The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
U OF F LIBRARIES
T. C. Skinner
FLORIDA COOPERATIVE EXTENSION SERVICE
INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES
UNIVERSITY OF FLORIDA, GAINESVILLE
JOHN T. WOESTE, DEAN FOR EXTENSION
Some Greenhouse Considerations*
T. C. Skinner 1
GREENHOUSE LOCATION AND ORIENTATION
Most greenhouses are erected to produce plants during the off-season;
therefore, they must provide a desirable plant environment. Correct location
and orientation of the house are of paramount importance in providing ideal
environmental conditions. Since location can also influence the heating cost,
labor utilization, and disease factors, economic success may also depend on
the site selection.
Recommendations for locating and orientating your greenhouse are given
in this publication. The following specifications are the ideal ones; however,
some builders may not be able to follow each suggestion given, depending on
individual limitations of his greenhouse. For example, houses used for display
and sales purposes are often connected to existing buildings; therefore, they
may have to sacrifice some of those factors described for production-type green-
houses. Bench culture or similar production practices may also justify deviation
from some of the standard requirements.
Location Of The Greenhouse
Sunlight provides energy for plant growth, and is generally the limiting
factor in greenhouses. When planning the construction, give primary considera-
tion to obtaining maximum sunlight exposure during those "short" days of mid-
winter when the sun is lowest in the sky. Maximum sun altitude (angle of sun
above earth's horizon) occurs at noon and varies from a high on June 21 to a
low on December 21. At solar noon, the sun is located due south. This means
that the building site should preferably have an open southern exposure. If
the land slopes, it should ideally slope to the south.
Do not build near large trees, buildings, or other obstructions which will
shade the building. Figure 1 gives the ratio of shadow length and height obstruc-
tions for selected solar altitudes. To determine how far away an obstruction
must be to prevent a shadow on the greenhouse, multiply these ratios by the ob-
struction height. As a general rule, no objects taller than 10 feet should be
within 27 feet of the greenhouse in either the east, west, or south direction.
Even objects this tall will cast long shadows in the early morning when the sun
is particularly low in the sky.
* Based on published information by J. N. Walker and G. A. Duncan, Department
of Agricultural Engineering, University of Kentucky, Lexington, Kentucky.
1 Extension Agricultural Engineer, Department of Agricultural Engineering,
IFAS, University of Florida, Gainesville, Florida.
- 2 -
Ang1e Ratio L/H
O OBSTRUCTION (H)
Figure 1. Ratio of shadow length and obstruction height for
selected solar altitudes.
When plants are to be grown in the soil covered by the greenhouse, select
a site where a deep, good draining soil is available. Avoid top soil below which
a tight hardpan is present. Although organic matter and artificial types of con-
ditioners can be added, problems are reduced if a site with good natural soil is
selected. Grading often produces uneven soil conditions within the greenhouse.
Careful soil analysis and preparation are necessary if even plant growth is to be
achieved. Before any organic matter or additives are mixed with the soil, make
a complete soil analysis and carefully follow the resulting recommendations.
Avoid areas where chemical residues which would injure greenhouse crops may
persist, including places heavily sprayed with damaging weed killers and herbi-
cides. If you question the degree of danger, grow selected plants in samples of
the soil to determine if any detectable injury occurs. Noxious weed seeds can
also be a problem, but generally proper sterilization methods will kill most
Select a site that is level and well drained to reduce problems with salt
build-up and insufficient soil aeration. A high water table may result in satura-
tion of the soil and prohibit effective use of the greenhouse. Ground water which
flows into the house may carry soil diseases. If necessary, tile drain the area
enclosed by the greenhouse.
Ground beds should be nearly level. If they slope in any direction, water
will tend to concentrate in the low areas, accentuating any problems of poor
drainage. Slopes within the greenhouse also allow hot air to rise and cold air
to settle, creating environmental problems. A greenhouse in a low, damp area
could be subject to higher humidities and dampness which accentuate leaf mold,
Although obtaining maximum sunlight should have first consideration,
placing the greenhouse in a sheltered area will reduce wind-induced heat
losses. For example, a wind barrier north of the greenhouse may materially
reduce heating costs; yet it would have little effect on the light received.
Trees are helpful in preventing heat loss, but deciduous trees which lose their
leaves in the winter are not effective when the heat loss potential is greatest.
A greenhouse requires a number of utilities, notably electricity, water,
and an energy source for heat.
Electricity: The electric service for ventilation alone will require
approximately 4 to 6 kilowatts for a 1/4-acre range. For a small hobby-sized
house, connected loads of up to 1 to 2 kilowatts are not unusual. If lights are
used for photoperiod control or supplemental lighting, the electric load will
increase significantly. Normally the electric power companies will willingly
supply the necessary service; however, the grower should attempt to anticipate
his intended electric usage and provide sufficient entrance capacity to allow
for full electric utilization.
Water: A reliable supply of clean water is mandatory. A water requirement
of up to 1/3 gallon per square foot per day may be needed. Depending upon the
soil type, up to 1 gallon of water per square foot may be put on the soil "at one
Energy Source: The availability of an inexpensive energy source is often
one of the most important factors in determining where to build a greenhouse
range. Natural gas is a widely preferred fuel because of its clean performance,
low maintenance, and relatively low cost. Not only is it one of the lower cost
fuels, but gas heating equipment is generally among the least expensive for initial
cost, annual maintenance, and operating costs. LP Gas and fuel oil are alternate
sources of fuel and can be transported to greenhouse locations that are not close
to gas lines. Since heating costs can be a substantial part of the production cost,
select a location carefully for its fuel availability and economy.
Standby emergency power equipment sized for electrical support of heating
equipment, air circulation, and minimum ventilation is vital when storms disrupt
local service for long periods (2 hours or more).
The consequences of a heating system failure during freezing temperatures can
be catastrophic. An alarm system which is independent of the electrical service
should, therefore, be provided. Place the alarm bell in a residence or location
where people are normally present.
Locate the greenhouse near your place of residence, if possible. This
will prove convenient for you and will facilitate care during weekends or
holiday periods. If the ventilation and watering systems are not fully auto-
matic, operator care will also be mandatory during sunny periods. Should a
heating failure occur, corrective action must be prompt.
