Ice, Snow Take Toll on Northeast Roofs, But Engineers Say Codes Are Adequate
A Nottingham, N.H., public works employee was gassing up his truck when he heard a noise and suddenly the roof of the town’s highway maintenance garage collapsed. That was at 3:30 p.m. on Feb. 8 just after the day shift had ended.
“Workers often gather after work, but luckily they didn’t that day,” says Charles Brown, Nottingham’s town administrator.
While the cause of the 2,520-sq-ft building collapse is still under investigation, Brown says snow accumulation on the roof likely contributed to failure of a truss. This is just one example of a structural collapse in New England following a series of severe snowstorms and freezing rain that have made accumulated snow denser and heavier, especially in southern New England.
Both New Hampshire and Vermont, which have received mostly light powdery snow, have experienced only a handful of collapses, including many barns. However, in Massachusetts, which received wetter, heavier snow, 172 roof failures were reported to state emergency officials from Feb. 1 to Feb. 9, including 98 commercial or industrial buildings and 10 institutions, including schools and churches.
Paul Brady, director of the American Council of Engineering Companies of Connecticut, said state fire marshals were gathering data on collapses of an estimated 75 structures. Most collapses in New England have involved structures with long-spans, including web-joist structures with flat roofs; modified designs or older abandoned buildings, according to Brady and other engineers interviewed for this article.
Matthys Levy, chairman emeritus at Manhattan-based Weidlinger Associates and author of Why the Wind Blows: A History of Weather and Global Warming, says he was not surprised by this winter’s weather since researchers expected a cold La Nina winter associated with the cooling of the Pacific Ocean. “While it is difficult to pinpoint the overall effects of global warming and La Nina, it is agreed that New England will generally be wetter in coming years,” he says.
Levy, who lives in Vermont, and other engineer sources for this article say existing building codes are adequate if designers allow for a factor of safety. But Levy recommends designers make proper allowances for snow accumulation such as when designing roof structures near parapet walls, especially those that are four feet or higher.
“We’ve had many roof collapses this year involving accumulation of snow on Main Street roofs when snow builds against the parapet,” he says. “You have to design for these kinds of snow loads,” he says.
Multi-level roofs with steps instead of roof flashing can also be problematic with snow accumulation on a first-story portion, for instance, piling up against the second story, Levy says. “It’s best to avoid stepped designs unless you design for them,” he says. While an average of 40 psf may be adequate for designing most of a roof, designers should allow 60 psf to 80 psf for the parapet wall, he says. And mountainous regions may require twice as much strength, he says.
Garrick Goldenberg, professor of structural engineering at Wentworth Institute of Technology in Boston and chief structural engineer at Chappel Engineering Associates in Marlboro, Mass., said that while state building codes address snow drifts with requirements for the shape and slope of a roof, this year’s record snowfalls and ice accumulation with little thawing has led to a greater number of collapses.
In sampling snow at Chappel Engineering in early February, Goldenberg says his group found that even two feet of freshly fallen snow or more totaling 16 to 20 lb/sf was not a danger to buildings. On average, people assume fresh fallen snow produces eight pounds of pressure on a roof per square foot.
However, invisible loads caused by accumulation of ice have been a serious problem since ice weighs 7.5 to 8 times more per cu ft than snow. “In many cases, even before reaching two-foot snow loads, we were in excess of 30 pounds because of the ice,” he claims.
Solidification of Melt
This occurs when two layers of ice form in the snow from solidification of melt off when the temperature drops at night, he explains. One layer of ice can form during a cold but sunny day in the upper section of snow from melt off that penetrates into the snow and the other forms at the lowest level while the building is heated. This invisible load has been overlooked by many property managers, Goldenberg claims, since this has not been a typical problem in prior winters with time for snow to melt between storms.
Recently Goldenberg has been busy consulting on snow removal for prevention of collapses. “Of the 12 schools and office buildings we have assessed, some had more snow than the roofs were designed for. We have prepared layouts so that in some cases, property managers only had to remove 20% of snow in critical areas,” he says. He suggests the state consider developing an educational program to educate building owners about unusual situations involving snow, wind and ice.
Donald Dusenberry, senior principal at Simpson, Gumpertz & Heger in Waltham, Mass., says of the 40 distressed roofs the firm has assessed in the past week, his group found only a few buildings where the total weight of ice and snow was approximately equal to or slightly above design code requirements. “The design code has not been proven wrong,” he says.
“Most collapses are related to details of design, structural deterioration or building modifications” says Dusenberry, who chairs the design-load committee for ASCE. “The latest code edition has not impacted construction in Massachusetts,” he says.
However, property owners should be cognizant of any structural changes and assess whether they have proper drainage to avoid refreezing of any melt off. “If an owner builds a new roof, they should be sensitive to issues that can affect the total load on the roof during the snow season,” he says.
