Choosing an Open Roof Greenhouse

June 11, 2002 - 12:22

With their many benefits, deciding to use an open-roof structure is easy, but choosing the right system can be a challenge.

One of the main advantages of a retractable roof system is
that the size of the structure is not limited by the restrictive capacity of
mechanical ventilation systems. This allows the total growing area to have
fewer “edges,” resulting in a more uniform growth response from the
entire crop. Originally designed for snow protection of forest seedlings in the
1970s, retractable roof systems are now used for all types of horticultural
production, with five types to choose from.

Types of Systems

Open-panel Systems.
Structures that use the entire greenhouse roof like a ridge-vent are available
in standard greenhouse sizes. These “open-panel” structures are
typically hinged at the gutters, allowing the glass, rigid-panel or double-poly
glazings to be raised to a vertical position. One design has hinges at the roof
ridge and one gutter, using a rack-and-pinion system to open the roof by
closing the peaks like an accordion. A twin-peak design reduces the height of
the panels, minimizing exposure to the wind.

Although they do not use a retractable plastic film, these
structures do provide almost the same amount of roof ventilation as other
retractable roof systems. open-panel systems require 4-15 minutes to open using
a rack-and-pinion system, and individual sections can be left closed if needed.
When closed, the structure functions similar to a covered greenhouse with
typical heating and cooling requirements and typical snow and wind loads. For
additional light and cooling control, a retractable shading system can be installed
above or within the open-roof structure.

Open-panel systems are generally the most expensive of the
“open roof” designs. A less expensive option uses double-poly film
glazing instead of rigid panels or glass, but regular replacement of the poly film
would be required.

Most of the open-panel designs do not use side wall
ventilation, which severely limits their natural ventilation. With roll-up side
walls, open-panel systems would provide nearly the same amount of total
ventilation that is available from other types of retractable roof designs.

Long term wear on hinges and rack-and-pinion drivers are one
concern with open-panel designs. Also, if the glazing is plastic, it may need
to be UV-treated on both sides (not just the top). When the roof is open, both
the undersides of the roof panels and the inside of the side wall glazing is
exposed to UV light, which may reduce the longevity of the glazing. Common to
all retractable-roof designs, the ability of a particular open-panel system to
operate in wind, snow and ice storms needs to be studied based on local weather
patterns. Like many other retractable roof designs, individual motors can
operate several greenhouse bays (over 1/2-acre per motor). open-panel systems
are presently the only retractable roof design that is compatible with the use
of glass as the glazing material.

Retractable-Film Systems. “Retractable-film” structure are the namesake
“retractable” roof systems and are typically mid-range in price.
Cost per unit area falls as the total amount of area installed increases.

Peak-roof film systems that “fold” the glazing
as the glazing is retracted are available in standard greenhouse widths from
20-48 feet, and custom widths can also be designed. With the Á roof
closed, the structures are typically engineered to handle snowloads of 20-40
pounds per square foot and wind loads of 100 miles per hour, and additional
load strengths are available. In the early and late 1990s, some growers
purchased untested designs that had insufficient wind resistance; the
structures failed during storms. Purchase systems that have already proven
themselves under grower conditions.

When open, retractable-film systems provide over 95 percent
ventilation. Only the rolling-roof systems can compete with retractable-film
systems for total amount of natural ventilation. With the roof closed, the structures
are over 98 percent airtight (properly designed and installed) and provide from
18-65 percent shade. Other shading levels are available. End and side walls are
often covered with single-layer polycarbonate or with roll-up sides
(preferred).

Without internal energy curtains, most retractable-film
structures have heating costs similar to the same-sized structure covered with
a single layer of corrugated polycarbonate. Energy curtains, which can double
as an additional layer of shading or as black-out cloth for photoperiod
control, greatly improve the heating efficiency of the structures.

The roof and roll-up side wall coverings are typically made
from a woven polyethylene, with a protective coating on both sides along with
anti-fog and anti-condensate coating on the underside. The plastic film glazing
is suspended from hooks that slide along heavy-gauge, stainless steel wires.
Make sure the design you purchase uses wires and an installation process that
prevents the wires from “stretching” after installation. A central
motor and drive-shaft pull the covering from truss to truss. A single motor can
operate up to five 10-meter-wide bays with a maximum length of about 88 meters.
The roof can retract or close in 2-3 minutes. Life expectancy of the covering is
typically 10 years, but may only have a 5-year guarantee. To operate bays
independently, individual motors are needed for each bay (increased
installation cost). One manufacturer uses an internal gutter design to help
prevent snow or ice “flooding” of gutters that can damage
structures and the crops inside.

Although originally designed with solid side walls, the
majority of retractable-film designs are now built with seasonally removed or
roll-up side walls (motorized or hand-crank). Using motorized and automated
side walls allows the structure to be ventilated in a controlled manner and
improves crop uniformity.