Orientation Of The Greenhouse
Light Availability and Shading Effects
Orientation of the greenhouse for
maximum light availability is also an
important consideration. For the southern
regions, a north-south orientation of the
ridge is preferable. With east-west ori-
entation, however, a problem is encountered
with ridge and furrow houses as shown in
Fig. 2. A definite shadow line develops
within the houses due to the north sloping
roof sections and the gutter between sec-
tions of the house. This shadow effect is
usually sufficient to result in reduced
plant growth in the region of the house
affected. Depending upon the width of
span, the shadow area can be 10 percent
or more of the house space. Although
shadows occur within north-south oriented
ridge and furrow houses, the shadows move
across the floor of the house as the day
progresses and noticeable reduction in
growth in one region of the greenhouse is
not normally apparent.
a SH-ADOW AREA
FIG 2 SHADOW EFFECT OF NORTH SLOPE AND GUTTER OF EAST-WEL
GREENHOUSES WHEN SOLAR ALTITUDE IS 30?
Ventilation, cooling and heating systems are noticeably affected by the
way the greenhouse is oriented and the equipment installed.
Ventilation: Ventilation air should not have to move more than 120 to
150 feet across the house between entrance and exit. Design and install fan
ventilation systems so that air moves with prevailing summer winds rather than
against them. This procedure will eliminate opposing air forces which decrease
the air flow rate by 10 percent or more. Usually, you should install exhaust
fans in the leeward end of the greenhouse and fresh air inlet shutters in the
windward end. However, sometimes a sidewall fan location in the leeward side
and fresh air inlets on each end are best for certain houses.
(end or side) of the
tiveness, air should
When pad cooling is used, locate pads on the north wall
greenhouse(s) to prevent shading. For best cooling effec-
not travel over 150 feet between pads and the exhaust fans
- 5 -
(200 feet absolute maximum). For long houses, over 150 feet in length, locate
the bank of pads in a sidewall at the center of the house with fans in each end.
(NOTE: A water supply of approximately 1 to 2 gallons per minute per 100 square
feet of pads is generally required. Also, 100 square feet of pad areas per 1,200
to 1,500 square feet of greenhouse floor area is typical. Obtain more detailed
information on pad cooling before making a decision to purchase.)
Heating System: The greenhouse heating system should provide adequate heat
supply and distribution throughout the house for environmental uniformity. Con-
sistent heat supply is especially important toward the northwest portion of glass
houses or those with sizeable cold air leakage and infiltration.
Grade and Fill
Prior to erecting the greenhouse, grade and fill those areas where changes
are needed to level the site, establish drainage, roads, parking, etc. If you
plan to practice soil culture, remember that poor existing soil must be removed
and replaced by 12 inches or more of good topsoil. Grade and fill to require-
ments, then replace the top soil without compaction. Any subsurface tiling or
urility lines can be placed during these operations.
Transportation and Parking
When selecting the site, try to locate near a good road so that materials
can be conveniently moved to and from the greenhouse. Sufficient room for
turning and parking vehicles is desirable for greenhouses of commercial size,
especially when bedding plants and flowers are sold on the premises.
Place headhouses on the north side of the greenhouse to avoid shading a
portion of the house. Attachment to the greenhouse or a connecting passageway
makes work, handling, and greenhouse operations more convenient. Processing
facilities, cold storage rooms, and other such facilities should be adequately
incorporated into the ultimate layout.
Expansion WIND BREAKS
When building, always keep future PAVED ACS
expansion in mind. Most successful
ranges are expanded several times after
the first house or small range area is / .--,TRUK ACESS
"'- OPTIONAL SEPA-
.------ RATE STORAGE
Figure 3 illustrates many of the ;8ARKING L ASREQUtE
factors of location, orientation, and EAD
/ L IHOUSE I .. ....
layout for one greenhouse or a range. -----U
S!i i FUTURE
o I i I I EXPANSION
FIG 3 TYPICAL GREENHOUSE RANGE LAYOUT ON A LEVEL
BUT WELL DRAINED, SOUTHERLY EPOSED SITE.
Important points to remember are:
-- Orient and locate the house for maximum sunlight. In southern latitudes,
the ridge should run north-south.
-- Avoid placing the house near objects east, west, or south which will shade
-- Place in an area sheltered from northerly and north-westerly high winds if
-- Locate on a deep good soil which is well drained and where surface water
does not run into the house.
-- Avoid sloping beds or floors in the greenhouse. Locate the greenhouse
near adequate and reliable sources of utilities--electricity, water and
-- Provide good access roads, parking, and turn-around area.
-- Position headhouses or supporting facilities on the north side.
-- Arrange initial construction so that the range can be expanded.
Greenhouses vary from small hobby types to ranges which cover several acres
in one enclosure. All of them have one thing in common, they are built to permit
the off-season production of plants. The basic purpose of the greenhouse struc-
ture is to provide a reliable enclosure within which an environment favorable to
plant growth can be created.
In commercial greenhouses, operations are generally permanent and long
depreciation schedules are acceptable. Annual cost is generally more important
than the initial cost; however, if capital is limited, the initial cost may still
govern the type of construction used. In determining the annual costs, deprecia-
tion, interest, maintenance, insurance, taxes and operating costs must all be
Regardless of its use, every greenhouse must meet certain functional require-
ments. For those planning to build, an understanding of these requirements will
help in selecting a design. In the discussion that follows, the remarks are largely
directed towards commercial greenhouses.
- 7 -
Every greenhouse must be designed to withstand the loads which will be
imposed upon it without failure or significant deformation. The primary load
which must be considered is wind.
For most sections of the United States, the major load which must be con-
sidered is wind. The wind speeds used in design of crop production buildings
should be the winds predicted for a 25-year recurrence interval. This means
that you would expect winds of the given intensity to occur once every 25 years.
For the majority of the United States, except for the two regions shown in
Figure 4, the maximum expected wind speed would be 80 mph. In the two shaded
regions, special consideration should be given to design, and the advice of
engineers who are aware of the local wind conditions should be sought.
