Farming Magazine - February, 2012
Preventing the collapse of greenhouses and tunnels
The ability of a greenhouse or tunnel to shed snow is a critical factor in reducing the risk of collapse. The shape of the frame, the temperature of the poly covering, and the location of hipboards all affect how much snow will be shed from the structure."
Photos by Vern Grubinger.
Most winters, some greenhouses and high tunnels in the Northeast collapse. Some winters, quite a few do. After last winter's snow plus rain events in my area, it seemed like they were dropping like flies, along with the roofs of barns and other buildings. To better understand how to avoid the loss of these structures when it snows, I asked Ed Person, of Ledgewood Farm Greenhouse Frames in northern New Hampshire, to share his insight into the anatomy of a greenhouse collapse. He sent me the following information.
The longer the snow sits on a frame the heavier it gets because it absorbs moisture out of the air even if it doesn't rain. If it rains, the snow gets heavier very quickly. A foot of wet snow can weigh 30 to 40 pounds per square foot. Wind can add to the load because it turns a static load into a bouncing load. That's like standing on a plank: it doesn't break when you're the stationary load, but if you jump up and down then it breaks.
The potential for collapse under snow and wind loads will vary among greenhouses and tunnels depending on the style of frame being considered, diameter of the bows, wall thickness of the tubing and depth of the ground posts. Homemade and less expensive frames are at greater risk because they generally use smaller diameter tubing with thinner walls, have shorter or non-existent ground posts, and very little bracing. Some of the key issues to consider are discussed in detail below.
Ed Person, owner of Ledgewood Farm Greenhouse Frames, explains how to build a structurally sound greenhouse at an on-farm workshop. Gothic-shaped frames with a slight peak, like those in the photo, are better at shedding snow than Quonset-shaped frames without a peak.
1. Quonset shape frames are most vulnerable at the top because the flat area along the ridgepole can carry huge snow loads, and if the poly is loose it can sag along the ridge and hold even more snow. The wider frames have greater areas of collection at the top. Sometimes the snow along the edges can give a little support against outward flex of the bows, which could add a little strength to the frame, but it's marginal. So, when clearing snow, start with the ridge, and then do the sides. If you can't get to the ridge, then put wood or metal posts under the ridgepole to help support it before clearing the sides. Generally, a post every 8 feet helps. After placing the posts get the snow off as soon as possible. Of course, it's a good idea to have the posts in place before the snow arrives.
2. Gothic or peaked frames have more load concentrated on the rafters (poles along the side of the house, below the ridge), and the mid-rafter is the weakest point. Clearing snow from the sides next to the house is most important so the snow on the roof can slide all the way to the ground and not get concentrated above the hip. Crossties and trusses help stiffen the greenhouse frame against wind loads and thus reduce movement of the frame, but they don't do much for increasing the ability to bear snow loads because the entire load of the top of the frame is transferred to the end of the crosstie, which is normally mid-rafter or lower. If you want to help a frame with crossties be stronger, place support posts under the end of the crossties, not under the peak.
3. Moveable frames are especially vulnerable because they do not have ground posts to keep the bows from bending outward at the lower curve. You must install posts and attach them to the frame at least every other bow. The best way to do this is with posts driven into the ground 2 to 3 feet and extending above grade about 1 foot. An extension should be added to the posts and then bolted to the bows 2 to 3 feet above ground level. This helps to support the bows, but is still not as good as a post every 4 feet.
Rolling greenhouses are more vulnerable to collapse because they typically lack ground posts to secure the frame in place, which also reduces the likelihood of hoops bending outward under snow load. Adding temporary posts to a moveable house in the winter can help prevent collapse.
4. A single layer poly cover is more likely than a double poly cover to allow ice to form on the outside surface of the poly. When ice forms snow will more easily stick to the poly, especially in houses that are minimally heated or where plants are kept "warm" with soil heat captured under row covers close to the ground. This occurs when the temperature is just warm enough inside to initially melt falling snow but then the snow soon falls too fast to remain melted and as the water freezes it sticks to the poly. Then subsequent snow builds up and won't slide off.
Double layer inflated houses tend to have a cooler outer poly surface. There isn't the initial thawing so the snow doesn't stick and it just slides off. If you are going to heat the greenhouse to melt snow or ice off the poly, then the greenhouse interior temperature needs to be above 70 degrees before snowfall starts and if it is not windy you would be best to shut off the inflation fan for faster heat transfer from the inside of the house to the outer layer of poly.
5. Now that many growers are using roll-up sides it is important to keep the hip board below the center of the hoop curves so it doesn't impede snow sliding off or rain shedding. Wiggle wire has only a slight elevated profile so if it is located below the curve it isn't a problem. In a board and decking screw installation, make sure that you use two pieces of wood, one under the poly at the top of the hip board and the actual cleat below it screwed to the hip board.
In some locations, excessive snow can be a real challenge for greenhouse growers.
6. The wider a greenhouse frame is, the larger the snow accumulation along its sides. A 30-foot-wide frame will pile up about 17 cubic feet of snow for every foot of snowfall each foot along the frame. Going from a 26-foot-wide to a 30-foot-wide frame increases loads about 15 to 20 percent. So, if you use the same tube diameter and gauge a 30-foot frame will be weaker than a 26-foot frame because of the longer span of the rafter and more snow accumulating along the span.
Plan ahead for heavier-than-normal precipitation and the load it may place on your greenhouse frame. It's important to protect such a large investment.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at firstname.lastname@example.org.