Institute of Agriculture and Natural Resources EC308 High Tunnel Design, Site Development, and Construction Stacy A. Adams, Associate Professor of Practice Kim A. Todd, Extension Landscape Specialist Figure 1. Common high tunnel terminology. Experienced specialty plant growers can increase the High Tunnel Design quality of their products and the duration of their market- Structural Components ing of high- value crops through the use of high tunnels. High tunnels give growers the opportunity to plant earlier Before purchasing a high tunnel, it is critical to know what parts are used in the structure, how to decide on the and include more sequential planting dates. This can result size and shape of the unit, and what options will be needed in early- season, high- dollar returns and the ability to offer for specific production methods. their products for longer periods. High tunnels also provide High tunnels can vary in size and shape, but all use similar environmental protection, which helps reduce blemishes and parts, materials, and construction methods. A high tunnel discoloration, improving visual appeal. For successful high is a series of hoops that are held in position with continuous tunnel crop production, decisions and actions before planting lengths of supporting structural members (purlins and girts) the first crop include choosing a structural design to meet and end walls with accessible doors. The exterior covering is specific needs, identifying the best site for locating the struc- typically a greenhouse- grade polyethylene plastic that can ei- ture, initial soil preparation, and the method of construction. ther remain on the structure year- round or be removed during Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska– Lincoln cooperating with the Counties and the United States Department of Agriculture. University of Nebraska– Lincoln Extension educational programs abide with the nondiscrimination policies of the University of Nebraska– Lincoln and the United States Department of Agriculture. © 2015, The Board of Regents of the University of Nebraska on behalf of the University of Nebraska– Lincoln Extension. All rights reserved. Figure 2. Common high tunnel profile designs include the hoop (left), raised hoop (center), and peaked hoop (right). the heat of summer. The hoop- style high tunnel in Figure 1 strength. Typical structures have three to five purlins, spaced identifies key parts labeled with commonly used terminology. at 4 to 8 feet apart. Girts are stronger than purlins and are generally manufactured of square tube or roll form metals. The Hoop Girts provide greater structural support for polyethylene A series of hoops or bows make up the primary skeletal retaining systems (poly- locks), roll- up wall components, or structure of high tunnels. These provide support for the outer other equipment. They are installed in positions similar to covering and maintain structural integrity against environ- purlins to maintain hoop spacing. If the supplier provides mental forces. The spacing of the hoops can vary, depending only one ridge purlin for a particular type of high tunnel, on the desired structural strength. Hoops are most common- additional purlins or wall girts must be added to prevent ly spaced at 4, 5, or 6 feet. Narrower hoop spacing provides structural failure. more support for the polyethylene covering, requires the use of smaller diameter metals, and allows easier shedding of Wind Bracing rain and snow. However, structure cost increases with more Overall structural integrity depends on the number of hoops, and there is a potential for increased structural shad- angular wind braces positioned within the structure. At a ing. If wider hoop spacing is used, larger outside diameter minimum, these braces should be positioned between the (OD) metals are needed for structural strength against wind, end hoops and the baseboard at each corner. Additional an- rain, and snow loads. gular bracing can be installed from the ridge and roof purlins and connected to the end wall framing to increase structural End Walls strength. This is especially important in high wind locations. The size and type of end walls are designed to meet accessibility requirements for people and equipment. Most suppliers carry a variety of end wall framing packages, from High Tunnel Profiles simple pass- through doors to framing with larger openings High tunnel suppliers offer a variety of profile designs that allow for the use of rototillers and tractors. It is import- (Figure 2). Growers should select the optimum high tunnel ant to remember to include a small door for access if the profile based on its intended use, the desired durability, and high tunnel will be used during the coldest part of the year budget. The following paragraphs summarize each profile since large openings will greatly reduce the ability to keep the and its specific features of