A high tunnel, often called a hoop house, is a passive solar structure designed to extend the garden’s growing season by several months. This unheated, polyethylene-covered shelter captures solar energy, creating a warmer microclimate that protects crops from harsh weather, frost, and wind. High tunnels allow gardeners to plant earlier in the spring and harvest later into the fall or even winter, significantly increasing both yield and crop quality. Building one requires careful planning and execution, but the payoff is a more productive and resilient garden space.
Essential Planning and Site Selection
Proper site selection is essential for the success of the high tunnel. A location with excellent drainage is required, as the structure sheds a large volume of water from the roof, which can cause internal flooding and soil erosion if not managed. The ideal grade should be less than a five percent slope. If the site has poor native drainage, the area should be slightly elevated or drainage systems installed to divert runoff.
The structure typically ranges from 14 to 30 feet wide and 50 to 150 feet long, with the length often dictated by the size of available plastic sheeting. Orientation should be chosen based on the local latitude and primary growing season goals. For northern climates (above 40° latitude) seeking to maximize winter light exposure, an East-West orientation is recommended to catch the low-angle sun.
Conversely, a North-South orientation is preferred in southern regions or where summer ventilation is a concern, as it provides a more even distribution of light throughout the day. The site must be clear of any nearby trees or buildings that would cast a shadow, which reduces the tunnel’s heating efficiency. Before construction begins, the selected area must be leveled and squared, often requiring a string line and batter boards to ensure the base frame is rectangular.
Gathering Necessary Materials and Tools
The structural skeleton of a high tunnel is composed of heavy-gauge galvanized steel tubing, which resists corrosion. The hoops and ground posts utilize larger diameters, while purlins and bracing use slightly smaller tubing. For the covering, greenhouse-grade, 6-mil polyethylene film is the standard material, chosen for its UV-stabilizing additives that prevent degradation for at least four years.
The base requires lumber, often 2×6 boards, to create a secure anchor point for the bottom edge of the plastic. Untreated lumber is preferred for interior framing to avoid chemicals off-gassing and damaging the plastic or crops. Specialized fastening hardware is required, including poly-lock channel and spring wire (often called wiggle wire) to hold the plastic securely against the frame under high tension.
Tools for assembly include a post pounder to install the ground anchors, a measuring tape, and a drill. Self-drilling screws, such as #14 hex-head TEK screws, are used extensively for metal-to-metal connections and for attaching purlin connectors without pre-drilling. A saw and basic woodworking tools are needed for preparing the end wall and baseboard lumber.
Constructing the High Tunnel Frame
Construction begins by accurately marking the location of the ground posts, typically spaced four feet apart. These posts are driven into the ground to ensure they are firmly anchored and level across the width of the structure. A small drainage hole should be drilled near the soil line on each post to prevent water accumulation inside the tubing, which can lead to premature rust and cracking.
The pre-bent steel hoops are then inserted into the tops of the ground posts and secured with self-tapping screws or bolts. Next, the purlins are installed horizontally along the length of the structure, starting with the ridge purlin at the peak, which provides initial stability. Purlins are attached to the hoops using specialized hardware, with fasteners positioned so the screw or bolt head faces inward, away from the plastic covering.
The end walls, which bear the brunt of wind loads, are framed using lumber and connected to the final hoop at each end. Diagonal bracing should be installed from the end hoop down to the baseboard at the corners to resist racking and increase wind resistance. Once the frame is complete, the baseboards are attached to the exterior of the ground posts, creating a continuous, flat surface for securing the plastic covering.
Installing the Covering and Ventilation Systems
The greenhouse film should be installed on a warm, calm day to allow for maximum tensioning and prevent wind damage. The film is draped over the entire frame, centered, and temporarily secured. To achieve a smooth, tight finish, the plastic must be pulled down and outwards, working from the center toward the ends, eliminating wrinkles before final fastening.
Securing the film involves installing poly-lock channel along the ridge, baseboards, and end walls, then inserting the spring wire (wiggle wire) over the plastic into the channel. This system creates a continuous lock that holds the plastic against high winds while allowing easy removal. At the base, the plastic should extend past the baseboard and be sealed by burying the excess film or attaching it to a metal skirt plate to prevent drafts.
Passive ventilation is managed primarily through roll-up sides and framed end wall doors. The roll-up system uses a metal tube inserted into the bottom edge of the side plastic to act as a roll bar. This roll bar connects to a manual hand crank mechanism, allowing the user to regulate internal temperature and humidity. Hinged doors, framed with lumber and covered in the same polyethylene film, are installed at the end walls for access and cross-ventilation.