Greenhouse. Structures and Coverings.pdf

OKLAHOMA COOPERATIVE EXTENSION SERVICE

F-6700

Greenhouse Structures

and Coverings

Michael A. Schnelle

Assistant Professor

Extension Ornamentals/Floriculture Specialist

Oklahoma Cooperative Extension Fact Sheets

are also available on our website at:

http://www.osuextra.com

John M. Dole

Assistant Professor

Floriculture

Many advances in greenhouse materials and designs have

occurred in the last few years. Traditionally, greenhouses were

made of glass and all had a similar structure. Today, however,

a great variety exists, and the materials chosen depend on

production goals and inancial constraints.

Unless the grower is constructing a greenhouse without

the use of a manufacturer, speciic recommendations are

usually provided for adequate durability of the foundation.

The foundation stage of construction is critical and consulting

with appropriate personnel, such as experienced builders, is

highly recommended.

Types of Greenhouses

Freestanding (single) or ridge and furrow (gutter con-

nected) greenhouses are two common styles of commercial

greenhouses. The freestanding style is often a Quonset, which

will accommodate many growing situations but presents height

restrictions near the side walls. Another freestanding style is

the single gable greenhouse and its many variations.

The ridge and furrow or gutter connected greenhouses

are joined at the eave by a common gutter. When several

of these buildings are joined they are often referred to as a

greenhouse range (Figure 1).

Structural Components

The major vertical support of greenhouses is provided

by rafters placed on two to four foot centers, depending on

the strength needed. Depending on the width of the structure,

truss or curved arch type rafters are used.

Purlins are arranged horizontally and connect the rafters.

Four to eight foot spacings are used depending on the size of

the greenhouse. Cross ties are also used in certain houses.

In areas where high winds occur, purlins are important in

maintaining structural integrity of the greenhouse.

Side posts and columns are vertical supports that dictate

the height of the production area. These range from one to

ten feet in height and should be given serious consideration

since they directly inluence eficiency (Figure 2). Quonset

style greenhouses are an example of a structure with low side

walls, making it dificult for laborers to work or grow plants in

that reduced area (Figure 1).

Foundations

The foundation must resist overturning and vertical pres-

sure from structural loads and snow, and should extend below

the frost line. Concrete is the most appropriate material for

permanent structures. A 2,500 PSI or greater mix should be

chosen when ready-mix concrete is used.

Figure 1. Commercial greenhouse structures: A) gutter connected, B) Quonset, and C) single gable.

Division of Agricultural Sciences and Natural Resources • Oklahoma State University

Framing Materials

Aluminum is the most economical material for constructing

the greenhouse frame. It can be shaped as needed to form

various structural components of the greenhouse and needs

no maintenance after installation. Aluminium framing also has

the longest life span and allows for light relectance.

Steel is commonly used but must be painted or galvanized

to resist high moisture conditions within the greenhouse. Steel

needs more maintenance than aluminum and is heavier,

requiring additional support.

Wood was once a common framing material, but it has

steadily lost popularity for a number of reasons. The main

disadvantage of wood is that it deteriorates over time. If wood

is desired, pressure treated lumber should be purchased

and then treated with commercially available coatings. Avoid

PENTA and creosote since they liberate fumes that are harmful

to plants.

guarantee their materials for up to four years. Look for ultra-

violet stabilized products that are slower to yellow and crack.

Single and triple layer polyethylene coverings are used less

commonly. Replacement of a polyethylene covering as a result

of wind, hail, or solar damage will be more frequent than any

other covering, but the low investment and the need for less

structural components make it a material to consider when a

temporary greenhouse is desired. Two additional products,

available commercially, to extend the life of a polyethylene

house are poly patch and anti-drip material. The anti-drip mate-

rial reduces condensation that can harm plants growing below.

Condensation should also be controlled to prevent lowered

light transmission, especially during the winter months when

low light levels may limit plant growth.

Fiberglass is another material that has gained popularity

over glass. It is very durable, rigid, and available in various

light transmission levels. Ultraviolet light will cause iberglass

to deteriorate in a few years from swelling and fraying of the

ibers. This in turn quickly leads to lowered light transmission

to the plants. However, there are products now available that

will help reduce the fraying. These coatings should be applied

prior to damage to the iberglass. Although the life span of

iberglass can be as short as ive years, choosing higher grade

products and applying a coating may result in a twenty year

life span.