A wind blowing at 80 mph develops a maximum possible pressure of 16.4 Ibs.
per sq. ft. on a flat surface perpendicular to the wind. However, the actual
wind loads on buildings are not this severe because of a height adjustment
= WIND VELOCITIES ABOVE 80 MPH
FIG.4- WIND MAP FOR A 25 YEAR RECURRENCE INTERVAL.
- 8 -
Wind speeds are measured and reported by the U.S. Weather Bureau for a
height of 30 feet above the ground. The wind is less intense closer to the
ground. The wind velocity reduction from 30 feet above ground to the ground
surface is shown in Figure 5. The pressure developed by the wind is related
to the square of the wind velocity. Therefore, the wind pressure reduces more
rapidly than the wind velocity as the ground surface is approached. As shown
in Figure 5, the pressure and wind velocity at 15 feet height are approximately
85% and 90%, respectively, of that at 30 feet. At 10 feet the values are 73%
and 85%, respectively. The height of the building will, therefore, affect the
E CASE 1.
H GREATER THAN E
EFFECTIVE HEIGHT = H
H CASE 2.
H LESS THAN E
:L W EFFECTIVE HEIGHT = H + -
Figure 5. Effective height of a building
for wind loading.
Figure 6. Wind velocity and pressure /
30 reduction near the ground
0 0.2 0.4 0.6 0.8 1.2
0 0.2 0.4 0.6 0.8 .0 1.2
WIND REDUCTION FACTOR
- 9 -
The effective height of a building is defined as the distance above the
ground at which the wind force acts. This is illustrated in Figure 6. For
many buildings, it is the eave height (H). However, for wide buildings or
steep-roofed buildings where the height from the eaves to the peak is greater
than the eave height, the effective height is the distance from the ground to
midroof (H + E/2). The eave plus mid-roof distance can become fairly large
and result in large effective wind pressures that are important in building
designs. For example, if an 80 mph wind speed, as reported by the U.S. Weather
Bureau, occurs and effective building height is 8 feet, the maximum potential
wind pressure would be reduced by a factor of .68 (see Figure 6) which results
in the wind pressure being reduced from 16.4 to 11,1 Ibs. per sq. ft. For a
building 100 ft. long, the forces against the building could possibly total
8,880 Ibs. However, one more factor in wind forces must be considered.
WIND DIRECTION -
H/W = .4
+ .7 1
H/W = .2
S.7 -- ----* -.40
___ W _
E/W = .3
E/W = .4
Figure 7. Typical wind force coefficients for various roof slopes.
- 10 -
The full wind pressure is not normally developed on the surface of the
building because of shape and orientation. The wind generally hits the surfaces
at some angle, depending upon the building profile, and an aerodynamic effect
develops. This is similar to the forces which are created over the surface of
an airplane wing. Due to the aerodynamic action, an uplifting or suction force
develops on many building surfaces. To describe the forces, wind force coeffi-
cients are used. These coefficients are multiplied by the wind pressure to
determine the design wind forces. A negative coefficient represents a suction
or uplifting force and a positive coefficient, a pressure or inward acting force.
Typical coefficients are shown in Figure 7. For each roof shape, the windward
roof section coefficient varies considerably with the height to width ratio and
roof slope. The other coefficients are affected only slightly by changes in
these values. For the upper roof sections and for the roof and wall sections
on the side away from the wind, uplift or suction occurs, the magnitude of
uplift being 0.50 to 1.0 of the potential wind pressure force. For example,
for a semicircular greenhouse with an effective height of 8 feet and E/ equal
to 0.4 (Figure 7), the lower 1/4 roof section on the windward side would have
an inward acting pressure of 5.4 Ibs. per sq. ft. (11.1 lbs./sq. ft. times 0.49).
For the same greenhouse, an uplift of 7.8 Ibs Ibs. per sq. ft. would occur in
the center section; and on the downwind side, an outward acting force of 6.4 Ibs.
per sq. ft. would be expected. Interestingly, the uplift force is the largest
force and is of a magnitude comparable to snow loads in most areas of the
United States. This means that anchorage of the building and covering to pre-
vent wind uplift and overturning of the building is just as important as providing
adequate supports for snow loading!
The problem can become even more critical if a large door is left open on
the windward side of the building during high winds. If this happens, a positive
pressure builds up inside the greenhouse so that not only is there an uplift
suction force due to the aerodynamic effect, there is also a pressure on the
inside equal in magnitude to about 0.7 of the potential wind force acting in the
same direction. These two combined forces, when they occur, are generally greater
than the full potential wind force!
If the greenhouse roof frame is also to support a crop load, such as hanging
baskets, additional strength may be required.
For any given wind condition, the loads imposed upon the structure must be
carried to the ground by the foundation and footings. The foundation and footings
must, therefore, resist uplift, overturning and downward acting loads. The down-
ward load includes not only possibly the crop weight, but also the dead weight
of the structure itself.
Though the size of foundations and footings would depend upon the size and
type of the greenhouse and the loads which occur, all foundations for permanent
greenhouses should be of a durable material and should extend to a minimum depth
of 18 inches. Little support can be expected from the loose topsoil and earth
within the top 6 to 12 inches. If post construction is used, where the main
side-wall members are set directly into the ground and where the soil around the
- 11 -
post is intended to prevent overturning, depths deeper than 18 inches are
required. The most common failure of greenhouses constructed in this manner
is the tipping of sidewalls with a subsequent dropping of the ridge line and
a weakening of the rafter-to-eave plate joint. Though the filling of post
holes with concrete is helpful this is not a substitute for placing the posts
to an adequate depth.
For permanent greenhouses, concrete is the most suitable foundation material.
A 2,500 PSI or better mix should be ordered if ready-mix concrete is used. If a
continuous foundation is used along the full length of the house, it is recom-
mended that 3/8" diameter reinforcing rods be placed horizontally in the founda-
tion approximately 1 1/2" from the bottom and top and 1 1/2" from the edges of
the foundation. This is particularly desirable in areas where high soil moisture
exists, where rock outcroppings occur, or where some fill has been used and the
foundation is near the surface of the original soil. Where fill has been used,
the foundation or footings should always be placed sufficiently deep so that
they bear on the undisturbed soil on which the fill was placed.
