Some roofs more likely to cave to winter stresses

CLEAN-UP WORK: The interior of the Pizzitola Sports Center at Brown University following this winter's roof collapse. / COURTESY BROWN UNIVERSITY
CLEAN-UP WORK: The interior of the Pizzitola Sports Center at Brown University following this winter's roof collapse. / COURTESY BROWN UNIVERSITY

Heavy snows this winter collapsed dozens of roofs in southern New England, with the structural design as much of a factor in many cases as the amount of snowfall.

Rhode Island’s winter 2015 snowfall as of March 6 averaged 73.2 inches, surpassing the state’s yearly average of about 47 inches. Although generally light in nature, the accumulating snow put added stress on building rooftops – commercial and residential – in both Rhode Island and Massachusetts.

In the Bay State, more than 200 buildings reportedly have suffered structural failure or had serious damage done to their roofs through the beginning of March, including a canopy-style roof covering the garden section of a Lowe’s in Seekonk and a portion of the roof at North Attleboro high school.

In Rhode Island, Brown University’s Pizzitola Sports Center fabric roof collapsed, giving way to mounds of snow that fell on unoccupied tennis courts. No one was harmed, according to the university, but the early morning collapse marks the second time the center’s roof has caved in since 2011.

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The current roof, which the university scrambled to repair, is made of an air-supported synthetic fabric, installed in 2012 after the original 25-year-old roof failed the year earlier, according the university.

“A stretched fabric roof is an effective way to cover a large area without posts or other supports, making it tight to the elements and allowing for heating and air conditioning,” Brown spokesman Mark Nickel said in an email. “It is less expensive and does not add the weight and stresses of a conventional roof.”

The material is also relatively easy to maintain and repair, according to Robert J. Dermody, associate professor of architecture, art and historic preservation at Roger Williams University.

“Fabric roofs are fine up until they’re not,” Dermody said pointedly, remembering the air-supported roof that once covered the iconic Hubert H. Humphrey Metrodome in Minneapolis, Minn.

Former home of the Minnesota Vikings professional football team, the Metrodome collapsed in December 2010 after sustaining more than 17 inches of snowfall in two days. The structure was ultimately razed in January 2014.

Despite the athletic-center mishaps, synthetic-fabric air-supported roofs are not the first type of covering that jumps to mind when Dermody thinks of structures susceptible to the dangers of heavy snow.

“Long-span, lightweight roofs,” typically seen atop box stores and commercial warehouses, are usually built more efficiently and have less structural redundancy, or support.

A run-of-the-mill New England residential building, on the other hand, is often built with a pointed roof and is integrally connected to the trusses of the house, giving it more structural redundancy and making it more likely to hold up under heavy weight.

And the theory seems to match the reality.

Alex Ambrosius, a spokesman for R.I. Emergency Management Agency, said through mid-February he hadn’t heard of any reported residential roof collapses.

“Sometimes we tend to forget, New England homes are built to withstand snow and large amounts,” Ambrosius said.

In most Rhode Island municipalities, roofs must be built to sustain at least 30 pounds per square foot, according to state building code. The code is based on the statistics of historical snowfall data, according to standards created by the American Society of Civil Engineers.

Specifically, the standards are created for an event that has a 2 percent chance of happening in any given year, based on a 50-year occurrence.

“The philosophy of the probabilistic approach used in this standard is to establish a design value that reduces the risk of a snow-load-induced failure to an acceptably low level,” according to the American Society of Civil Engineers.

But still – statistically – there’s a 2 percent chance for winter conditions to exceed the building-code requirements in any given year, which has happened in Massachusetts, where some municipalities received more than 100 inches of snow.

Also, the fact remains that not all roofs are built the same.

Materials and a building’s structural design play a large part in how strong a roof will hold. Nooks and crannies, or gables, are popular rooftop locations for snow drifts to accumulate in heights greatly exceeding average snowfall.

“The building code is still all done by humans and it’s an inexact science,” Dermody said. “The shape of the roof, the temperature, the drifting and the structure all play a part.”

RIEMA urges building owners to remove accumulated snow totaling 2 feet in height or more, but Ambrosius says it’s merely an estimate, as snow density fluctuates based on temperature, which can add to structural strain.

And as days get warmer, Ambrosius warns against the winter-spring “freeze-thaw” cycle in which hidden ice sometimes blocks drainage systems – preventing natural melted-snow runoff. The blockage can cause pooling, flooding and additional weight. •

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