Polycarbonate is one of the newest materials available

and is still being evaluated. This material is rigid but also

lexible enough to be used in a Quonset style greenhouse.

Although the initial cost of polycarbonate is high, a ten to

15 year life span can be expected. Polycarbonate can be

purchased in double and triple walled forms which are highly

impact resistant.

Acrylic is also new and expensive but it has a minimum

ten year warranty from some manufacturers. Although many

advantages exist such as high light transmittance, high impact

resistance, and great strength, costs have proven prohibitive

in most cases.

Coverings

Glass allows maximum light transmission in greenhouse

production. Despite this, there are several disadvantages to

consider. Glass is expensive and, because it is fragile, has

to be replaced more often than many other materials on the

market today. Also, consider that when using glass, the cost

of structural components will be expensive because of the

added weight which must be supported. When glass is desired,

check with the manufacturer for double and triple strength

ratings available. Also, “hammered” or “frosted” panes will

distribute incoming light better by dispersing the rays, result-

ing in fewer shadows. This type of glass is not transparent

from the outside, which can be an added security beneit in

certain situations.

A double layer of polyethylene, inlated with air, is another

option to consider for covering the greenhouse. One advan-

tage of using this material is that it is relatively inexpensive.

Materials have greatly improved and some manufacturers

Figure 2. Basic structural components of a greenhouse: A) rafter, B) end wall, C) side posts, D) side wall, and E) purlin.

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Remember to check with the covering manufacturer for

several factors.

1. Combustibility - Some materials may result in lower insur-

ance premiums if they are ire retardant. Other materials

are highly lammable, such as iberglass reinforced panels

(FRP), and that must be considered. Fire retardant FRP

panels can be purchased.

2. Durability - Not only will this differ among materials, but

also will be determined by whether they are one, two, or

three ply.

3. Insulation - Note insulation “R” factor and compare for

fuel savings. Heat retention from highest to lowest:

• Acrylic (double layer)

• Polycarbonate (double layer)

• Glass (double layer)

• Polyethylene (double layer)

• Fiberglass

Oklahoma Greenhouse Companies

Aqua-Hort Systems, Inc.

P.O. Box 57197

Oklahoma City, OK 73157

Acme Engineering & Mfg. Co

P.O. Box 978

Muskogee, OK 74402

Out of State Greenhouse Companies

B.F.G. Supply Company

1455 Kinsman Road

Burton, Ohio 44123

Stupp GH Mfg. Inc.

P.O. Box 12456

N. Kansas City, MO 64116

Keep in mind that this is relative to the number and thick-

ness of layers used.

4. Life span

5. Care - Maintenance may be required to realize life span

claims from manufacturer.

6. Guarantee - Read warranties carefully as most will be

limited.

Poly-Tex, Inc.

P.O. Box 458

Castle Rock, MN 55010

Jaderloon

P.O. Box 685

Irmo, SC 29063

Shading Compounds

During the summer months the solar radiation in Oklahoma

is too intense for many crops to be grown to acceptable qual-

ity. Shading is necessary to make greenhouses usable and

to prevent overtaxing the cooling and ventilation systems.

Many commercial shading compounds can be applied in

the spring and removed as fall approaches. Instructions are

provided to reach the desired percentage of radiation blockage.

Liquid formulas are commonly used for glass and sometimes

iberglass. However, frayed iberglass may absorb the liquid,

making it impossible to remove when maximum solar radiation

is again needed.

Another alternative is to use shade cloth. It can be pur-

chased for the percent of solar radiation reduction desired,

depending on the crops grown.

Keeler-Glasgow Company, Inc.

80444 C.R. 687

P.O. Box 158

Hartford, MI 49057

National Greenhouse Co.

P.O. Box 100

Pana, IL 62557

V & V Noordland

P.O. Box 739

16 Commercial Boulevard

Medford, NY 11763

Atlas Greenhouses

Dixie Mfg. Company

Rt. 1, Box 339

Alapaha, GA 31622

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