\0 3k NORTH FLORIDA EXPERIMENT STATION
April 5, 1961
NFES Mimeo Report 61-4
Plastic Greenhouse Construction
H# W. Young and C. E Dean
Many growers could utilize greenhouses in their operation, however, the
high costs have made permanent glass houses prohibitive. Plastic greenhouses,
because of their low cost and ease of construction compared to glass greenhouses,
offer to both the commercial grower and the home owner an inexpensive way of
growing plants the year round. Although greenhouses are usually thought of as
important in more northerly areas, they have found wide utility in southern areas
for modifying environmental conditions so that crops can be grown over a longer
period of time. While considered as a semipermanent structure they will give many
years of service if constructed properly of treated framing material.
The principal advantage of plastic over glass as a greenhouse covering is
the low initial cost. A plastic-covered-wood framed house can be built for about
one-tenth the cost of a steel-framed glass house. Other advantages include:
comparatively simple construction which makes it possible for erection by unskilled
labor, and the use of locally available materials for framework.
The greenhouse should be located away from trees or buildings which might
shade it during any part of the day. Care should be taken to select a site with
good drainage, and as an extra precaution, the floor of the house should be
slightly higher than the surrounding area.
The structural description which follows may be adapted to plastic green-
houses of various sizes, and embodies some improvements not included in two houses
which have been constructed at the North Florida Experiment Station.~.:Figures 1-3
show an end view, cross section and side view of these houses. ,Square posts,
measuring 4" by 4" are placed 2' in the ground and 8' apart. /Additional support
is provided by 2" by 2" posts placed on 2' centers between th8lzarger posts. 'These
smaller posts are secured at the bottom to a 2" by 8" board rled to.the.'uutside
of the larger posts at the soil line, and at the top to a 2" 41y "j plate nailed/on
the top of the larger posts. All of these materials should be treated with a .wood
preservative and the underground portions embedded in concrete. '-*,'.
The height of the building to the eaves can be varied to'meet-t he demands
of the plants to be grown. Heights from four to six feet have proven satisfactory.
The rafters and ridgeboard of the house may be made from 2" by 4" lumber,
although 2" by 2" material may be substituted for the rafters in a smaller house.
The rafters must be spaced in accordance with the width of the plastic used. A
spacing of 2t is satisfactory for plastic from 51 to 54 inches in width. This
allows ample material for pulling wrinkles out of the sheets and for lapping the
edges. The pitch or angle of the roof is very important in a plastic greenhouse.
A rather steep. slope is preferred so that the water which condenses on the interior
of the plastic roof will roll down to the sides rather than fall on the plants
below. A steep slope also facilitates the recirculation of warm air when a
perimeter type heater is used, and reduces the rate of heat accumulation when
sunlight is intense.
For additional support, it may be necessary to place 4" by 4" posts
equidistant down the center of the greenhouse, embedded in the floor and extending
to the ridgeboard. The number of these support posts will depend on the length of
the house. As a general rule, posts spaced 20' apart will provide the necessary
support. For rigidity the structure should be braced with 8 guage wire attached
near the top of each 4" by 4" posts, extended under and diagonally across the
rafters, and fastened to the ridgeboard. Turnbuckles provide a means of tightening
Most wood preservatives will damage plants if there is contact between the
treated wood and the plants, however, they can be relatively safe if certain
precautions are observed. First, treat the lumber before the house is erected in
a location other than the greenhouse site. Second, allow the treated lumber to
dry before use. Remember that pressure treated lumber will require considerably
longer to dry than will lumber with the preservative painted on. Unless the
material is properly dried, plants may be damaged from moisture condensation on
the roof and rafters which drips on the plants, or by the accumulation of toxic
vapors from the preservatives in the closed greenhouse. Wood preservatives
commonly in use include chromated zinc chloride and pentachlorophenol. It is
recommended that pentachlorophenol be used only on the material which is to be
below-ground,and chromated zinc chloride used on all above-ground structural
material, since the latter preservative will not damage plants if properly applied.
Several companies manufacture plastic materials for covering greenhouses,
hotbeds and other plant growing structures. Although many types are available,
the most common in use are polyethylene, polyvinyl and polyester.
Polyethylene is the least expensive but has the shortest life, due to the
deteriorating effects of ultraviolet rays from the sun. Its useful life in the
South is usually considered about one year. This material can be obtained in 2,
4 and 6 mill thickness and in sheets up to 40' wide and 1001 long.
Polyvinyl, with an ultraviolet inhibitor, is more durable than polyethylene,
and also more expensive. This material costs approximately five times as much per
square feet as polyethylene, however, two or three years of continuous use can be
expected under average conditions.
11 mil equals 1/100 inch.
The most practical thickness of this material seems to be 8 mil.
Polyester materials are more durable and are also more expensive than the
two previously mentioned materials, costing nearly 15 cents per square foot.
Polyester materials are sold under the trade name of Weatherable Mylar and as such
are available with an ultraviolet inhibitor on one or both sides to retard the
deteriorating effects of the suns rays, There are indications that it may last up
to four years on roofs and seven years on sidewalls. It is currently available in
widths up to fifty-one inches and a thickness of 5 mils, however, other thickness
are being tested.