Special precaution must be taken with foundations if rigid-frame construc-
tion is used. As loads occur on rigid frames, there is a tendency for the base
legs of the frame to spread, and horizontal forces as large as the vertical loads
can be developed. This spreading tendency must be resisted if the frames are to
carry the loads without failure. This means that foundations such as masonry
block, which have low resistance to horizontal loads, should not be used for
supporting rigid-frames unless proper reinforcement is done.
Maximum Light Transmission
In most greenhouses light becomes the limiting factor in growth during much
of the off-season production period. Consequently, everything which can be done
to obtain maximum light intensity within the greenhouse should be accomplished.
Orientation is important and is discussed elsewhere. As a general rule, the green-
house ridge should run north and south in the southern parts of the United States.
The roof slope should also be about 280 (6:12 slope) or more whenever possible.
The most important features in obtaining maximum light transmission are to
minimize the number and size of structural members in the roof area and to use a
highly transparent glazing material. It is for these reasons that wide span glass
and certain plastics are used. The use of stronger, wide-span covering material
reduces the number of supporting members required, and thereby reduces shading.
Wooden truss houses, though providing clear span unobstructed interiors, have the
disadvantage that there are more members to cause interference with light and thus
are not recommended. Only properly designed and fabricated steel or aluminum
trusses should be used. Overhead heating, irrigating, and electrical lines should
also be kept to a minimum to prevent light blockage.
Structural Influence on Heating and Ventilation
The final success of a greenhouse will generally depend upon the ability of
the operator to control the environmental conditions within the greenhouse. Though
any shape structure can be successfully heated or ventilated, some designs greatly
increase the difficulty or cost in providing an adequate system. In these cases,
- 12 -
a less than optimum system is often installed, which then creates problems in
In greenhouses used the year around, the solar intensity during the winter
will often become sufficiently intense to require ventilation even when outside
temperatures are near freezing. If this air is brought into the house and
directed on the crop without first intermixing with the air within the green-
house, growth will be hurt. Similarly, if hot air from heating units is allowed
to come in direct contact with the plants, rapid drying and poor growth will also
occur. In most greenhouses, attempts are made to mix ventilation or heating air
with the greenhouse air in the greenhouse space above the crop. In bedding plant
and other low crop production, adequate space exists within even low profile houses
to effectively achieve this mixing above the crop. However, with tall crop pro-
duction on benches, adequate space may not be available. This is particularly
true in narrow houses where the rise from the eaves to the peak is small. Some
growers argue that they want low houses to minimize the volume of air they must
heat, but this volume is not a truly important factor. The important factor in
the cost of heating is the amount of exposed wall or ceiling area, since any heat
added to a greenhouse remains in the house until it passes through the covering
material. Though an increase in height does increase the wall area, this is
usually a very small increase in the total amount of exposed area. As a general
rule, at least 1/3 of the total house volume should be unoccupied if it is intended
to use this space for the introduction and intermixing of ventilation or heating
air with the greenhouse air.
Ventilation and evaporative cooling both require the introduction of large
quantities of outside air during bright warm days. A common method of doing this
is to place fans in one sidewall or end and introduce the air through baffles,
pads, or louvers in the opposite wall or end. When this is done, the air picks
up the solar heat in the house as it moves across the house, resulting in the air
gradually increasing in temperature as it nears the fan. If this temperature
rise and air velocity are to be kept within reasonable limits, the distance
across the house from the fans to the air inlet openings should not be more than
100 to 150 feet (200 feet absolute maximum). The house length or width in the
other direction can be any desired dimension depending on the size of greenhouse
The height of the house in the walk areas should never be less than 6'6".
This allows a working man to move conveniently through the house. For tall crops
such as tomatoes, 6 feet should also be the minimum height at the eaves, and 7 feet
is commonly considered as being the minimum desirable height. For low crops, the
eave height can be as low as 4 feet, as long as greenhouse workers do not have to
regularly move back and forth in this area of the house. Some quonset type houses
severely restrict plant growth around the outer walls due to the low curvature.
One other important factor is the roof slope of the greenhouse. The roof
slope affects the run-off of condensed water from the ceiling. Slopes of 280
(6:12 slope) are generally considered as minimal if run-off without severe
dripping is to occur. With lower slopes, run-off is restricted, and dripping is
a serious problem. With some of the plastic covering materials where drops occur
more readily than they do on glass, even greater slopes would be desirable.
- 13 -
In most greenhouses it is necessary on occasion to remove large amounts
of plants, and in some instances, to remove the soil or rooting media. On
these occasions, large doors in the end of the greenhouse will prove useful.
This permits the use of tractors or large wheeled carts or wagons. Such doors
also facilitate the use of large equipment for tillage operations within the
greenhouse. If walking tractors are to be used, 4-foot wide doors are adequate,
but 6 to 8 foot doors are desirable if standard-size, four wheel tractors and
wagons are to be used within the greenhouse.
Types of Greenhouse Structures
Greenhouse structures come in a variety of shapes and styles. All are
acceptable if properly designed and erected. The features discussed in the
previous section should be considered in selecting a specific type of greenhouse.
In addition, cost, aesthetic appearance, flexibility and availability should be
The various common shapes and advantages
are listed below:
--Simple and efficient construction
using thin-wall electrical conduit
for houses up to 10 to 12 feet wide
(build yourself) or galvanized steel
pipe commercially formed for wider
houses up to 36 or 40 feet.
--Primarily used with plastic covering
which is applied externally. Strong
fastening at the ends and edges (and
ridge sometimes) is very important.
Various types of extruded metal bars
with rod inserts and screw-down clamps
are available. Wider spans with larger
curvatures bendable can be used with the
or special features of each type
--Internal layer of plastic is difficult to apply and therefore external double-
layer with the air inflation technique is recommended.
--Provides clear-span interior with minimum shading but has some side-wall height
restriction on tall crops unless higihr- foundation supports are used. Higher
foundations increase significantly the strength required and the potential for
--Not suited for ridge and furrow designs. Each house should be separate from
CIRCULAR PIPE FRAME
\i TEEL PIPE,
TREATED WOOD, OR
- 14 -
--Usually have to use extra wooden construction for ends, doors, fan-shutter-
louver framing, etc. End-wall covering may be the same as rest of structure.
