Greenhouse Structures and Design
Open-roof Greenhouse Systems
Open-roof greenhouses are becoming more popular with commercial growers. These structures have entire roofs that can be opened and closed. Open-roof roof greenhouses are designed for crops such as bedding plants, perennials and field grown cut flowers that would prefer to be outdoors but can’t take the rain and cold early in the season. In this case, the entire roof can be open when weather conditions are favorable for growing and closed when the weather starts to get inclement. Open-roof greenhouse systems include open-panel, retractable-film, flat-roof, low-profile, and rolling-roof systems.
Open-panel Greenhouse Systems
Open-panel greenhouse system has roof segments that are hinged at the gutter and open at the peaks (See Figure 1.13). The entire roof area can be opened by engaging the electric motors to drive rack and pinion systems. 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.
Retractable-film Greenhouse Systems
Retractable-film greenhouse systems act by “folding” the glazing as the glazing is retracted (See Figure 1.14). 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. End and side walls are often covered with single-layer polycarbonate or with roll-up sides (preferred). 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.
Flat-roof Greenhouse Systems
The flat-roof greenhouse system is very similar to retractable-film systems, but they do not have trusses, which significantly reduces structural, installation and glazing materials costs(See Figure 1.15). In addition, 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. The glazing is usually retracted during heavy wind, snow or ice storms, or during severe wind storms, to prevent damage to the structure.
Low-profile Greenhouse Systems
Low-profile greenhouse systems are either very wide-span systems with low-pitch roofs, or they may have sawtooth-style roofs (See Figure 1.16). 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.
Rolling-roof Greenhouse Systems
The rolling-roof greenhouse system typically has bows mounted on lattice trusses spanning each bay (See Figure 1.17). Tube-type motors drive aluminum pipes that roll the covering up or down on each side of the bow. The covering is typically a nylon mesh film enclosed within two layers of plastic and provides from 15 to 25 percent shade when closed. Along with flat-roof systems, rolling-roof structures are often much lower in price compared to other designs.
Advantages of Open-Roof Greenhouses
Open-roof greenhouse technology is having an increasing impact on the way growers cultivate plants. Open-roof greenhouses have several advantages:
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