Ask OGS: Repairing a Snow-Damaged Greenhouse
Dear OGS,
My greenhouse collapsed under the recent snow and the Quonset shape now looks more like a capital “M”. Any suggestions on repairs?
- Perplexed Grower
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Dear Perplexed Grower,
We’ve seen poly houses collapse or blow away on many occasions. Each time, they have been built back stronger than before. The first suggestion is to look for the points of failure so that the repair can eliminate that particular failure next time. With our changing climate, we expect more large snow events. Here are some common causes of polyhouse failure and possible preventive strategies to consider:
Wind: Greenhouses look remarkably like the leading edge of an aircraft wing. As high wind moves across the hoops, pressure is down (to collapse the structure) and lifts (to pull up ground stakes and make it airborne). It pays to either secure ground posts with concrete or put tabs on the posts underground to resist an upward pull. Mobile home anchors might be a good idea, particularly for a retrofit.
Rain: Loose poly can create bags where hoops intersect the purlins. In heavy rain, hundreds of pounds of added load can take down a greenhouse. Keep the poly tight and also check the wiggle wire channels from time to time and make sure the screws attaching them to the frame are still solid.
Snow: The traditional approach used for snow protection is 2X4 posts under the center of the house every couple of hoops. That system can fail in two ways: in some cases, the supports are driven into the soil and allow the structure to deform enough to fail; in other cases, the 2X4 splinters under the load. For heated houses, the strategy is typically to turn off the inflation blower and keep the house warm enough to melt snow as it falls. Another approach to consider next time, if behind in removing snow, is to pull the wiggle wire and get the poly off the frame.
Some lessons learned by staff:
Use bigger boards than 2X4. In one house, 2X6s deformed but did not fail.
Make better use of the poplars, which are common on some farms, and cut beefy posts instead of relying on dimensional lumber. Then, put those posts on a footing like a Trex platform. (Pressure-treated wood is not allowed in organic houses.) Trex is a composite wood made from sawdust and recycled plastic. Composites won’t leach toxins or rot.
Repair suggestion
We recommend cutting out the kinks and breaks in the old frame with a Sawzall or similar reciprocating saw. A circular saw with an abrasive metal blade also works but is a little slower. Cut enough of the bend out so that the cross-section of the pipe is round again. Large pipe wrenches or a section of larger pipe can often be used to fix minor bends. The key to substantial repairs is finding a pipe slightly larger or smaller than your hoops. Cut enough for about a foot overlap on either side of the repair and slip the other pipe over (or inside) the cut ends. Tek (self-tapping metal) screws can hold it in place. We have seen this method used before and never seen that same spot fail again. The double metal probably makes the repair stronger than the rest of the frame, even with the screw holes (which generally are not a good idea for hoops). We anticipate about 40 hours to reconstruct each house. The cost of that time compares favorably to the cost of hauling off the old frame and buying a new one. Another thought to consider as you reconstruct is to see if you can change the shape from Quonset to Gothic. Gothic designs with a steeper roof will shed snow better and cause condensation to slide off the interior instead of dripping on crops (a leading cause of tomato leaf mold!).
Happy farming!
This blog post, written and copyrighted by Tom Elmore, was originally published as part of a previous Organic Growers School article series called "Ask Tom," which addressed the growing concern. Some updates may have been made by OGS staff to keep the content current.