When should you shovel your roof? Snow load is something to worry about, but maybe you should call a professional

Feb. 12--YAKIMA, Wash. -- As the owner of several historic buildings in Yakima, Joe Mann might worry about the snow piling up on all those expansive flat roofs. Or not.

Mann isn't concerned at this point, he said early Monday afternoon.

"We didn't do anything back in '96," Mann said, referring to the historic December when two major storms slammed the Pacific Northwest within a week.

That week, heavy snow overwhelmed the roofs of the new Entiat High School gym and structures in Manson and Chelan. A barn collapsed in Outlook and snow finished off the abandoned 90-year-old Marcus Whitman School in Naches Heights.

Much more recently, a family of five was displaced when heavy snow partially collapsed the roof of their home in Outlook late Saturday. No one was injured and the family is being temporarily lodged in the Sunnyside area, the Yakima County Sheriff's Office reported.

For now, Mann is waiting but also watching the weather closely.

"If we get another dump like we did last week ... it could be a problem," he said.

Only minutes earlier, Cliff Moore, Yakima's city manager, urged property owners to keep an eye on their buildings and homes amid the continuing snowfall.

"You should monitor the snow load on your roof," Moore said during a news conference in City Hall.

Homeowners -- particularly those with flat roofs -- who think they're getting too much snow piling up overhead should hire professionals to remove it, Moore said. "We do not suggest that you get up on the roof yourself," he said.

And there's the challenge -- what too much snow looks like. Modern roofs built to code in areas that get a lot of snow could theoretically hold up to 4 feet of snow, but snow's weight varies. Looks can be deceiving because wet snow and powdery snow have different densities and weights. Water and ice under the snow, especially ice dams, further stress a roof.

According to the Insurance Institute for Business & Home Safety, unless the roof structure is damaged or decayed, most residential roofs should be able to support 20 pounds per square foot of snow before they become stressed.

Snow load construction requirements vary, with roofs in snow country designed to hold much heavier loads. The Structural Engineers Association of Washington -- founded in the wake of the 1949 Seattle earthquake -- supplies guidelines to the Washington State Building Code Council, said Glenn Denman, building official and supervising code inspector for the City of Yakima.

For the city of Yakima, that's 30 pounds per square foot, according to Denman. That figure is based on area case studies.

Heavy snow can further stress flat roofs, most often protected by membrane roofing. But again, snow removal should be left to professionals.

"We've shoveled and then you have a thousand leaks because you break that membrane," Mann said.

Do-it-yourselfers who want to tackle the snow load on their roofs may use roof rakes, which are a safe and effective way to lighten the structural load and prevent ice dams. It will take some practice, and property owners should be mindful of gravity and where snow and ice may fall.

"Take small bits around the edges first," Denman said.

City code officials red-tagged a commercial building Monday morning at 505 S. Third Ave. because a wall on the north end is pushing out, Denman said.

"Red-tagged" means that area of the building can't be used until the wall is stabilized, Denman said. Though the stress could be caused by a frost heave issue -- the upward swelling of soil during freezing conditions -- code officials are still requiring the business owners hire a structural engineer for an evaluation, Denman said.

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