Sometimes solid material is used on a north end-wall.
--Several commercial pre-fab packages are available for this type of greenhouse.
--Fabricated by bending pipe or lami-
nating wood strips in a gothic shape.
--Has pleasing aesthetic appearance.
--Construction and covering features
similar to quonset above.
--Good height near side-walls but has
ratio which allows more heat loss than
other designs. Good air circulation
is required to prevent air and heat
stratification in the gable. The extra
volume allows adequate space for mixing
ventilation or heated air with the
--Usually built of wood from blueprints.
limited in availability.
Commercial pre-fab packages are
--Simplest construction, but requires
more wood or metal than some other
--Clear-span interior, but limited in
width to approximately 20 or 24 feet
with wood due to rafter size and
strength. Wider houses can be built
with steel or aluminum, depending on
--Wood construction is primarily used with low-cost film plastic coverings.
--Requires strong sidewall posts and deep post embedment to withstand outward
rafter forces and wind pressures.
--Smooth interior and exterior for easy covering.
- 15 -
A-Framle: COLLAR BEAM
--Comparable to the wooden post-rafter RAFTER
construction above except the collar RAF
beam strengthens the rafter construc-
tion for wider houses but hinders EEMBEDDED
placement of an inner liner. POSTS
--Provides high strength per unit of
wood used and is suitable for self RAFTER
construction with proper materials GLUED OR NAILED
and blueprints in widths of 10 to PLYWOOD GUSSETS
--Used in many commercial designs of
steel or aluminum structures,
especially glass covered houses. CONCRETE
--Unobstructed clear-span interior.
--Smooth interior and exterior for ease of covering with film or rigid plastic
materials for most build-your-own designs.
--Excellent strength permits a relatively small number and size of wood or metal
members to be used, resulting in minimum shading.
--Proper foundation piers or concrete wall required for adequate support of large
lateral loads developed with such frames.
--Suitable for ridge and furrow type construction.
--Wood trusses not recommended due to CA A TR
excessive shading. Use only steel or
aluminum designs having adequate
strength and minimum shading. _"
--Clear-span floor area, but truss -- EMBEDDED
members obstruct gable space. POSTS
--Commercial designs available in widths
up to 60 feet or more.
--Well suited to covering with rigid TRUSS
plastic panels or glass panes, de-
pending on sash bars used. Not
often covered with film plastics.
- 16 -
--Sidewalls must resist lateral wind loads which require strong posts and
adequate embedment, and, normally, concrete backfill around posts.
--Suitable for ridge-and-furrow construction.
The most common structural materials are wood, galvanized steel, and
The most commonly used material with home- or local contractor-constructed
greenhouses is wood. It is also used to a limited extent by some commercial
greenhouse manufacturers for the gothic shaped structure and for glass supporting
bars. Regardless of where wood is to be used in a greenhouse, the high moisture
environment which exists within a greenhouse makes it mandatory that only treated
lumber be used if reasonably long life is desired.
When wood is used for the main structural frame, care should be taken to use
only high grade wood. Many of the designs are based upon select structural quality,
which is the highest grade normally used in construction. Some of the designs
further specify that any defects in the lumber be placed out of the high stress
areas. With the rigid-frame designs, this high stress area is the stud and rafter
closest to the eave joint.
When plywood is used for gussets with trusses or rigid-frames, it should be
exterior grade, and one of the faces should be a smooth face at least "C" grade
or better. Also, the plywood should be preservative treated for long life com-
parable to the treated wood. The edges of plywood should be well painted to
prevent moisture from entering into the edges and causing delamination.
Gluing of wooden joints is frequently specified and used because of the
superior strength obtained per unit of wood used. Though casein or "white" glue
is commonly used for structural gluing, the high moisture condition in greenhouses
requires an adhesive highly resistant to moisture. Thus, the casein or "white"
glues should never be used for greenhouse construction. The adhesive which has
the moisture resistance capabilities desired and which can yet be used under
noraml conditions is resorcinol resin. This adhesive will cure at temperatures
at 700F or above and is marine-rated for use in "humid" environments. It is a
reasonably good gap filler and is tolerant of some minor surface irregularities.
This adhesive is actually stronger and more durable than wood. Resorcinol adhesive
is not available in all localities; but, due to its exceptional performance regard-
less of the exposure condition and its ease of use, no other adhesive should be
substituted without an engineer's approval. Tests on the gluing of plywood to
treated lumber indicate that adequate joint strength can be obtained with resor-
cinol adhesive with penta-chlorophenol or the salt-type preservative treated
lumber if visible oil and preservative crystals are removed by sanding or wire
brushing prior to gluing.
- 17 -
Steel is commonly used for commercially manufactured houses. Occasionally
growers bend steel pipe to form small quonset or gothic shaped houses. The high
moisture within a greenhouse can result in excessive rust. All steel should be
painted or galvanized. If galvanizing is done, it should preferably be done
after all cutting and welding has been performed. Those areas where bare metal
is exposed by cutting or welding should be painted. If steel is kept painted,
it is a highly durable material and should last indefinitely. Care must be
taken when cleaning dirty glass with acid, due to the corrosiveness of the acid
on galvanized coatings and paint. Acid should be kept away from the steel as
much as possible. After completion of the cleaning, the metal affected should
Aluminum is being used more extensively by commerical manufacturers due to
its light weight and excellent durability. Aluminum generally requires no main-
tenance and is very attractive. Its high strength makes it possible to use
small roof support members, minimizing shading problems. Aluminum has not yet
been used to any extent in owner-fabricated, build-your-own type greenhouses.
The type of structure used must be compatible with the covering desired.
All the quonset and wooden structures can utilize film plastic; some can use
corrugated fiberglass. Most all the commercial steel and aluminum designs use
fiberglass or glass materials for covering.
The type of greenhouse one builds should depend upon its use, location,
size, and the grower's preference. The grower should consider both the initial
and annual costs. If capital is limited, initial cost is very important. Pos-
sibly one of the designs which would permit an initial covering with low cost
film but a later covering with a rigid plastic would be preferable. As a general
rule, the increased durability of well constructed glass houses and the reduced
annual maintenance costs due to the elimination of recovering results in glass
greenhouses and many of the plastic greenhouses having comparable annual costs.
