Choosing an Open Roof Greenhouse By Sven E. Svenson

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 isthat the size of the structure is not limited by the restrictive capacity ofmechanical ventilation systems. This allows the total growing area to havefewer “edges,” resulting in a more uniform growth response from theentire crop. Originally designed for snow protection of forest seedlings in the1970s, retractable roof systems are now used for all types of horticulturalproduction, with five types to choose from.

Types of Systems

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

Although they do not use a retractable plastic film, thesestructures do provide almost the same amount of roof ventilation as otherretractable roof systems. open-panel systems require 4-15 minutes to open usinga rack-and-pinion system, and individual sections can be left closed if needed.When closed, the structure functions similar to a covered greenhouse withtypical heating and cooling requirements and typical snow and wind loads. Foradditional light and cooling control, a retractable shading system can be installedabove 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 filmglazing instead of rigid panels or glass, but regular replacement of the poly filmwould be required.

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

Long term wear on hinges and rack-and-pinion drivers are oneconcern with open-panel designs. Also, if the glazing is plastic, it may needto be UV-treated on both sides (not just the top). When the roof is open, boththe undersides of the roof panels and the inside of the side wall glazing isexposed to UV light, which may reduce the longevity of the glazing. Common toall retractable-roof designs, the ability of a particular open-panel system tooperate in wind, snow and ice storms needs to be studied based on local weatherpatterns. Like many other retractable roof designs, individual motors canoperate several greenhouse bays (over 1/2-acre per motor). open-panel systemsare presently the only retractable roof design that is compatible with the useof 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 glazingas the glazing is retracted are available in standard greenhouse widths from20-48 feet, and custom widths can also be designed. With the Á roofclosed, the structures are typically engineered to handle snowloads of 20-40pounds per square foot and wind loads of 100 miles per hour, and additionalload strengths are available. In the early and late 1990s, some growerspurchased untested designs that had insufficient wind resistance; thestructures failed during storms. Purchase systems that have already proventhemselves under grower conditions.

When open, retractable-film systems provide over 95 percentventilation. Only the rolling-roof systems can compete with retractable-filmsystems for total amount of natural ventilation. With the roof closed, the structuresare over 98 percent airtight (properly designed and installed) and provide from18-65 percent shade. Other shading levels are available. End and side walls areoften covered with single-layer polycarbonate or with roll-up sides(preferred).

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

The roof and roll-up side wall coverings are typically madefrom a woven polyethylene, with a protective coating on both sides along withanti-fog and anti-condensate coating on the underside. The plastic film glazingis suspended from hooks that slide along heavy-gauge, stainless steel wires.Make sure the design you purchase uses wires and an installation process thatprevents the wires from “stretching” after installation. A centralmotor and drive-shaft pull the covering from truss to truss. A single motor canoperate 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 istypically 10 years, but may only have a 5-year guarantee. To operate baysindependently, individual motors are needed for each bay (increasedinstallation cost). One manufacturer uses an internal gutter design to helpprevent snow or ice “flooding” of gutters that can damagestructures and the crops inside.

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

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

Open-panel designs allow direct sunlight to reach the croponly 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 directsunlight to reach the crop throughout the day. Unlike open-panel orrolling-roof systems, there is some concern about the removal and installationof the glazing when the glazing has aged beyond usefulness. Bothretractable-film and open-panel designs can be easily fitted with internal”black-out” curtains for photoperiod control, along with anyequipment normally used in stationary roof greenhouses.

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

First, flat-roof systems do not have trusses, whichsignificantly 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 weatherexposure to snow or ice storms should not be grown in flat-roof systems.Rainwater drips through the glazing, or the glazing is retracted to allowrainfall to naturally irrigate crops. The glazing is usually retracted duringheavy wind, snow or ice storms, or during severe wind storms, to prevent damageto the structure. In this way, severe storms that destroy stationary roofstructures and some retractable roof designs can be “survived” bythe retractable or roll-up roof systems.

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

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

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

Some nurseries have started installing flat retractableroofs with two layers of glazing. One glazing layer is used for shading or coldprotection (often a white woven film with a shading factor of about 50percent), while the Á other is used for light transmission and heatretention (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 heatretention and summer shading at two different levels. The grower obtains theability to force the crop with heat retention, slow the crop with coolertemperatures or manipulate development with DIF (the difference between the dayand night temperature). Flat-roof systems have been installed as”black-out” compounds to protect container-grown forest seedlingsfrom long photoperiods.

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

Low-Profile Systems. Theseare either very wide-span systems with low-pitch roofs, or they may havesawtooth-style roofs. The low-pitch roofs often use specialized film glazingsthat contain hundreds of small channels that direct rainfall or meltedprecipitation to the gutters. They typically require the use of expensiveenergy-trusses to design them for reasonable wind and snow loads. Theirventilation can be as good as retractable-film systems if proper side wallventilation is installed.

Aside from snow and hail loading, the main concern withlow-pitch roofs for the low-profile design is the “failure” of thechannels in the glazing. In some situations, condensation and algae growth inthe glazing channels have contributed to reduced light transmission. Otherconcerns would be similar to open-panel or retractable-film systems. The mainadvantage of low-pitch roofs is the limited number of posts required to supportthe roof, which may improve labor efficiency inside the structure. However,since many automated systems are designed for structures of typical postspacings, the low-pitch roof may have limitations related to customizedaccessories. Depending on the strength of the trusses, low-profile systems maynot be compatible with all types of greenhouse automation equipment, or installationadjustments may be needed to use automation equipment in a manner similar tothat of flat-roof systems.

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

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

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

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

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

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

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

Common to all open-roof designs, the ability of a particularrolling-roof system to operate in wind, snow and ice storms needs to be studiedbased on local weather patterns. Depending on the design, rainfall or overheadirrigation water can be “dammed” behind the pull bar, which thenpours 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 glazingdownward from the peak toward the gutter, there is less risk of forming”spill” water on the glazing. However, if ventilation is neededmid-day, plants would be subject to direct sunlight under conditions whenshading may be required.

Depending upon the load strength of the bows, rolling-roofsystems may or may not be compatible with typical greenhouse automationequipment.

Additional Equipment

Roll-up Side Walls.Roll-up side walls greatly improve the growing environment produced byretractable-roof systems and can reduce labor costs. Some growers are soimpressed by the improved plant growth provided by roll-up side walls, they areremoving the solid plastic and glass side walls from their existing greenhousesand retrofitting them with roll-up side walls. Side wall retrofits have alreadybeen completed at bedding plant, perennial and forest seedling facilities inOregon and Washington.

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

Automation does not replace the need for you to operate yourretractable-roof system. Remember that the computer only assists the grower inoperating the system or growing the crop. These are “computer-assistedgrowing systems,” not computer-controlled systems. Growers must know croprequirements and must make routine or seasonal adjustments to the computersettings as needed.

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

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



Sven E. Svenson

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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