What should you know before purchasing or building a greenhouse?

Avoid greenhouse construction headaches by outlining its purpose before you build.

By Amy Rigazio

When the public has access to a building, regulations become more stringent. A greenhouse must meet certain building safety codes concerning structural loading, construction materials, location and number of exits and accessibility, which normally would not apply to a production facility.

In addition to the final use of the greenhouse, site preparation, water management and the availability of utilities must be established so that the building being constructed meets the legal and physical requirements of the site.

Site preparation
Before greenhouse construction begins, the site must be reviewed to find its optimal location.

Orientation. Ideally, a greenhouse should receive the maximum amount of direct sunlight without excessive shading. A free-standing structure running east and west allows for the greatest surface area with a southern exposure. A gutter-connected structure running east and west would also bear a sidewall with a southern exposure, but shadows produced by the gutters would be stationary as the sun moved throughout the day. To avoid these “dead spaces,” a north and south orientation allows for shadows to move throughout the day, so that no part of the house is perpetually shaded.

Drainage/runoff. Once the physical location of the facility has been established, the ground needs to be prepared. Adequate drainage is vital. Not only does rainwater need to be collected and moved away from the building, but also runoff produced from daily operation must to be considered. Most U.S. greenhouses are operated as open systems, which means that irrigation runoff is allowed to leave the greenhouse and enter the outside environment.

To avoid water backing up into the greenhouse, the site must be level and graded away from the structure in all directions. The soil should be checked for adequate water movement and gravel and sand fill used to compensate for any drainage problems.
Water table/rainfall. The level of the local water table should be established to help predict future problems. A high water table can lead to erosion around concrete anchors or under slabs, which can compromise the structural integrity of the house. Knowing the level of the water table will help the builder determine the size and type of anchors most appropriate for the site.

Rainfall statistics for specific geographic locations should be assessed before any structure is built. Gutter-connected buildings must be equipped with appropriately sized gutters and down spouts so that water collected on the roof can be effectively removed.

Once provisions have been made to remove water off of the structure, a system for transferring the collected water, along with the surface water, should be established. An irrigation basin or pond is frequently required to collect the runoff and can often be used as irrigation water for crops.

Utilities
All greenhouses require water and energy, so the availability of utilities on site is critical. Before construction begins, the electrical requirements of the operation should be established to allow for future expansion.

Once the power requirements have been determined, the local electric supply company can be contacted to determine availability and pricing. The supply company can help design the most efficient distribution system from the service drop location and advise you on the type of electric power system (three-phase vs. single-phase) that can be installed.

A fuel source, usually natural gas oil or propane, will also be required on site to run hot air heaters, boilers and furnaces for year-round production. The determination of which fuel to choose will be based on availability, price and fuel consumption.
Utility questions should be answered before construction begins or in the early stages. Severe problems with the water supply or site drainage may require significant excavation on the property, which can disrupt the soil stability while the work is being done.

Any utility lines that need to be laid will be less intrusive if the trenches are dug before foundations are poured. Proper planning of all groundwork will provide for a faster initial construction and easier future expansion.

Water supply
Adequate water supply is the most important requirement of any greenhouse site. Not only is the quantity of available water critical, but the quality of the supply as well.
The water supply, whether from a source on site or from a city reservoir, must be tested before any construction is done. The pH, electrical conductivity (EC) and mineral content all affect plant growth. If a site’s water supply is found to be contaminated, steps must be taken to compensate for the problem before the water is used or a new site must be found.

The pH and EC are usually adjusted to meet a crop’s specific needs regardless of their original levels. Initial mineral content is of more concern because it will affect the type and quantity of fertilizers that can be used. The water supply should also be checked for physical contaminants, such as sand, which can damage irrigation equipment.

Permit changes
As the role of greenhouses has changed, the legalities concerning structure construction have changed also. In the past, all agricultural buildings were exempt from local construction and usage permit regulations. This is no longer the case. Many jurisdictions now require greenhouse operators to apply for permits, regardless of whether the greenhouse will be used for retail or for production.

The first step in determining what permits, if any, are required for a greenhouse is to establish if the site has been zoned agriculture or commercial. An agricultural site will often not require construction or usage permits as long as the building is not accessible to the general public.

In situations where permits are required for production houses, the regulations are adapted to reflect the low liability of the structure. Many building codes have a section dedicated specifically to “membrane” structures. This type of building has vastly reduced structural requirements such as environmental loads and anchoring techniques, as well as reduced fire safety requirements. Membrane structures are considered to be a type of long-term storage facility and therefore apply to most production-only polyethylene-covered greenhouses.

Most municipalities recognize a greenhouse as a special type of structure, so the applied environmental loads are not as stringent as those for a typical commercial building.
Retail greenhouses can be considered commercial buildings or agricultural buildings, depending upon the individual jurisdiction. As a commercial building, the greenhouse must meet higher structural requirements, as well as fire and accessibility rules for public spaces.

Emergency exits, sprinkler systems and fire alarms are often mandatory in retail greenhouses. Aisle widths and lengths also must comply with all accessibility laws, as well as exit ramps and handrails.

Proper paperwork
It is vital that the local building department be contacted to determine what permits are required before any construction begins. Not only will penalty fees be avoided, but construction delays as well.

Once it has been determined that a permit is mandatory, the building inspector can explain what specific information must be supplied. Often, a drawing of the proposed building, which has been signed and sealed by a professional engineer, must be submitted. The drawings usually note the loads for which the building was designed, as well as identify the structural members that comprise the greenhouse. Some towns may require copies of the engineering calculations in conjunction with the drawings, while others may only need a letter from the professional engineer stating the loading capacity of the structure.

The greenhouse manufacturer should be able to provide the required engineering certification, but it is the responsibility of the customer to request the information. As every municipality requires different documentation, it is vital that the appropriate paperwork be determined before the purchasing process begins.

As not every greenhouse is designed for every environment, understanding what is legally required will ensure that a suitable structure be purchased. It is more economical to purchase a structure that will meet the local building codes than to try and adapt one afterwards.