In determining what type of greenhouse to build, a grower will need to consider
Initial cost--Is capital limiting or is the long term future unclear?
Annual cost--How much are the actual annual costs which must be borne by
the production income?
Insurance--Can the house be insured against fire, and is the insurance high?
Some of the plastic houses cannot be insured for fire and the poorer plastic house
designs cannot be insured for structural failure.
- 18 -
Taxes--Are plastic houses not taxed or taxed at a low rate as temporary
structures in the community in which the house is to be built?
Heating costs--The use of a double layer of film can reduce heating costs
by 30 to 40%. Double covering reduces light and requires some extra labor for
annual or every-other-year installation.
Environmental control--Does the design lend itself to easy installation
of heating and ventilating equipment and provide an environment suitable for
the crop being grown?
Flexibility--Can the crop being grown be changed if the economic picture
changes and can the structure be easily expanded for a larger range?
Structural strength--Is hail a problem in the area in which the house is
to be built? Are there other physical hazards or unusual circumstances?
Labor simplification--Does the design permit the use of mechanical equip-
ment? Will it allow future installation of mechanization?
There are not simple, clear-cut answers to the above questions, and the
grower must select the type of house he feels will best meet his needs. Still,
he should select a sound design which can withstand the wind and snow loads in
the area the house will be built. The failure of a greenhouse structure can
have catastrophic consequences, and a poorly-designed house will almost always
end up being the most expensive.
The use of greenhouse benches has several advantages over planting directly
in the greenhouse soil or setting pots in the soil: 1) plants are at a more
convenient height to work comfortably; 2) benches permit a more effective display
of plants; 3) benches provide improved air circulation and environmental control
around the plants; and 4) benches permit better disease and growth control.
Greenhouse benches elevate plants and flowers closer to eye level where they
can be better observed and tended without the inconvenience of stooping or bending.
This is especially important for many flower crops where plant height is not too
Air circulation is very important for good plant growth. Though plants might
dry more rapidly when grown on benches than when placed on or in the ground, this
is often an advantage due to the better control over watering which it permits.
With control over watering, fertilizers can be properly applied with regard to
both time and amount. Increased air circulation also helps to minimize root and
foliar diseases since it causes the plant surfaces to dry more rapidly after
watering instead of allowing moist areas where spores can germinate. Also due
to air circulation under benches and around pots, the roots of plants on benches
are warmer than the roots of those in the cooler soil of ground beds, resulting
in a greater growth for the benched plants.
- 19 -
Plants grown in pots placed on
the floor of a greenhouse also have
a tendency to root through into the
greenhouse floor soil unless a barrier GALV. STEEL BRACE-
is provided. And problems with dis- GALV. STEEL STRAP-
eases and insect pests are greater --. CORRU3ATED CEMENT / '
with such culture than when using \ ABESTOSBOARD J
Greenhouse benches should be cus- GALV. PIPE
tomized for the individual operation.
The best size depends on several fac-
tors, such as the height and reach of
persons working around the benches, the
type of plants grown on the benches, CLOSED BENCH
and aisle access to one or both sides
of the benches. A tall person with
correspondingly long reach could comfortably work with higher and wider benches
than a short person. In commercial or other houses where several people are to
be working with the plants, the benches should obviously be of a height and width
suitable to an "average-sized" worker. This means, approximately, a 32- to 36-inch
As for width, benches generally should be 42- to 48-inches wide if they are
to be worked from both sides, and 30- to 36-inches if they are accessible only
from one side. Many establishments use benches wider than this, but difficulty
may be encountered when plants are handled frequently.
Some exceptions do occur. For example, tall flowering plants should be
grown in lower bench-beds so the stems and flowers will not be out of the reach
of workers. Also, temporary benches for growing, bedding, or other small potted
plants could be lower and wider where once-on and once-off handling is practiced.
For example, temporary benches as wide as 5- to 6-feet and only 18- to 24-inches
high are used successfully for bedding plants.
In instances where shade plants are to grow under the benches, bench heights
as high as 48 inches have been used. With these high benches, however, some dif-
ficulty in inspecting and handling plants on the bench is encountered and therefore
such height should normally be avoided.
The arrangement of benches within a greenhouse should depend on several
factors, including 1) the dimensions of the house, and walk-way location; 2) heating
and air circulation patterns; and 3) materials handling into and out of the green-
In any arrangement, the bench-to-aisle space ratio should be considered. For
efficient production houses, the aisles should be less than 1/4 to 1/3 the total
- 20 -
The types of equipment used will cause some exceptions.
In commercial houses the standard arrangement is to run benches the length
of the house, as shown in Figure 8. This permits long continuous runs of watering
lines, heating pipes, and/or plant support techniques. This arrangement often
uses more floor space for aisles than shown.
Normally, benches should not be placed directly against exterior walls,
since this interferes with maintenance of the house. Also, such benches will
likely be cooler than benches elsewhere in the house and uneven plant growth
will occur. Allowing a sidewall space of 6 to 12 inches permits better air
circulation around and under the benches.
In hobby houses, where a great number of different plants having different
temperature requirements are grown, the cooler outside bench may be utilized to
obtain a desirable difference in environmental conditions for particular plants.
The "peninsular" type bench arrangement shown in Figure 9 gives the greatest
amount of bench area per unit of aisle space and yet permits convenient access to
all areas. In this arrangement there is a single, wide center aisle with narrow
aisles branching from it. The center side is made wide enough for all greenhouse
FIG. 8 LONGITUDINAL BENCH ARRANGEMENT PERMITS LONG CONTINUOUS
WATER AND HEAT LINES, AND PLANT SUPPORT TECHNIQUES.
FIG 9 CROSS OR "PENINSULAR" BENCH ARRANGEMENT FOR GREATER
BENCH-TO-AISLE RATIO, AND EASY ACCESS FROM CENTER AISLE.
equipment, while the side aisles are only wide enough to permit a person to
enter. The center aisles would normally be 3 to 4 feet wide and the side aisles
only 1 1/2 feet wide. With this arrangement it is allowable to have a consider-
able amount of bench area adjacent to the outside walls of the house. Where it
is desired to have better air circulation, the benches would have to be moved
in 6-12 inches from the walls to permit free movement of air between the walls
and the benches as described above.
