Snow Loads: Holding The Winter Weight
Growers and structures experts explain best practices when snow begins to build on your greenhouse.
May 25, 2010
A brutal winter can be a real wake-up call for growers. Forcing you to ask questions: Are my greenhouse structures up to par? Are they strong enough to withstand heavy snow from a late and unexpected storm while we’re in the middle of production?
The questions are legitimate ones, and industry veterans provide some answers. They explain how a little practicality, heat and attentiveness can go a long way – and may even save the season for your operation.
Along with keeping a close eye on the local and national weather reports, Jaderloon’s Neil Devaney advises removing all exterior shade cloth before anything else.
“The snow will grab that cloth a lot more and hold,” he says. “If that happens, it won’t go anywhere and just turn into ice.”
Once the snow starts coming down, Devaney says long-handled brooms and PVC pipes are excellent for manually working the snow down and off of a structure, but be sure to work in teams.
“What you don’t want to do is have a person walk down and clean off just one side,” he says. “You want a person on each side – working at the same time. If there’s a lot of snow and you clean off one side, it might just shift the structure.”
When dealing with heavy, wet snows and snow-loaded greenhouses, Devaney advises against walking inside the structure until some of the snow and weight have been removed from the roof.
“There’s not a meter on there that reads, ‘Hey I’m getting ready to fall,’” he says. “It can be a domino effect if one part of the structure collapses, because that places a ton of weight and stress on the remaining bolts and supports.”
For growers with ground-to-ground structures, two-by-fours are an easy way to provide additional support to the frame, says Burt Moore, Atlas Greenhouse Systems engineering manager. Having these posts notched, pre-fit and ready for severe weather will do a lot of good, he says. “Place these support columns down the center of the buildings during the winter months, every 10 or 12 feet. When the weather warms up again, they can be easily taken down and stored.”
Sans Penny Pinching
At Jaderloon, Devaney has experienced many growers trying to get the least amount of greenhouse for the least amount of money. “I tell people all the time: The frame, which is your strength, is about 20 to 25 percent of the overall greenhouse project costs, and if you increase that frame cost by 10 percent, you get a much stronger greenhouse and you’re not increasing the overall project cost by very much.”
Steve Mercer, manager at Preston Greenhouse in Louisville, Ky., completely agrees with Devaney’s advice. In the last 30 years at Preston Greenhouse, Mercer has never experienced a collapsed structure, even during winters that have loaded his greenhouses with 3 feet of wet snow.
“To start with, we try to overbuild our houses,” Mercer says. “We try to construct them by using vendors who know how to build structures for northern climates. And even though some people in our area would build a southern-style greenhouse, we like to build to northern specs.” One example of that is column spacing. In gutter-connect housing, Mercer says some growers will space their support columns 12 feet apart, while Preston Greenhouse works on 9 feet of spacing.
“We build for worse-case scenarios,” he says. “We never want to put ourselves in a position where we have a house caving in on a crop. Not only do you have to experience the labor to tear down the damaged house and take the time to rebuild it in the middle of a production cycle, you’re also at risk of losing the crop. I don’t see how anybody can justify being pennywise and pound foolish in this situation.”
Mercer notes that Preston has one weaker house that is coming down this year. “When we build it back, it’s going to be built a whole lot stronger.”
Keeping on top of snow removal is important because of its ability to insulate and also block light. In the same breath, Atlas’ Burt Moore says keep the structure well heated, especially before the snow hits.
“We typically recommend to keep heating at 50 degrees Fahrenheit at 3 feet above the finished floor,” Moore says. “Naturally, heat rises, and that means if you can keep it at 50 degrees at that level, at head-level it’ll be 60 degrees, and up in the top it’ll be in the 70s.”
Devaney adds that increasing a greenhouse’s temperature costs money, but he asks what’s more expensive: Running heat for six hours or replacing a structure?
Moore and Devaney agree, the main goal in a snowy situation is maximum heat loss. Growers should pull back all interior shade systems, and those with double-poly houses need to turn off the inflation system to increase heat loss.
“We turn the heat up, bringing the temperatures up to 70,” Mercer adds. “Do you really want to bet the company on a weaker house and being in the middle of a production cycle and lose the house and the crop, too, and not be in business when the customers start coming around? That’s a pretty big bet.”