Long-term wear on drive lines, pulleys and cables, or
rack-and-pinion systems are one concern with retractable-film designs. Unlike
open-panel systems, the underside of the glazing of retractable-film systems
are rarely exposed to UV-light, but side wall glazing may be Á exposed
to UV-light on both sides. Common to all open-roof designs, the ability of a
particular retractable-film system to operate in wind, snow and ice storms
needs to be studied based on local weather patterns. Like open-panel designs,
individual motors can operate several greenhouse bays (over 1/2-hectare per
motor).

Open-panel designs allow direct sunlight to reach the crop
only during hours near mid-day (when you often need to shade to control heat),
but retractable-film and rolling-roof systems can be operated to allow direct
sunlight to reach the crop throughout the day. Unlike open-panel or
rolling-roof systems, there is some concern about the removal and installation
of the glazing when the glazing has aged beyond usefulness. Both
retractable-film and open-panel designs can be easily fitted with internal
“black-out” curtains for photoperiod control, along with any
equipment normally used in stationary roof greenhouses.

Flat-Roof Systems. A
third type of retractable roof system, the “flat-roof” system, is
very similar to retractable-film systems, but they have two major differences.

First, flat-roof systems do not have trusses, which
significantly reduces structural, installation and glazing materials costs.
Second, the glazing is typically designed to allow water to pass through.

Crops that are not tolerant to rainfall, or cannot weather
exposure to snow or ice storms should not be grown in flat-roof systems.
Rainwater drips through the glazing, or the glazing is retracted to allow
rainfall to naturally irrigate crops. The glazing is usually retracted during
heavy wind, snow or ice storms, or during severe wind storms, to prevent damage
to the structure. In this way, severe storms that destroy stationary roof
structures and some retractable roof designs can be “survived” by
the retractable or roll-up roof systems.

Crops grown in small container sizes are not normally
produced in flat-roof retractable systems. The water dripping through the film
during heavy rains can wash-out seedlings or plants in smaller cells.

Flat-roof systems are typically used as shading systems
during spring, summer and fall, and for unheated or heated cold protection
during fall, winter and spring. They can also be used as on-demand protection
from hail, wind or short-duration ice storms. Compared to outdoor growing
areas, flat-roof systems provide a year-round “protected” growing
compound. Crops are protected from unexpected or unseasonal winds, cold, frost,
freeze, heat and other short-term weather patterns that can destroy unprotected
crops.

Properly managed, the flat-roof systems can be used for
“heat-trapping” or “cold-trapping” to regulate crop
development. Open-panel and retractable-film structures are less effective for
cold-trapping but may be able to use cold-trapping if equipped with energy
curtains.

Some nurseries have started installing flat retractable
roofs with two layers of glazing. One glazing layer is used for shading or cold
protection (often a white woven film with a shading factor of about 50
percent), while the Á other is used for light transmission and heat
retention (often a clear IR woven film with a shading factor of about 20 percent).
The combination provides two layers for cold protection, two layers for heat
retention and summer shading at two different levels. The grower obtains the
ability to force the crop with heat retention, slow the crop with cooler
temperatures or manipulate development with DIF (the difference between the day
and night temperature). Flat-roof systems have been installed as
“black-out” compounds to protect container-grown forest seedlings
from long photoperiods.

Without trusses, flat-roof systems may not be compatible
with all types of greenhouse automation equipment, or installation adjustments
may be needed to use automation equipment. For example, boom irrigation systems
cannot hang, so they must be mounted to side wall posts.

Low-Profile Systems. These
are either very wide-span systems with low-pitch roofs, or they may have
sawtooth-style roofs. The low-pitch roofs often use specialized film glazings
that contain hundreds of small channels that direct rainfall or melted
precipitation to the gutters. They typically require the use of expensive
energy-trusses to design them for reasonable wind and snow loads. Their
ventilation can be as good as retractable-film systems if proper side wall
ventilation is installed.

Aside from snow and hail loading, the main concern with
low-pitch roofs for the low-profile design is the “failure” of the
channels in the glazing. In some situations, condensation and algae growth in
the glazing channels have contributed to reduced light transmission. Other
concerns would be similar to open-panel or retractable-film systems. The main
advantage of low-pitch roofs is the limited number of posts required to support
the roof, which may improve labor efficiency inside the structure. However,
since many automated systems are designed for structures of typical post
spacings, the low-pitch roof may have limitations related to customized
accessories. Depending on the strength of the trusses, low-profile systems may
not be compatible with all types of greenhouse automation equipment, or installation
adjustments may be needed to use automation equipment in a manner similar to
that of flat-roof systems.

