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US (MI): Ensure your greenhouses withstand the snow load

Heavy snow loads can cause greenhouse structures to collapse if proper snow removal techniques are not practiced or extra support is not provided.

Photo 1. Heavy, wet snow accumulation on the double-poly greenhouse resulted in collapse of the greenhouse structure. All photos by W. Garrett Owen.

Strong winds, gray skies and cold, snowy days are here in Michigan. During this time of the year, we experience extreme cold temperatures, low light levels and inches upon inches of snow. As a greenhouse grower and operator, are you prepared to remove heavy, wet snow that accumulates in greenhouse gutter-connects or blankets poly-covered greenhouses? If you do not have a plan, you may find yourself in a scrambling predicament to save your greenhouse from collapsing (view larger version of Photo 1).

The moisture content of snow can vary dramatically. Dry snow typically occurs over interior sections of the state or at very low temperatures. Snow is “wet” after it picks up moisture from large bodies of water or when the temperatures hover around 32 degrees Fahrenheit. Wet snows can weigh four times as much as dry snows, causing a considerable amount of weight baring down upon every square foot of greenhouse roof surface.

Snow load of a greenhouse is based on expected ground accumulation, greenhouse roof slope, whether the structure is a gutter-connect or free-standing greenhouse, and if the greenhouse is heated or unheated during the time of snowfall. For more information regarding snow load of your greenhouse structure, refer to the National Greenhouse Manufacturing Association.

Glass-glazed greenhouses
The most common method for melting snow in high-tech glass-glazed greenhouses is to open the energy curtains, which allows the heat to rise to the peaks of the greenhouse before snowfall. By doing so, the heated air warms the greenhouse glass so the snow melts upon contact. However, if snowfall exceeds the rate of snow melt, snow will begin to accumulate on the greenhouse, forming an insulation barrier and reducing heat loss. Although an insulation barrier is created, the snow contacting the greenhouse glass will melt and run off.

Another problem among greenhouse operators who manage gutter-connect greenhouses is bridging. Bridging occurs when snow slides towards the gutter, accumulates and the heat transferred from the gutter melts the snow and creates a bridge between the two greenhouses. In many cases there is not adequate heat exchange to collapse, melt and run or slide off the snow.

So how do you prevent bridging? Most recommendations are to place a heating source sideways and under the gutters (Photo 2) to cause the bridge to collapse and melt the snow.

Photo 2. To collapse snow bridging and melt the snow, a heating source such as heating pipes or fins are installed sideways and 18 inches below the greenhouse gutter.

Double poly-covered greenhouses
The greatest snow removal challenge is with double poly-covered greenhouses. Although an advantage of double poly is to reduce heat loss during the growing season, it delays heat transfer and the rate in which snow melts during heavy snowfall events. Greenhouse operators can reduce the air pressure between the two layers by manipulating the blower fan speed, thus allowing for the greenhouse to become deflated to a single layer. However, operators should be cautious of this method because on windy days, when the layers are deflated, rippling of the poly can place stress on the point of attachment, i.e., wiggle wire and tracks.

In other instances, growers may use snow rakes to pull the snow off. Operators should again be cautious to not pierce the poly or allow snow to accumulate along the sidewall (Photo 3), which could crush it in.

Photo 3. Avoid snow accumulation along the sidewall as it may crush it in.

To provide additional support in double poly-covered greenhouses, two-by-fours can be placed under weight-bearing bow and purlin connections (Photo 4). However, this only provides additional support to the greenhouse infrastructure and snow removal may still be required.

Photo 4. To provide additional support in double poly-covered greenhouses, two-by-fours can be placed under weight bearing bow and purlin connections, however snow removal may still be required.

Overall, a plan should be established by greenhouse operators before winter weather arrives. It is recommended to frequently check snow accumulation on greenhouse structures and maintain proper heating or snow removal equipment.

Source: Michigan State University (W. Garrett Owen)
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