Structures – Back to Basics with NGMA: Part I By NGMA staff

Over the past four decades, the NGMA's structural manufacturers have worked with growers from all over the world. And whether it's at a greenhouse or a trade show, there are some common questions growers ask that fall into eight general categories.

So the NGMA collected the most frequently asked questions about structures, venting/cooling, heating, insect control, curtain systems, environmental controls, glazing and electrical systems to pass on to growers.

The questions are basic, the answers simple, but the information should be helpful. So read on to find out more about purchasing a greenhouse and greenhouse electrical systems. Feel free to contact any NGMA structural manufacturer for further assistance.

Purchasing a Greenhouse

What should a grower do first before

purchasing a greenhouse?

The first thing to do before purchasing a greenhouse is to contact the local authorities. Because zoning varies from one county to the next, one greenhouse may have a totally different situation than a greenhouse just across the street. Simply put, each location is different and there is no one answer. While the United States recognizes 50 state governments, there are more than 87,000 local governments. The possibilities of code and zoning combinations are endless. Zoning codes and building requirements can be any mix. The laws are becoming more strict and the negative consequences of noncompliance worse. We cannot stress enough the importance of checking with your local authorities before doing anything.

What does a greenhouse cost?

Probably one of the most common questions the association receives is regarding costs. Unfortunately, there is no quick answer to the question. A better question is, “What would a greenhouse cost that fits your specific needs?” Cost is not easily determined because greenhouses vary due to local codes and zoning, the purpose for the greenhouse, its geographical location and future expansion goals.

What authorities do I contact before

purchasing a greenhouse?

First, check with your zoning authorities for your specific location's setbacks, building material requirements and allowed businesses. Second, talk with the planning commission regarding building codes. Ask them what code body they recognize and for which year. Also, determine if stamped and licensed drawings, by an engineer, are required. If applicable, also ask what the wind and snow load requirements are for a greenhouse. Then, check with the fire authorities on what they require for greenhouses. Although greenhouses are unique, they often are not treated as such. Find out all their fire codes, specifically sprinkler system and covering requirements.

The tough questions should be answered before designing a greenhouse, not after. While these questions may seem overwhelming to the new grower, there is help available through any of the National Greenhouse Manufacturers Association's structural members.

What exactly is a non-code house?

The simplest commercial greenhouse is referred to as a cold frame. Cold frames are designed for wintering-over crops and for shade. They are considered temporary and do not meet codes. Cold frames are very basic, but can have some significant variables. While they are non-code houses, cold frames can still have structural strength as they are built with steel tubing.

How do non-code houses vary?

When growing crop on the floor, sidewall height is not necessary. However, if you're growing crop on benches, sidewall height is very important. Height is also an issue if any overhead equipment is to be hung from the structure.

What is a basic greenhouse?

Although there is no such thing as a 'basic' greenhouse, a standard greenhouse requires permits and must be built to local building codes. These greenhouses are the heart of the industry and come in all sizes and shapes. They are not necessarily flashy, but are designed to provide years of efficient and dependable crop protection. These code greenhouses have the added benefit of working well with environmental controls and can stand alone or be gutter-connected.

What are the high-end greenhouses?

The top of the line greenhouses can be just the answer for certain growers. These greenhouses are fully automated using sophisticated controls that open roofs, close vents, survey outside weather, etc. These high-end greenhouses can control any environment to the strictest of margins. Controls have become so finite that a good grower can program and monitor activity and results in any given zone, at any given time. The NGMA's structural members strive to keep up with these technological advances and make them readily available to growers.

Do most growers stick with one type of greenhouse?

No, many growers will have a mix of all levels of greenhouses. They will often integrate different types and styles of greenhouses, which enables them to optimize crop quality without compromising economics. Thus, a grower must know where the greenhouse will be built, local zoning and codes, weather patterns and what time of the year the crop will grow.

Are greenhouses insurable?

Yes, look into the insurability of a greenhouse before purchasing. Ask whether your insurance company will provide replacement cost coverage for the structure. What perils do they insure? Some companies will only provide a very limited number of insured perils and may exclude those of most concern. Do they offer coverage for plant material and how will they value the crop should there be a loss? As with purchasing an automobile, insurance information allows you to make an educated decision.

What exactly is zoning?

