Snow can be a major problem for greenhouses. If the houses are unheated and melting does not occur, or if the greenhouses are connected (ridge and valley or gutter-connected), then the potential for high snow loads significantly increases. Heavy snows have often damaged or destroyed greenhouses in the state when greenhouse operators were not prepared for snow storms.
The snow loads which can be expected on a 50-year recurrence interval in the central states are 10 to 15 pounds per square foot of the greenhouse. Thus, the National Greenhouse Manufacturers Association recommends a minimum value of 15 pounds per square foot of covered area for snow load. Any greenhouse used in Kentucky should meet this minimum standard for strength.
Remember that a snow heavier than these 50-year probabilities can occur in any given year. A "wet" snow can weigh as much as 20 lbs/sq ft of horizontal area for a 12-inch deep layer. For a 30' x 100' quonset greenhouse, the top portion of the plastic covering that equals about half the floor space could retain this "wet" snow. The snow would put a load of approximately 1200 lbs on each bow spaced four feet apart (20 x 15 x 4) or 1500 lbs on each bow of a five foot spacing! Is your bow design capable of this load? Remember that heaters or other equipment hanging from some bows could greatly increase this load on some bows and cause failure.
Since high winds would result in snow being blown from the greenhouse roof, designs normally do not need to consider both snow and wind loads simultaneously. Greenhouses which are designed for a 10 to 15 lbs. per sq. ft. snow load and are well anchored to resist the wind uplift forces have generally proven satisfactory. Since winds cause an overturning action as well as uplift, lateral bracing is also important and must be included. This is of particular importance in long houses.
Solutions for heavy snows:
1. Have 4x4's, or two 2x4's nailed together to form a 'T' member, handy to set under the peak of each bow to reinforce the bow strength. Cut the members to the right length to fit snugly under the bow.
2. Turn up the heat ten degrees or more to help melt the snow (and/or ice) and let it slide off, assuming the plastic will hold up. Generally, heavy snows compress the inflation layer so snow will melt relatively quickly if the greenhouse is heated. If necessary, use a broom or an inverted rake to pull snow from the greenhouse roof. You may have to move snow away from each side foundation to let the snow slide off freely and hope slivers of ice do not tear the plastic.
3. If purlins or other longitudinal bars of the greenhouse trap snow and/or water due to sagging plastic, use a large soft-headed device like a floor mop to gently push the plastic from inside the greenhouse to let the water drain from the pocket or cause the snow to slide off.
4. In an emergency, a greenhouse operator should understand that it may be necessary to cut and destroy the plastic cover in order to prevent major damage to the greenhouse structure.