As the climate changes, nature seems to be getting more violent in recent years with increased numbers of hurricanes and record-breaking storms. The International Building Code has revised upward its wind and snow loading requirements for some areas of the U.S.  

Damage to greenhouses from storms can include racking of the frame, bending of hoops, broken glass or torn plastic and uplifted foundation posts. Preparation ahead of time can minimize the damage.

Wind force

The wind passing by a greenhouse creates a positive pressure on the windward side and a negative pressure on the leeward side. An 80-mph wind on a 10’ by 100’ sidewall of a gutter-connected greenhouse would have to resist a 16,000 lb force. Do the following to prepare for wind force:

• Tighten loose truss and frame connectors.
• Check that cables are tight and rigid braces secure.

Wind uplift

Wind can also create an uplift. An 80-mph wind blowing perpendicular to the side of a 28’ x 100’ hoophouse can create a lifting force of 220 lb/sf of length or 22,000 lbs of uplift on the whole structure. The following help prepare for wind uplift:

• Close all openings including vents, louvers and doors. The effective force of the wind is doubled when it is allowed inside the building.
• Increase the inflation pressure slightly on poly-covered houses by opening the blower’s intake valve. This will reduce the rippling effect. Check to see that the plastic is secure.
• Disconnect the arm to the motor on all ventilation intake shutters and tape the shutters closed.  Then turn on enough exhaust fans to create a vacuum in the greenhouse. This will pull the plastic tight against the frame.

Snow load

Snow can be light and fluffy with a water equivalent of 12” of snow = 1” rain.  It can also be wet and heavy with 3” of snow = 1” of rain. Snow with 1” rainwater equivalent = 5.2 lb/sf.  This amounts to 6.5 tons on a 25’ x 96’ greenhouse. Prepare your greenhouse using these steps:

• Collar ties and post connections should have adequate bolts or screws. This is a weak point in some greenhouse designs with brace bands.
• Clean snow between closely spaced houses or cut plastic to prevent hoops from being bent.
• In hoophouses, install 2” x 4” posts under the ridge every 10’ when heavy snow is predicted.
• Maintain 80ºF inside temperature to melt snow and ice. Add temporary heaters if necessary.  It takes 250 Btu/hr per square foot of glazing to melt wet snow falling at a rate of 1”/hour. Turn heat on several hours before the storm begins.
• Energy screens should be retracted to allow heat to the glazing.

Heavy rain

A 1” rainfall is 27,150 gallons on an acre of roof. Do the following prior to rain:

• Clean gutters and downspouts of leaves and debris before the storm.
• Remove obstructions in drainage swales.

Backup power

 A reliable standby generator is needed to power heaters, fans, blowers, and pumps.

• Have adequate fuel for the duration of the storm.
• Operate once a month.

References

Bartok Jr, J. W. (1991). Reduce Storm Damage to Your Greenhouses. Yankee nursery quarterly (USA).