Zoning is defined as any section or district in a city restricted by law for a particular use like homes, parks, and businesses. Zoning is thus very location oriented. In Santa Fe, N.M., a building can be no higher than the Cathedral Tower. In Washington, D.C., nothing can be higher than the Capitol dome. In small towns and counties, where there is limited manpower, zoning is often outdated or non-existant. A grower that ignores local zoning laws can literally be shut down.

Does it matter what materials I build a greenhouse with?

Yes, zoning also includes building materials. Government regulators can dictate exact specifications for such items as brick, coverings, glass and what the overall building must look like when erected.

What should I look for in a location?

As with any real estate decision, location is everything. There are actually cases where the location can mean a reduction in code regulations, which is referred to as an agricultural reduction. For example, a 25 percent agricultural reduction on a #20 wind load code would mean the greenhouse needs to be only #16. This means less expense for the grower and would affect how a manufacturer would design a greenhouse. Be sure to check with local planning and zoning authorities.

Do I need a licensed engineer to stamp finished greenhouse drawings?

Yes, some states require that a licensed engineer stamp your drawings. If this is a requirement of your state, the engineer must be licensed in that state. An engineer can be licensed in numerous states, not just where they are located. To be licensed they must be tested and/or pay a fee.

Should I be concerned about expansion?

Planning ahead is always beneficial. Be sure to convey to your NGMA greenhouse manufacturer what your future plans for expansion will entail. Placement and types of equipment depend greatly on whether a grower plans on expanding. Most houses can be lengthened or added onto the side with advance planning.

Is there a difference between production and retail greenhouses?

Yes, the codes are very specific as to the usage of a greenhouse with respect to the general public. Fire code issues are an example of one of the major differences. A production house is defined as a greenhouse that is occupied for growing a large number of flowers and plants on a production basis or for research without public access. A retail greenhouse is occupied for growing large numbers of flowers and plants while having general public access for the purposes of viewing and purchasing various products. Included in this category are greenhouses occupied for educational purposes.

Do setbacks affect greenhouses?

Yes, they can, depending on your location. A setback, which is more common in larger cities, is the amount of feet from the street a greenhouse must sit to be built. For example, a building may be required to set back 50 feet from the curb for what they call “street appeal.” Zoning setbacks are also lot specific and can vary from one lot to another.

Are there any fire-code issues with greenhouses?

Yes, there are fire codes specific to greenhouses and they vary depending on the area. Many growers have elaborate drawings made up only to find out that none of the fire codes have been addressed. It is not uncommon for a retail greenhouse to be required to have a sprinkler system, even though most greenhouse insurance companies do not offer a premium reduction for their installation. Before you invest in drawings, find out what fire codes affect a greenhouse at your location. Fire codes are becoming stricter, particularly for greenhouses that welcome the public. Nothing will bring your building to a halt faster than not meeting a fire code.

Does the crop determine the type of greenhouse needed?

Crop is important when choosing a greenhouse, but seldom does a grower start and stay with the same crop. We often see growers completely change or add to their existing crop line. When it comes to type of crop, make sure your greenhouse is designed to be flexible.

Does the NGMA offer any publications regarding greenhouse


Yes, the NGMA has published a notebook of standards that includes curtain systems, design loads, electrical design, environmental control, glazing, heat loss, heating systems, insect screening, ventilation and cooling. The documents can be downloaded off the NGMA Web site ( or purchased from the NGMA for $25 (includes shipping within the United States).

Electrical Systems & The Greenhouse

How does electricity work?

Electricity moves through wires, pushed along by a force called potential. Electric potential is measured in units of volts. The voltage forcing electricity through the power wiring comes from a utility company’s generating plant. Large wires are needed to carry large currents. Electricity faces resistance to its flow through a wire, and in fact, resistance is the electrical term for the force that works against the free-flow of electricity. Resistance is measured in units called ohms. Electricity, voltage and current all work together to form power, which is measured in watts.

What is voltage drop?

In a greenhouse, equipment can easily be hundreds of feet from the circuit breaker panel that feeds it power. Wires have resistance and resistance works against voltage. This results in voltage drop and relates directly to the efficiency of the equipment we connect with electrical wiring. Voltage drop, in turn, has another effect: the voltage doesn’t just disappear, it gets converted into heat that is wasted, even though you pay for it on your utility bill.

How do I eliminate voltage drop?

