Chapter 1

Greenhouse Structures and Design

Types of Greenhouses

Greenhouse styles can vary from small stand-alone structures to large gutter-connected greenhouses. Free-standing greenhouses are completely independent structures that come in a variety of different designs. The gutter connected greenhouses are joined at the eave by a common gutter. Greenhouse designs can be simply classified as attached (i.e., lean-to), free standing (i.e., even-span, uneven span, Quonset, arch), and ridge-furrow or gutter connected (i.e., sawtooth, venlo). There are many designs and structures to select from, thus it is important to become familiar with the advantages and disadvantages of each. Here is a brief rundown of some of the styles available to growers.

Free-Standing Greenhouses

A free-standing greenhouse can have a Quonset (hoop), A-frame, gothic, or gable roof shape. This type of structure is what one usually envisions when the word greenhouse is mentioned. The free-standing design is usually the best choice for the small grower planning on less than 10,000 square feet of growing space.

Advantages and Disadvantages of Free-standing Greenhouses

One advantage of a separate house is that it is easier to program and maintain the temperature to meet specific crop requirements. In addition, free-standing greenhouses are easier to ventilate without exposing the plants to erratic temperature fluctuations or harsh blasts of cold air. The light in a free-standing greenhouse is uniformly distributed over the entire growing area. In gutter-connected greenhouses, the roof gutters cut down on the light admitted and shade the crops, affecting their growth.

Quonset Greenhouses

Quonset greenhouses, also known as hoop houses or polyethylene tunnels, are constructed from pipe, either PVC or metal, bent into hoops and draped with polyethylene film (See Figure 1.2). Quonset greenhouses are ventilated through roll-up sidewalls and large open doors at both ends. Some hoophouses are fitted with end walls that frequently can be opened for ventilation. Because hoophouses generally lack foundations and permanent environmental control systems, regulations and permits in some states are typically less rigorous for these types of structures than for greenhouses.

Gothic Arch Greenhouses

Gothic arch greenhouses have an arched roof line and walls that form a continuous shape (Figure 1.3). The Gothic arch design of greenhouses has many advantages, making it a popular choice among horticulturists. Firstly, it is known for its ability to withstand strong winds and heavy snowfall, thanks to its half-teardrop shape of the roof. The pointed arches of Gothic greenhouses can accommodate a larger air volume than other greenhouse designs, resulting in better heat retention and energy-saving. Additionally, the uniform slope of the arch allows for highly uniform light distribution, ensuring equal sunlight exposure for all plants. The slope of the arch allows highly uniform light transmission and decrease interior condensation.

Gabled Roof Greenhouses

Gable-style greenhouses have sloping, flat roofs connected to vertical sidewalls (Figure 1.4). Gable roof style greenhouses use traditional welded or bolted trusses to support the roof. These are more expensive and complex to build, but they do provide plenty of internal strength to support curtain systems, boom irrigation, automated basket systems, and other equipment. The use of trusses allows gable style buildings to span widths up to 60 feet (18m).

A-Frame Greenhouses

The A-frame, also referred to as an even-span structure, represents a traditional greenhouse design, with a peaked roof of two equal-length slopes (Figure 1.5). The A-frame is constructed of trusses in the shape of an elongated A (thus the name) either welded or bolted together and affixed to gutter posts spaced on center. Additional members, referred to as purlins or girts, are placed perpendicular to the trusses along the slope. The pitched roof (often 6/12 pitch) is the perfect angle to facilitate winter light transmission and increase interior condensate control.

Gutter-Connected Greenhouses

GGutter-connected greenhouses consist of several even span greenhouses attached by a common gutter only at the lower edges or eaves of the roof. These greenhouses are sometimes called ridge and furrow. Gutter-connected greenhouses are composed of several bays or compartments running side by side along the length of the greenhouse. Typically, these compartments are approximately 120 feet (37m) long by 21 to 25 feet (6.5–7.5m) wide. The production area is completely open between the bays inside the greenhouse; there is no division of the structure by inner walls. With a gutter-connected greenhouse, the lowest part of the roof are the gutters, the points where the adjacent arches begin and end. The roofs of these greenhouses can either be curved (Figure 1.6) or gable (Figure 1.7).

Greenhouse Height

The height of a greenhouse based on the gutter height (distance from the ground to the bottom of the gutter) rather than the peak height. Typical gutter heights for greenhouse structures are 13 to 14 feet (4–4.25m) and are quite suitable for greenhouse crop production. The trend for future gutter height is to increase it further, with new construction designs moving to 16 to 18 feet (4.9–5.5m) for several reasons. The reason for this is two-fold; firstly, newer vegetable crops like peppers have a requirement for a higher growing environment. Peppers will often reach 12 feet (3.5m) in height during the production cycle.

Light Distribution Patterns

Gutters or curtains placed in a north-to-south orientation will cast a shadow that is constantly changing while the sun moves east to west throughout the day. In contrast, gutters or curtains placed in an east-to-west orientation cast a relatively fixed shadow as the sun moves through the sky. Fixed shadows create a poorer light distribution pattern that results in poorer uniformity for flowering and for water usage.

Advantages and Disadvantages of Gutter-Connected Greenhouses

Gutter-connected greenhouse structures occupy less land and have no sidewalls; therefore, fewer materials are needed for construction. Because there are no walls where the gutters are, more interior space is available than in several free-standing structures. Less energy is required to heat and cool the greenhouse because the exposed wall surface area is reduced. It is cheaper, and thus more feasible, to automate the single consolidated space inside a gutter-connected greenhouse than the multiple equivalent spaces in several free-standing greenhouses.

Sawtooth Greenhouses

Sawtooth greenhouses are constructed with one roof slope having a vertical side, all above the gutter (Figure 1.8). Depending on manufacturer, the roof can be either arched or straight. In both designs, the distinguishing trait is that one roof slope is shorter than the other, creating a space for a vertical roof vent at the peak where the two slopes of varying heights converge. Louvers or roll-up curtains at the top of each vertical wall can be opened or closed as needed. Sawtooth greenhouses are often naturally ventilated.

Venlo Greenhouses

The Netherlands developed a ridge-and-furrow structure called the Venlo greenhouse (Figure 1.9). The galvanized steel superstructure supports a gable glass roof. The Venlo was designed to use single panes of glass from the peak to the gutter rather than lapped glass and uses no purlins or girts in the roof structure, thereby maximizing energy efficiency and light transmission into the structure. With the roof glazed exclusively in glass, the Venlo offers maximum light to the greenhouse crop-especially important in low-light northern regions and on crops that require maximum light. However, the need for an internal shade system does exist, as heat buildup will occur because of prolonged intense sunlight.

Click on the following topics for more information on greenhouse structures and design.