (AP Photo/Elise Amendola)
A flat roof does not allow snow to easily work with gravity and slide off the rooftop and as a result the snow and ice can make the load on the rooftop heavy, thus weakening the house's internal structure over time and eventually causing it to break and collapse.
Other than the shape of the roof, what's positioned on top of it can influence how well it holds up in ongoing winter weather.
The worst situations are flat roofs with a parapet, or roofs with a wall around the edge, or those with heating, ventilation and air conditioning, or HVAC, units because these trap the snow when the wind blows and cause a snow drift to build on one side of the rooftop, according to Wistar.
Besides the rooftop itself, the type of snow dumped during a storm can greatly effect the likelihood of a collapse.
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Storms that bring heavy, wet snow to an area are the most dangerous due to the high water content in the snow. The higher the water content, the more weight the snow puts on a rooftop.
However, rain after accumulating snowfall can also be harmful as the snowpack will then absorb the water and become even heavier.
While roof collapses today are much less likely than they used to be due to updated building codes requiring every roof be built to hold a specific amount of load, there are three factors that make a rooftop more susceptible to collapsing.
Rooftops on unheated structures like barns or garages are more prone to collapse when compared to heated structures, according to Professor of Civil Engineering at Rensselaer Michael O'Rourke.
"Heat flow up through a roof reduces the roof load over time and thus reduces the susceptibility," O'Rourke said.

Snow falls into the field from a hole in the collapsed roof of the Metrodome in Minneapolis Sunday, Dec. 12, 2010. The inflatable roof of the Metrodome collapsed Sunday after a snowstorm that dumped 17 inches (43 cms) on Minneapolis. (AP Photo/Ann Heisenfelt)
Built before any building codes, older buildings also have increased instability, because over time the structural integrity of the roof lessens and things rust and rot, according to O'Rourke.
Although many buildings today are built with steel or concrete, wooden roofs are also prone to collapses.
"Unlike steel and concrete, wood gets weaker over time when subjected to significant loads," O'Rourke said.
Even if a collapse does not occur, leaking and ice dams are other threats that accumulating snow on a rooftop can cause if not properly taken care of.
To safely remove snow from a rooftop or detect potential problems with a roof, see the tips below from Professor of Civil Engineering at Rensselaer Michael O'Rourke.

Tips to Safely Remove Snow From a Roof:

1. If the roof has an access point, look and see if any components are sagging, creaking or cracking. 2. Turn up the heat in the house, as this can help melt the snow on the roof.
3. Get a snow rake and reduce the snow load by standing on the ground and pulling snow off the roof.

Have questions, comments, or a story to share? Email Kristen Rodman at Kristen.Rodman@accuweather.com, follow her on Twitter @Accu_Kristen or Google+. Follow us @breakingweather, or on Facebook and Google+.

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