Sawtooth retractables are a hybrid of stationary
sawtooth-greenhouse designs and retractable-film systems. The short wall above
the gutter can be rolled up to provide ventilation similar to a
“typical” sawtooth structure (standard saw-tooth operational
procedures), which allows roof ventilation with less risk of precipitation
landing on crop foliage. This is the main advantage of this design, and it is
the only open-roof system that provides for roof ventilation without risk of
allowing precipitation to reach the crop. The entire roof glazing can be
retracted to provide complete ventilation and crop exposure, if needed.
Retractable sawtooth systems are very versatile but are usually more expensive
than most other retractable roof systems. The mechanical wear concerns are
similar to other open-roof systems.

Rolling-roof Systems.
The rolling-roof is perhaps the oldest design concept in use. One of the
earliest automated open-roof systems used for “seedling” production
in the 1980s was a rolling-roof design (Northwest Transplants, Woodburn, Ore.).
Growers have been using hand-crank, roll-up roofs on hoop or quonset structures
for decades.

Flat- and peaked-roof systems that “roll” film
plastic glazings are available in standard greenhouse widths from 5-10 meters,
and custom widths can also be designed. Most rolling structures have peaked or
quonset (igloo) roofs. Ventilation capacity can be as good as retractable-film
systems.

Rolling-roof structures typically have bows mounted on
lattice trusses spanning each bay. Tube-type motors drive aluminum pipes that
roll the covering up or down on each side of the bow. The film glazing is
usually rolled up from the gutter toward the ridge. Each bay can be up to 300
feet long and is operated independently using four motors (sometimes only two
tube motors have been used). The roof can be retracted or closed in 2-7
minutes.

The covering is typically a nylon mesh film enclosed within
two layers of plastic, with a life expectancy of 4-6 years. The covering
provides from 15-25 percent shade when closed. Less-expensive coverings can
also be used with more frequent replacement.

Rolling-roof end walls are typically covered with single- or
double-layered polycarbonate, and roll-ups are often used as side walls when
gutter-connected structures are installed. Along with flat-roof systems,
rolling-roof structures are often much lower in price compared to other
designs. However, rolling-roof systems require a large number of motors that
require more wiring if full automation is desired.

Durability of motors and physical stress on the glazing are
concerns with rolling-roof designs. The glazing may need to be UV-treated on
both sides (not just the top). When the glazing is being rolled, the undersides
may be exposed to UV light, which may reduce the longevity of the plastic.

Common to all open-roof designs, the ability of a particular
rolling-roof system to operate in wind, snow and ice storms needs to be studied
based on local weather patterns. Depending on the design, rainfall or overhead
irrigation water can be “dammed” behind the pull bar, which then
pours down onto the crop when the glazing is lowered. The ‘spill’
water can wash-out seedlings or plants in smaller cells.

When the rolling-roof is designed to roll the film glazing
downward from the peak toward the gutter, there is less risk of forming
“spill” water on the glazing. However, if ventilation is needed
mid-day, plants would be subject to direct sunlight under conditions when
shading may be required.

Depending upon the load strength of the bows, rolling-roof
systems may or may not be compatible with typical greenhouse automation
equipment.

Additional Equipment

Roll-up Side Walls.
Roll-up side walls greatly improve the growing environment produced by
retractable-roof systems and can reduce labor costs. Some growers are so
impressed by the improved plant growth provided by roll-up side walls, they are
removing the solid plastic and glass side walls from their existing greenhouses
and retrofitting them with roll-up side walls. Side wall retrofits have already
been completed at bedding plant, perennial and forest seedling facilities in
Oregon and Washington.

Computer Assisted Growing. A computer operation system is an essential component for proper
operation of a retractable-roof system. Trying to keep the roof and side walls
in the correct position by hand is a full-time job, and other chores will
usually cause the worker in charge of the structure to “fail” on a
regular basis. A one-time purchase of a good computer control system is much
less expensive than a permanent, full-time worker.

Automation does not replace the need for you to operate your
retractable-roof system. Remember that the computer only assists the grower in
operating the system or growing the crop. These are “computer-assisted
growing systems,” not computer-controlled systems. Growers must know crop
requirements and must make routine or seasonal adjustments to the computer
settings as needed.

Automation Equipment.
Generally, retractable-roof systems are compatible with all types of automation
used in common greenhouse systems, but certain types of automation may not be
compatible will all designs. Irrigation booms, monorail systems and rolling
benches are commonly installed in retractable-roof greenhouses.

Before purchasing a retractable-roof system, visit with
growers who are already successfully producing crops using the systems you are
considering.

About The Author

Sven E. Svenson is an assistant professor at the North Willamette Research and Extension Center at Oregon State University, Aurora, Ore. He may be reached by phone at (503) 678-1264 ext. 14 or E-mail at sven.e.svenson@orst.edu.

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