interest to the grower. inside temperature warm. End walls can be custom framed using lumber or metal piping if the polyethylene can be Hoop or Half- Round Style securely fastened to prevent air leakage or being blown in by This is the most common high tunnel profile since it re- high winds. Polyethylene zip entry end walls are an available quires the smallest number of structural members and can be option from some high tunnel suppliers, but these are not easily manufactured and built by individuals with little con- recommended for High Plains growers due to the risk of pre- struction experience. The support hoops of the half- round mature failure of the closure system from strong winds. profile have a smooth, gradual bend that limits the number of stress points on the plastic covering. The hoops bend from Purlins and Girts the ground on one side of the structure to the ground on the Purlins run horizontally from end to end of the high other side to form a continuous arch. This design may limit tunnel, maintaining hoop spacing and increasing structural access or standing room along the sidewalls. 2 © The Board of Regents of the University of Nebraska. All rights reserved. Raised Hoop polyethylene splices are installed to connect multiple lengths The raised hoop design is a modified hoop structure with of polyethylene. It is important to remember that the larger the sidewalls created by extending the vertical ground posts to polyethylene sheet, the greater the difficulty of installation, and the desired height (typically 36 to 60 inches). The straight the greater the weight of the covering on the structure. sidewalls allow for greater accessibility along the wall and provide more usable growing space; however, raised hoop Determining High Tunnel Width structures are taller than typical hoop profiles. The raised High tunnel width is a grower’s personal decision based hoop design is preferred when using roll- up sidewalls be- on the desired interior layout and placement of growing beds, cause it prevents rainwater from draining directly from the walkways, and support trellises, and the type of equipment roof into the growing beds, which can negatively affect plant that will be used inside the structure. The only physical lim- or flower quality. itation on the area being covered is the size of the available site. The most affordable option is to purchase “off the shelf” Peaked Hoop high tunnel packages as these often use standard- sized steel These structures are constructed by adding a bend at the or aluminum and polyethylene sheets that result in the most apex, or top point, of the hoop or bow to create a distinct floor space at the lowest per foot cost. ridge. This ridge provides additional strength against cross- A typical half- round high tunnel is usually half as tall as winds and creates a steeper roof pitch that sheds rain and its width. For example, a tunnel 20 feet wide will be approxi- snow more quickly. Peaked hoop structures typically have mately 10 feet high at the center ridge. If the structure is nar- distinct roof and sidewall surfaces that are divided by a wall rower than 12 feet, ground post extensions may be required girt or eave board. This allows for the easy integration of roll- to allow for more accessible headroom and more usable space up or drop- down sidewalls for natural ventilation. next to the sidewalls. Peaked hoop designs have the widest amount of usable floor space, but overall height decreases as the structure is widened, which may result in low headroom High Tunnel Size space near the sidewalls. Wider tunnels have a flatter roof Suppliers of high tunnels offer structures in various pitch that may negatively affect the shedding of rain and widths, lengths, and heights, all designed for the efficient snow, potentially leading to structural collapse. use of industry standard- sized polyethylene coverings and lengths of metal tubing. Exterior Coverings Overall Dimensions The roof and sidewalls of high tunnels are typically cov- Structures appropriate for growing home garden crops ered with polyethylene (poly) plastic. Greenhouse- grade, 6- may be as small as 8 feet wide by 10 feet long. Commercial mil ultraviolet light (UV) protected poly should be used. Do production tunnels can be 20 to 30 feet wide and any length. not use construction grade poly found at construction supply Growers often use high tunnels that are 48 feet, 96 feet, or 144 or hardware stores. The UV polymer resins in greenhouse- feet long so that standard 100- foot or 150- foot lengths of poly- grade plastics increase the longevity of the covering to more ethylene can be used efficiently. If a longer structure is needed, than four years when securely fastened to the structure. Poly Figure 3. Twin wall (left) and corrugated polycarbonate panels (right) provide options for walls. © The Board of Regents of the University of Nebraska. All rights reserved.