Both the commercial and peninsular bench arrangements may be either flat
or stepped. Flat benches are simply "tables" upon which plants are placed. Step
benches are just what the name implies--"stair-steps." The stepped arrangement
permits a better display of plants for in-house sale and marketing. With wide
or high-stepped benches, it will be necessary to work from one side. The step
bench may be quite light in construction and can be built as a part of the green-
house. It is possible with step benches to get a few more mature plants in the
same bench area than with "tables." One disadvantage of step benches is that
plants on them get more light from one direction. This results in uneven plant
growth unless they are rotated occasionally. Step benches can be built quite
tall and, when so constructed, provide considerable area underneath for shade-
With stepped and flat benches, either a solid or open bottom can be provided.
Slat and wire benches are two types of open benches. Open benches provide the
maximum amount of air circulation among the plants. Insect and disease problems
are also reduced. For example, with some wire benches it is almost impossible
for slugs to travel from one pot to another. If wood is used, slat construction
is generally more resistant to rotting than closed or solid-bottom construction
due to better aeration and drying.
Open benches may or may not have sides. Where sides are used, they serve
primarily to keep plants from being brushed off the benches. This is particularly
necessary in commercial walk-in greenhouses.
Closed benches are used whenever a crop is to be planted directly in the
soil contained on the bench. This system has been successfully used with a
number of cut flower crops. Many growers also use flat benches containing a
few inches of sand or gravel for pot plants. Since the sand or gravel provides
a solid base in which the pot can be set, problems with tipping are minimized.
The sides and solid bottom of such benchesreduce air circulation to a large
extent and thereby slow the drying of pots placed on them. Though this may be
advantageous in some instances, disease organisms and insects harbored in this
moist layer of gravel or sand generally are more serious than any drying problem.
Solid-bottom benches in which a layer of sand or gravel is to be placed must be
strongly constructed in order to carry the weight of these materials. Closed-
bottom benches with sides are not particularly suited to the step-type construc-
* For copies of plans for the benches illustrated, contact your local Cooperative
Extension Office or the author. Ask for Plan Number 6163.
- 22 -
Types of Bench Construction
Corrugated Asbestos-Cement Bench:
The major advantage of the corrugated asbestos-cement ("Transite") bench
is its excellent durability. It comes close to solid concrete in its permanence.
The material is strong, though it will crack or break if abused. However, cor-
rugated asbestos-cement will not rot or deteriorate. These benches are easily
installed, with no special tools or skill required for fabrication. The material
comes with corrugations every 4.2 or every 2 1/2 inches. The 2 1/2 inch material
should be used when available since it is somewhat easier to work with.
For small pots it may be necessary to lay wire mesh over the corrugations
before setting the pots. The wire mesh provides a reasonably flat surface upon
which to set the plants yet permits good air circulation around the pots.
In some cases gravel is placed on corrugated benches to support the small
pots, but this has the disadvantage of reducing air circulation and increasing
the problem with disease and insects.
When sides are used, they are usually 6" to 8" high and normally made from
flat cement-asbestos material. When this material is used, the sides have the
same permanence as the bottoms. It is important that the sides be fastened to
the bottom in such a way that a gap is left between the corrugated bottom and
the sides. This gap is necessary to allow excess water to drain freely from
the benches. Glavanized bolts and straps are normally used for attaching the
sides. The bench is then filled with a stuiable growth media, as required by
the plants being grown. A gravel or coarse sand bottom layer is necessary to
provide good drainage.
The major disadvantage of benches of this type is their comparatively high
Other Corrugated Materials:
Other types of corrugated materials are sometimes used but generally do not
prove as satisfactory. The corrugated plastics lack the strength of the cement-
asbestos materials. Problems with deterioration of some materials are also
experienced. Aluminum would have reasonably good permanence, but the normal
roofing forms would be comparatively weak. If aluminum is used, one should
choose the 0.024 inch thick aluminum in preference to the more commonly avail-
able 0.019 or 0.0215 inch thick material. Galvanized steel, though available
in sufficiently strong thicknesses, would be a poor choice for benches due to the
rusting problem which will be encountered. The zinc-galvanized coating may also
be toxic to plants in many situations.
Flat Cement-Asbestos Board:
Flat cement-asbestos board can also be used for constructing a bench. As
with the corrugated form, the material should be 3/8" thick. Such benches do not
have the strength of corrugated benches, but have the advantage that small pots
will set on such benches without tipping. Flat cement-asbestos boards should not
be used for benches which will be filled with soil.
- 23 -
Several different types of supporting frames can be used, as shown in
Figures 10 and 11. For each type, the leg supports should be spaced four feet
II GA. GALV. STEEL OR
3/8" ASBESTOS BOARD
SIDES 6" TO 8" HIGH
3/8" x 5"x5" GALV. STEEL OR
ALUM. ANGLE -SPACED 2'O.C.
S8 "DIA. x2"DEI
1 1/4' GA LV.
I1/4" PIPE FLANGE ANCHORED TO CONC.
OR PIPE EMBEDDED FOR RIGIDITY
CORRUGATED ASBESTOS TRANSITITE)
BENCH WITH PIPE COLUMN AND
(NOTE: CAN USE BLOCK 8 WOOD
SUPPORTS SHOWN AT RIGHT)
Corrugated asbestos transitite) bench
with pipe column and rail supports.
- 24 -
Wood can be used for flat or step benches, but unless the wood is treated
or is a very durable species, such benches can deteriorate rapidly and lack
permanence. Some preservatives can damage the roots of plants that grow in
Contact with the wood. Thus, for good performance and permanence, use wood
treated with water-borne, salt-type preservatives.
li GA. GALV. STEEL OR
3/8" FLAT ASBESTOS SHEET
3/8"x 5"x5" GALV. STEEL OR
ALUM. ANGLE SPACED 2 0.C.
