Collapsed greenhouses in Waspik, a cracked wall due to a collapsing roof in Tilburg and several sports halls that were closed as a precaution due to a thick layer of snow on the roof. On Wednesday evening, several roofs in Brabant and beyond seemed to collapse. But how is that possible? University lecturer in architecture Arjan Habraken explains.
“Snow is always a difficult situation,” says Habraken, assistant professor of Architecture at TU Eindhoven. In the Netherlands, strict rules apply that designers and engineers must adhere to when it comes to snow loads. “A flat roof must be able to carry approximately thirty centimeters of snow. With the safety margin added, you are approximately forty-five centimeters.”
However, the winter weather of the past few days has proven disastrous for several roofs. For example, a Praxis branch in Tilburg closed its doors as a precaution: employees did not trust the amount of snow on the roof.
Sports halls in Waalwijk also closed. The walls of a commercial building on the Gesworenhoekseweg in Tilburg tore under the weight and part of a greenhouse complex in Waalwijk collapsed under the load of the white mass. Also outside the province in Utrecht a sports complex partially collapsedno one was injured.
According to Habraken, the weather change in recent days plays a major role. “It first snowed a lot, then it started to thaw and now snow is coming back on top. That combination can cause a lot of extra weight on a roof.”
How can a roof collapse?
“When snow falls on a roof, the roof deforms a little because the snow remains there.” The weight of the snow causes a roof to bend slightly, creating a hole. Meltwater remains in there and that only increases the burden. “If the snow then melts and the water has nowhere to go, it will collect in a hole like this. That adds even more weight.” In engineering this is called the ‘ponding effect’.
Ponding can only be prevented with a roof that is built according to the rules and that can drain water well.
Although each roof of a public place is officially rated for about thirty centimeters of snow, such a pit filled with meltwater can significantly increase the load. “The more water remains, the greater the pressure. The deformation becomes greater and greater,” Habraken explains. “These holes often occur in the middle of the roof. Then a roof can collapse under the pressure.”
Engineer and former dean at the faculty of architecture Elphi Nelissen also sees that things are going wrong with this weather. “But if roofs are already collapsing, they simply do not comply with the regulations,” she firmly told Omroep Brabant. “In countries like Russia, the standards are much higher, sometimes five times higher. There is much more snow there.”
Guidelines
In the Netherlands, every building must meet the snow load guidelines. This not only looks at the amount of snow on the roof itself, but also at situations in which snow can fall from a sloping roof onto a lower roof, for example.
“You can’t say: ‘the roof can handle thirty centimeters and that’s it.’” According to Habraken, this requires sharp design work. The drainage of the roof must also be properly arranged. “Designers have to take all those factors into account. That could be a little more important.”
Four times as heavy
“Snow can also slide or be moved by the wind.” In buildings with multiple pointed roofs next to each other, such as glass greenhouses in Waspik, snow can accumulate in one place. “Then the local tax could be up to four times higher,” he says.
According to Habraken, it is not inconceivable that snow rules will also be tightened in the Netherlands in the future. “I’ve been in the business for a long time and little has changed as far as the snow code is concerned,” he says. “But we see that the weather is becoming more and more extreme. We may have to look at that again.”
