Greenhouse - Some important facts

Greenhouse: A framed or inflated structure used for cultivating plants. It is covered with a transparent material that allows for optimum light transmission of the appropriate wavelengths
(i.e., photosynthetically active radiation or PAR). It protects against adverse climatic conditions and control of the environment to achieve goals (e.g., opt. yield, etc.). One of the first recorded greenhouses was built during the first century A.D. It was covered with “transparent stone”, probably sheets of mica, to grow cucumbers out of season for the Roman Emperor, Tiberius. A greenhouse must provide protection from adverse “abiotic” conditions such as heat cold rain wind sleet hail snow salt blowing sand
NOTE: Structures can also be built to protect plants against “biotic” factors, for example, cages covered with insect or bird netting to protect against insect and bird predation, respectively. However, these structures will not be considered here.

Structural members must be strong enough to prevent structural failure during adverse weather conditions but be kept to a minimum size and number to reduce the amount of shading and to provide for maximum light transmission. Greenhouse structures are rated for certain “design loads” (the load or weight supported by the structure):

1. Dead Load = the greenhouse framing and everything hanging from it including the glazing (covering), pipes, heaters, fans, pads, shade cloth, motors, support cables AND any hanging crops or baskets in place more than one month.
2. Live Load = transient greenhouse assembly or repair equipment, people (not swinging from the rafters!) who must climb onto the structure to perform various repairs, cleaning, servicing, etc. AND any hanging crops (e.g., tomatoes, peppers, cucumbers) or baskets in place less that one month.
3. Wind Load = the load, in pounds per square foot, placed on the exterior of the greenhouse by wind. This will depend on :
   

• The angle at which the wind strikes the greenhouse.
• The shape of the greenhouse (height, width, number of bays, etc.).
• Whether or not vents, doors, etc. are open or closed.
  

*NOTE: If a sufficient wind strikes the side of a greenhouse it could rip the roof off! (Local windbreaks – trees – can help.) Depending on the location, a typical “wind load” is 80 mph or 16 lb/ft2. The greenhouse frame needs to be secured to the ground against wind. With permanent structures, anchor the supports in concrete.
With temporary structures a cork screw device is used to anchor the greenhouse to the ground.
4. Snow Load = the load, in pounds per square foot, placed on the exterior of the greenhouse by snow accumulation. The type of snow makes a difference:

• 12 inches of dry snow equals 5 pounds per square foot of load.
• 3 inches of wet snow also equals 5 pounds per square foot of load.
• and 9 inches of wet snow can collapse a greenhouse
  

When it starts to snow hard – increase the heat in the greenhouse to melt it. Early snow will melt easily. Succeeding snows will slide off. Building Codes:

• Each state/country will have its own codes.
• Sometimes agricultural buildings will be exempt from the codes or be treated as “special structures”.
  

*Example: Greenhouses can be built very cheaply in Mexico because Mexico has no building codes. However, these greenhouses may also not be as safe as if they were built to USA code.
*Always make sure the builder/contractor is insured
.