3/8" ASBESTOS BOARD
SIDES 6" TO 8" HIGH
FLAT ASBESTOS TRANSITITE) BENCH
WITH CEMENT BLOCK PIERS AND
WOODEN RAIL SUPPORTS
Flat asbestos transitite) bench with cement
block piers and wooden rail supports.
- 25 -
For greenhouse use, the treatment should be one of the commercial, water-
borne, salt-type preservatives such as Chromated Copper Arsenate ("Osmose K-33",
or "Green Salts"), Ammonical Copper Arsenite ("Chemonite"), or Fluro Chrome
Arsenate Phenol ("Tantalith", "Wolman Salt", or "Osmosalts"). Wood properly
treated with such material will last for 20-30 years or more in greenhouse use.
(NOTE: Some of the above treatments are not rated for "ground contact" use.)
Since fumes and vapors from wood treated with the more toxic oil-borne materials
like penta or creosote can be damaging to plants, these preservatives are not
recommended for greenhouse benches.
II GA. GALV. STEEL OR
I" THICK BOARDS
3/8 x 5 Ox 5" GALV. STEEL OR
ALUM. ANGLE- SPACED 2'0.C.
I" BOARD SIDES
6" ro 6" HIGH
4" BLOCK OR EQUAL BASE,
LEVELED AND FIRM
WOODEN- BOARD BENCH WITH
SIDES AND SUPPORT FRAMEWORK
Figure 12 Wooden-board bench with sides and support framework.
- 26 -
Wood treated with all of the water-borne, salt-type preservatives can be
effectively painted once the wood has been redried. If possible, 6 months to
1 year weathering is recommended, since tests have indicated this period will
eliminate any toxicity of these materials to plant roots.
Figure 12 shows a wooden bench with side boards. For flat-topped wood
benches, a 1/8-inch to 1/4-inch crack should be left between the bottom and
side boards to allow water to drain from the bench and to prevent damage to the
bench if the wood swells due to moisture absorption. Cross supports should be
spaced not more than 4 feet apart.
I x2"x 14GA. OR 2"x 4x 12 1/2 GA.
GALV. WELDED WIRE FABRIC ATTACHED
WITH I" OR I 1/4" GALV. STAPLES
OUTSIDE 3/8" EXT. PLY. OR
GUSSET GALV. METAL PLATE
ALTERNATE CROSS -#
OR EQUAL GALV,
WELDED WIRE FABRIC
BENCH WITH WOODEN
Figure 13. Welded wire fabric bench with wooden framework and supports.
- 27 -
Benches of the wire-mesh type shown in Figure 13 are widely used in pot-
plant culture. They give excellent air drainage and simplify insect control
problems. The construction is simple. The framework can be made of 2" x 4"
wood, species rot resistant or treated. Welded wire fabric is then stapled
to this framework. The mesh should be one of the heavy wire types, since
sagging is a problem with even the best materials. A 1" x 2" welded mesh,
12 1/2 or 14-gauge, has been used with good results. Staples should be 1 1/4
to 1 1/2-inch long. The use of galvanized wire and staples will delay the
rusting problem and thus lengthen the life of the bench.
To minimize sagging, cross supports should be spaced 2 feet apart, though
some sagging will eventually occur and the pots will not set evenly. However,
the cost of such .benches is generally low when compared to other types.
BASE BLOCK LEVELED
AND FIRMLY PLACED
BENCH WITH PIPE RAIL AND CONCRETE
BLOCK SUPPORTS FOR BEDDING PLANTS
IN TRAYS OR FLATS
Figure 14. r-..,nm:..y lath-fence bench with pipe rail and concrete
hl~.'i supports for bedding plants in tranv or flats.
- 28 -
Wood-Slat (Lathe-Fence) Bench:
When using small plants in pots and these pots in trays or flats, an
economy bench top as shown in Figure 14 can be made of lathe-fencing placed
on treated 2" x 4" supporting rails or pipe rails. The lathe-fence allows
good air circulation around the pots. A problem would be encountered with
pot tipping if small bedding-plant size pots were used alone on the fence.
Wire mesh of 1" x 2" size should then be used over the slats to prevent pot
tipping. Since the lathe-fencing is not generally preservative treated, the
permanence of the bench top is not good in comparison to benches made completely
with treated wood. The lathe fence is, however, easily replaced when deteri-
oration requires its replacement. No practical method of providing side walls
for lathe-fence benches now exists.
I x 2 OR Ix 3
/ IL-OPTIONAL FOR
- 2 x 4 ON BACK SIDE OF I x 12
CUT TO STAIR STEP PATTERN
4 STAKE OR BOLT TO FLOOR
HEIGHT AND NUMBER OF STEPS OPTIONAL.
Figure 15. Wooden step-bench.
- 29 -
Wooden Step Benches
Wooden step benches like the one shown in Figure 15can be made any size,
width, angle or height. The size of plants to be placed on the bench and the
greenhouse size will largely dictate the specific dimensions of the bench.
Slats are most commonly used and they should be treated with a wood preserva-
tive. Painting the slats will reduce toxicity problems and will also increase
the light reflected below the bench. Step benches are usually built to face
south so that they will receive maximum sunlight. If two-sided step benches
are used, plants on the northern side will receive considerably less sun than
those on the southern side. Thus, orientate the benches north-south so the
morning and afternoon sun will shine on each side equally.
Step benches require more labor for construction than some of the other
bench types, but this seldom is a factor in determining what type of bench to
build. Shading problems, work efficiency, and space utilization should be the
primary considerations. Remember, if you want to have uniform plants, pots on
step benches will need to be turned occasionally when light intensity is higher
on one side of the bench than the other.
Single copies are free to residents of Florida and may be obtained from the County Extension Office. Bulk rates are available upon
request. Please submit details of the request to C.M. Hinton, Publication Distribution Center, IFAS Building 664, University of Florida,
Gainesville, Florida 32611.
This publication was promulgated at a cost of $378.66, or 12.6 cents per copy, to provide Extension personnel and
greenhouse operators with current information on greenhouse orientation, types of construction, materials for con-
struction, bench and bed layout and bench construction.
COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
Cooperative Extension Service, IFAS, University of Florida
and United States Department of Agriculture, Cooperating
K. R. Tefertiller, Director
TEC HIG REEARC EXTNSIO