Two industrial facility roofs near the lakeshore and one in Kentwood suffered partial collapse recently as the snow kept piling up, but a structural engineer and an architect, both with many years of experience in large construction projects, say roof collapse is still a fairly rare occurrence.
On Jan. 25, a roof at the Haworth plant in Douglas with more than three feet of snow on it collapsed, causing extensive damage and rupturing a natural gas line. No one was in the building at the time, and the building is slated for closing this spring anyway.
Then on Monday, Jan. 27, part of the roof at The Stow Co. factory on 3311 Windcrest Drive in Holland collapsed under the weight of snow that had accumulated in one area due to high winds.
“It was relatively minor and no one was injured,” said company spokesperson Bobbie Locke. “Operations were only interrupted for a few hours; the plant is fine,” she said.
The collapse ruptured fire sprinklers in the ceiling, which soaked part of the office area, according to an Ottawa County dispatcher.
Finally, a roof covering a Lacks Enterprises industrial facility in Kentwood partially collapsed Wednesday night. No one was injured, but work at the assembly and distribution facility was stopped. Lacks officials say snow may be to blame for the damage.
A roof collapse is “fairly rare,” said Dan Vos, a vice president, senior structural engineer and head of the structural department at Fishbeck, Thompson, Carr & Huber in Grand Rapids. “It makes news when it happens,” he said, but considering how many large commercial/industrial roofs there are in West Michigan, it is rather uncommon.
In Grand Rapids, Vos said, engineers design a roof to hold at least 25 to 35 pounds of snow per square foot, which is based on the average snow depth on the ground here. But the average depth is higher along the Lake Michigan shore north of Grand Haven, which is reflected in the building code there.
In areas of a roof where wind currents may be expected to cause the accumulation of a larger amount of snow, the code requires a stronger design.
Vos said what really matters is the density of the snow on a roof. Dry snow may only weigh 10 pounds per cubic foot, but the same volume of wet snow can weigh 20 pounds. When rain is predicted on top of a lot of accumulated snow — as was the case during the recent holidays in West Michigan — some contractors will recommend removal of snow from roofs. The use of a snow rake is recommended because it’s too dangerous to try climbing up on a pitched roof with snow on it.
According to Jim Horman, an architect with Progressive AE, Grand Rapids happens to be in the northern end of ASHRAE Climate Zone 5, which stretches into southern Indiana and is a guide for how much insulation should be used in construction. ASHRAE stands for the American Society of Heating, Refrigerating and Air Conditioning Engineers.
Insulation is a key issue in roof design because it has a lot to do with how much snow will accumulate on the roof and how much melting will occur, causing ice build-up on the edges of the roof.
Horman said he is seeing fewer ice dams and big icicles hanging off roofs this winter because the intense cold has prevented any heat under the roof from melting the snow.
Horman said sometimes cleats are attached to metal roofs to catch and hold snow that might otherwise avalanche off onto sidewalks or entryways.
“We are not doing a very good job of collecting snow and reusing it,” quipped Horman.
He said using electric heating cables on edges of roofs are now seen as detracting from the building’s sustainability.
“We also complicate our roofs, sometimes, in a positive way,” he said, referring to live roofs that contain plants growing in containers to improve the roof’s R value for cooling in the summer. Those roofs also are designed to prevent rainwater from entering the city storm sewers and treatment plant. Instead, they funnel runoff into cisterns that store the water for irrigation, which is precisely the situation involving the live or “green” roof at Grand Rapids’ new Downtown Market.
And as for so-called “flat” roofs: Both Horman and Vos pointed out there is no such thing as a really flat roof. The building codes always require a slight pitch in every roof so that rainwater does not pool on it, which certainly would cause as much strain as the wettest snow.
“The vast majority of flat roofs really aren’t flat,” said Vos.
Vos has been involved in roof design for Meijer stores, which can cover 200,000 square feet.
An engineer would like to design for more pitch to a roof, said Vos, but that adds to the cost. Owners of commercial buildings usually opt for the roof design that meets minimum building code requirements — meaning less pitch to keep costs down.