Fastening the Piadtici
The method of covering Will depend on the type and width of plastic used.
When using polyethylene in wide widths it is best to run the plastid the length of
the greenhouse, pulling and securing on every two or three rafters. If polyethylene
is installed in warm weather care should be taken not to stretch it too tightly over
the supports, since it contracts during cold weather and may become too tight,
resulting in splits and more rapid deterioration. A hand stapler may be used to
fasten the material and a lath tacked on each rafter with small nails spaced 8" to
10" apart to prevent wind damage to the plastic. When using polyethylene, it may
be advisable to use a lath that is at least i" wider than the rafter. This shades
the plastic at points of attachment and retards deterioration. When using plastics
in narrower widths an alternate method of fastening will require starting at the
ground level on one side of the house and extending over the ridgeboard to the
ground level on the other side. It is secured to the sides and rafters in the
manner previously described. Care should be taken to insure that the plastic sheets
overlap at least two inches on the sides and rafters where they come together.
Rigid plastics such as Mylar can be installed the length of the house. By
pulling the film tight and allowing at least a four inch overlap, it is not
necessary to seal the seams to make it water tight. Overlapping of the plastic
should be such that water will not enter the seam but will run off.
For the best retention of heat, two layers of plastic should be used,
particularly on the roof. A dead air space two inches thick will provide the
maximum amount of insulation, however, heat loss due to convection in an air space
four inches thick is not great enough to make the cost prohibitive. More distance
between the two layers will result in heat loss due to convection. In addition to
reducing heating costs, an inner layer of plastic will largely prevent moisture
condensation on the inner roof surface. Results obtained in other states indicate
that in cool, humid areas, two layers of plastic transmit more light than a single
layer because of the reduction in moisture condensation. Inexpensive plastics of
two mil thickness are adequate for the inside layer.
Proper ventilation is the most difficult problem in the construction and
operation of plastic greenhouses. Most of these structures are nearly air-tight,
and as a result, the humidity is almost always higher than in a glass house. Also,
carbon dioxide given off by the plants inside the house accumulates, and for the
safety of workers a method of ventilation should be provided. Various types of
vents which either open on hinges or can be removed entirely have been tried.
These vents have been located in the gable ends, the roof or the sides. Ridge vents
appear the most desirable, however, this type of ventilation in general does not
seem to be well adapted to plastic greenhouses because of the structural materials
used. Moreover, it is difficult to prevent water leaks around the vents. Another
way to provide air exchange is the use of exhaust fans. The fan or fans should be
placed in one gable end and shutter type aluminum vents placed in the other. The
fans may be controlled by a thermostat or time clock to provide ventilation when
SFor small greenhouses, unit heaters equipped with fans to circulate the heat
are quite satisfactory. Larger houses require a more uniform source of heat, and
for this purpose, one of the perimeter heaters will give more satisfactory results.
This type of heater also provides for heat at the soil surface which is necessary
in certain operations. These systems consist of a heater connected to hot air ducts
extending around the inside edge of the house. Hot air is circulated through the
ducts and the products of combustion are removed from the house by an exhaust or
vacuum fan at the end opposite the heater. Heaters of this type, with a capacity
of 160,000 B.T.U., are being used in the greenhouses at the North Florida Experiment
Station. It is possible to maintain a temperature of 600 F inside the house when
the outside temperature is 300 F. Thermostatic control makes the system automatic.
Gas is generally used as a source of heat in plastic greenhouses, with either
bottled gas or natural gas being suitable. Bottled gas, such as propane, produces
only carbon dioxide and water if combustion is complete. If carbon dioxide does not
become too concentrated there are no harmful effects on plant growth. Natural gas,
on the other hand, may produce products of combustion in addition to carbon dioxide
and water which are harmful to plants. When using natural gas, care should be
taken to remove all of the products of combustion from the house. This can be
assured by sealing the joints of heating pipe with furnace cement.
It may be desirable, under some conditions to continue growing plants in
plastic greenhouses during the summer months. Because of the difficulties in
providing adequate ventilation in these houses, they may become too warm for normal
plant growth during this season. A system of evaporative cooling involving movement
of air through a water-saturated fiber pad has reduced temperatures sufficiently
under most conditions. These pads, usually constructed of aspen wood excelsior or
similar material should be located on the side or gable end toward the prevailing
wind. A water source from above the pads keepsthem saturated when the system is in
operation. A fan or fans in the opposite side or end of the house pulls the cooled,
moistened air through the house, providing a cooling effect. This system has kept
greenhouse temperatures 150 to 200 degrees cooler than the outside air during warm
periods without supplemental shading. In bright sunlight when the outside air
temperature is 950 F, it is usually possible to maintain a temperature of 750 to
850 F inside the greenhouse. The capacity of the fan or fans and the size or area
of the cooling pad will depend on the size of the greenhouse. The manufacturer
should be consulted when planning a cooling system.
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