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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Prevent Greenhouse Collapse

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Greenhouses, high tunnels and hoop-houses warrant some advance planning to keep them from collapsing in winter storms. Many growers have learned the hard way that these investments are vulnerable to failure due to some combination of ice, rain, snow, and wind.

Much of the following information has been provided by John Bartok Jr., agricultural engineer and University of Connecticut Professor Emeritus. Some of the advice also comes from Ed Person of Ledgewood Farm Greenhouse Frames.

Move stuff out of harm’s way.  Tidy up ahead of storms, so that in the event of high winds items such as trash bins, pots and trays, supplies and the like will not be blown into, or through, your greenhouses. To prepare for snow removal, move all of the accumulated equipment and supplies out of the way along the sides and around ends of your greenhouses and tunnels.

Know snow. Snow can vary in consistency and weight. When light and fluffy, a foot of snow may only contain as much water as one inch of rain. But when heavy and wet, it only takes 3 to 4 inches of snow to equal to 1 inch of rain. For each inch of rainwater that snow is equivalent to it will load a structure with 5.2 pounds per square foot. This amounts to about 6.5 tons on a 25 by 96 foot greenhouse!

An uneven snow load makes a structure more likely to collapse because the pressure is not distributed evenly on the bows. This can happen if wind lifts snow and deposits it more heavily on one side of a greenhouse. The weight of snow may bend the side of a greenhouse frame if greenhouses as so close together that snow builds up between houses when it slides off the roof. If there is not enough space to get in with a plow or bucket loader to remove the snow, then cutting the plastic to let the snow fall into the greenhouse can relieve the pressure.

snow on greenhouse

In some locations it can be a challenge to remove heavy snowfall from greenhouses and tunnels.

Consider the frame. Check bolts, screws and clamps on the frame for tightness. Note that holes and screws in tubing create weakness, especially at the bottom of frame members, and this is where greenhouses often fail when a heavy load is applied. If lacking, install diagonal braces from near the peak at the endwall to the baseboard, about 16 to 20 feet from the endwall, on all four corners. This provides stability and keeps the frames vertical. Frames lose considerable strength when they are not vertical. Install tubing or a 1- by 4-inch board and secure with a U-bolt at each hoop. Check for weak welds that are not continuous or that have burned through the metal. Examine truss braces, welds between sections of gutters and tubing sections that are welded together without an insert.

Different shape frames have different weaknesses in a storm. The weakest point of a Quonset-shaped frame is at the ridge, so extra support (2x4, etc.) should be placed under the ridge pole and secured in place. A Gothic/peaked frames’ weak point is mid-rafter, so place supports under purlins or at the end of crossties.

gothic tunnel frame supportWooden supports placed under the ridge pole of Quonset shaped tunnels can help them withstand the weight of snow and ice.

Plan for wind. When heavy wind is expected keep the plastic tight by increasing inflation; open the blower’s intake valve. Make sure any holes or rips are taped. Make sure the inflation fan intake won’t get blocked by snow.  The effective force of the wind is doubled when it is allowed inside a greenhouse. Secure doors, vents and shutters so that they cannot open. Roll up sides should be completely closed and tightly secured.

Melt the snow. If your greenhouse has heat, turn it on when heavy snow is predicted. Set the thermostat at 70F or higher. The extra fuel you burn is less expensive than replacing a collapsed greenhouse. A portable propane heater (no power needed) is a good item to have on hand for unheated tunnels or if a furnace fails. They are pretty inexpensive and can run for a few hours off a small gas tank.

Snow removal. A greenhouse is most stable when the snow is balanced on each side. Since houses are not as strong in the middle (because they do not have end wall construction for support) it makes sense to start at the middle and work in both directions if you are removing a lot of heavy snow. In this case it is also important to unload the snow from each side as you go.  If you clear one side and the other side is still loaded up this causes uneven stresses in one direction on the bows.

Ice removal. Sometimes ice accumulates on the plastic. The best way to deal with it is to melt it with a heat source. If you have double poly and it is not windy, shut off the inflation fan and the heat from inside will be more effective.  If you break the ice free mechanically there is always the risk of the sharp edges of the ice cutting the plastic. Also, every time ice slides or you use a device on the outside of the plastic to pull snow or ice off the tunnel you run the risk of abrasion which makes the poly rough, so the snow won't slide off as well in the future. 

Check your policy.  I have heard from growers after a greenhouse collapse that their policy did not cover greenhouse structures, or that only part of the structure was covered (plastic vs. frame) or that only the content of the greenhouse (a crop) was covered. You might want to check your insurance coverage before it is needed.

Published: December 2014
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