While there’s no way to completely eliminate voltage drops and other inefficiencies, you can minimize them by working with a design engineer who is familiar with greenhouses. A design engineer will provide an electrical system that meets you requirements for safety, efficiency, adequacy, convenience and space capacity. You’ll need to coordinate both the design and installation phases of your electrical system with your local electric utility as only they can tell you if they can provide the type of system you require with the capacity you need.

What are the most important things to consider when creating an electrical greenhouse system?

Demand that all work be done in accordance with the National Electric Code and local codes. List all equipment requiring electricity. Have a design engineer prepare a load schedule, main service and branch layout, power diagrams, wiring layout and control diagrams.

What is a load schedule?

This is an inventory of all the electrical equipment in your project. The total electrical demand of this equipment determines the size of electrical service you’ll require.

What is the main service and branch panel board layout?

This shows the location and size of your service entrance components and any additional circuit breaker panels you will need. The service entrance is where the wires from the electric utility enter your property and include a large switch or circuit breaker that can disconnect your entire premises from the electrical grid.

What are one-line power diagrams?

These diagrams show how your equipment will be assigned to individual circuit breakers and breaker panels. These plans show the number and the sizes of the wires that will carry power to your equipment.

What is a conduit and wiring layout?

This diagram shows where conduits will be routed, the sizes of the conduits and the number and sizes of the wires that will be run into the conduit.

What are control diagrams and schematics?

These plans show the interface between control devices and the equipment they control.

Where should I locate my main service?

The location of your main service entrance must be coordinated with your electric utility. A central location minimizes wire lengths and reduces both the costs of voltage drop and of the wire itself. A centrally located main service is your most efficient and economical alternative.

What type of power system should I purchase?

One-phase systems are commonly used in houses and small businesses. Three phase systems are used in schools, medium to large businesses and industrial plants. Though three-phase power is not available everywhere, there are technical advantages that should be considered. Do not be afraid to go for high voltage. The higher voltages mean the voltage drops are less significant. Because power is the product of volts and amps, a higher voltage means we can deliver the same total power while using fewer amps. The practical benefit of reduced amperage is reduced wire size.

What are the advantages to three-phase power?

Three-phase motors are less expensive than single-phase motors when the motor is one horsepower or larger. Also, they are mechanically and electrically simpler than single-phase motors. A three-phase system can use smaller wires and conduits than a single-phase system Ð and are more reliable.

Do I need a standby generator?

Because a greenhouse can lose and gain heat so rapidly, it is critically dependent on a constant supply of power for its heating and cooling systems. Because a power outage of more than a few minutes can mean the loss of your livelihood, you must consider a standby generator when you plan your electrical system.

What is an automatic transfer system?

Automatic transfer systems are available that start the generator, allow the engine to warm and the generator to come to full power, and then connect the generator to your greenhouse equipment. When utility power returns, these systems reverse the process and shut down the generator.

How do voltage drops affect crop lighting?

Voltage drops have a disproportionate effect on the light output of incandescent lamps. If the electrical system delivers a voltage below the rated operating voltage of the lamps, they cannot deliver the intended amount of light. The light loss can be significant and is often undetectable to the human eye.

What are the benefits of high-efficiency motors?

Though high-efficiency motors cost more than standard motors, the energy savings they offer easily outweigh the cost difference. A one-horse power, high-efficiency motor can pay for the cost difference in only four months of typical greenhouse use. A high-efficiency motor is built using more copper wire and iron than a standard motor, and it runs cooler than a standard motor.

What is a ground-fault circuit interrupter?

A ground-fault circuit interrupter, or GFCI, is a safety device that protects people from possible electric shock hazards. A typical GFCI is built into an electrical outlet to kill the power once an electrical fault is detected. The National Electrical Code (NEC), which regulates electrical wiring, has begun to require ground-fault circuit interrupters in more and more locations, particularly in damp locations in contact with the earth or concrete.

More Information

How can I find out more about the topics discussed in this article?

When choosing a greenhouse manufacturer look to any NGMA structural manufacturer members. All of our structural greenhouse manufacturers promote code compliance and promulgate industry standards that ensure code compliance.

For information on electrical systems, you can refer to the free NGMA electrical guideline that you can receive by contacting the NGMA office or by downloading it off the Web at

Stay tuned for next month's installment, which will address greenhouse ventilation systems, heating systems and environmental control computers.

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NGMA staff

All information provided by the NGMA, Littleton, Colo., phone (800) 792-NGMA, fax (303) 798-1315

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