From David Kuack, Greenhouse Grower, 6/4/2013:
John Bartok Jr., an agricultural engineer and University of Connecticut Professor Emeritus, says snow varies considerably in consistency and weight. It can be light and fluffy with a water equivalent of 12 inches equal to 1 inch of rain. Snow can also be wet and heavy with 3 to 4 inches equal to 1 inch of rain. Snow having a 1-inch rainwater equivalent loads a structure with 5.2 pounds per square foot. This amounts to about 6.5 tons on a 25- by 96-foot greenhouse.

Bartok says there are 10 reasons for structure failures during snow storms.

1. Drifting Snow: In nor'easter storms, adjacent greenhouses or bays of gutter-connected houses that have a north-south ridge orientation tend to collect more snow on the leeward side. Snow that is lifted over the ridge of the first house can be dumped on the windward side of the second house. This creates an off-center load on the roof.

2. Proximity Of Adjacent Greenhouses: Building greenhouses too close together is a common cause of failure. This is especially the case with overwintering structures that are only 4 to 6 feet apart. When snow slides off the greenhouse roof, it fills the space and crushes the house's sidewall frame. Usually there is inadequate space to get in with a bucket loader to remove it. To save the structure, some growers cut the plastic covering to allow the snow to flow into the greenhouse and relieve the pressure. Other growers install two-by-fours to brace the side frame.

3. Greenhouse Frame Shape: The gothic-shaped greenhouse was developed to eliminate the flat spots that can collect snow on the top of hoop-shaped structures. Since 1994, when the nursery industry changed from the hoop design to a gothic design for overwintering structures, there have been fewer structure failures.

4. Poor Frame Connectors: Check and tighten all bolts and tek screws before the winter season. These fasteners tend to loosen over time. Brace bands and u-clamps can slip if they are not held in place with tek screws. The screws should be at the side of frame members, not at the bottom. Several greenhouses have failed at the point where tek screws were placed at the bottom of hoop tubing. This created weak spots.

5. Greenhouse Frame Racking: Many manufacturers do not include bracing with their greenhouse kits. All greenhouses should have 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.

6. Poor Welds: Welds that are not continuous or that have burned through the metal are weak spots. Areas that should be checked include truss braces, welds between sections of gutters and tubing sections that are welded together without an insert. Although expensive, professional inspection and x-ray testing may be worth the added expense.

7. Inadequate Air Inflation: Heavy wind can create rippling of plastic coverings causing failure at the structure attachments. This can be prevented by increasing the polyethylene film's inflation slightly by opening the blower's intake valve. Make sure any holes or rips are taped. Check to see that the inflation fan intake cannot be blocked by snow.

8. No Heat Or Inadequate Heat: Most greenhouses that fail don't have heat or were heated to 40ºF or less. When heavy snow is predicted, the greenhouse heating system should be turned on and the thermostat set at 70ºF or higher. Energy screens should be left open. The few extra gallons of oil or therms of natural gas burned are less expensive than replacing a collapsed greenhouse.

9. Open Vents, Doors Or Louvers: The effective force of the wind is doubled when it is allowed inside a greenhouse. Latch doors and tape vents and shutters so that they cannot open.

10. Plugged Gutters And Downspouts: Once snow has accumulated, it is important to have provisions for its removal. Removing snow from the roof lets some light in, allowing warming of the inside. This causes some melting, which helps to reduce the snow load. Once the snow is melted, it is important to keep the gutters and downspouts free of ice. Ice socks filled with calcium chloride placed in gutters and downspouts will melt the ice. If available, magnesium chloride or sodium acetate is more environmentally friendly and reduces corrosion.

Shoveling is not always the answer. It can be very expensive to remove the snow. You also need space to dispose of it. If the snow is light, there is not much danger of collapse. If the snow is heavy, some growers have found that as it settles, melts and refreezes, it forms a cocoon next to the plastic covering and doesn't put a lot of pressure on the greenhouse. Removing it may cause more damage.