Attaching polyethylene film to a metal greenhouse frame requires careful preparation and the correct hardware. Proper installation is a structural necessity that determines the greenhouse’s ability to regulate internal temperature and withstand wind and weather extremes. Selecting an appropriate attachment method ensures the film remains taut, preventing flapping that leads to premature material failure.
Essential Preparations Before Covering
Before unrolling the plastic, thoroughly inspect the metal frame to prevent immediate film damage. All metal components contacting the polyethylene must be free of sharp edges, burrs, or rust, as these abrasions create stress points where the plastic will tear prematurely. Clean the frame to remove any oil or dirt residue that could compromise the film’s integrity over time.
To mitigate heat-related degradation, apply white latex paint or specialized anti-hot-spot tape to the upper surfaces of the metal frame members. Metal absorbs solar radiation, and the resulting high surface temperature can accelerate the breakdown of the plastic film where they touch. Installation should ideally take place on a day with low wind, preferably in the morning when temperatures are between 59 and 70 degrees Fahrenheit. This temperature range allows the plastic to be stretched taut without the risk of over-stretching.
When handling the film, pay attention to its orientation; the UV-stabilized side must face outward toward the sun. This protective layer is typically indicated by a printed logo or label on the film, which should be readable from the inside after installation. Accurate measurement is also important, requiring the total length over the bow or arch plus an ample overhang. This overhang, typically one to two feet on each side, allows for tensioning and securing the ends.
The Wiggle Wire and Lock Channel System
The industry standard for durable attachment to metal structures is the Wiggle Wire and Lock Channel system. The Lock Channel is a galvanized steel or aluminum track affixed directly to the metal frame using self-tapping screws. This channel provides a continuous, secure mounting point along the frame’s perimeter, hips, and baseboards.
Once the channel is secured, the polyethylene film is draped over the structure and pulled taut over the open channel. The Wiggle Wire, or spring wire, is a flexible, pre-bent steel wire that is then inserted into the channel, trapping the plastic firmly against the metal track. This method creates a continuous, weather-tight seal that can withstand significant wind loads because the force is distributed evenly.
An advantage of this system is the ability to easily remove and re-install the plastic without causing damage, which is useful for seasonal ventilation or replacing damaged sections. Specialized double-channel tracks are also available, often used along the hip rail to secure two separate plastic layers simultaneously, such as the main cover and a roll-up side curtain. The system’s integrity relies on the channel being securely fastened to the metal frame using specialized self-drilling screws designed to tap directly into the steel tubing.
Alternative Attachment Methods for Metal Frames
While Wiggle Wire is preferred for long, structural runs, other methods are used for specific applications. Plastic snap clamps are a simple, reusable option for securing plastic to round or square metal tubing, particularly for temporary covers or roll-up side installations. These UV-stabilized polypropylene clamps snap directly over the film and the pipe, maintaining pressure without damaging the plastic, though they are only effective on specific pipe diameters.
For smaller, non-structural areas, thin wooden or plastic batten strips can be secured over the plastic film and directly into the metal frame. This method requires self-tapping screws to fasten the batten strip through the plastic and into the steel tubing beneath, effectively sandwiching the film. A drawback is that batten strips create permanent stress points where the plastic is pierced by the screws, making film removal and re-installation difficult without causing tears.
In high-wind areas, woven poly-strapping (a heavy-duty, UV-resistant webbing) is sometimes used as a secondary measure. These straps run over the entire structure, perpendicular to the main hoops, and are secured to the foundation or ground anchors. This external strapping holds the film tightly against the frame, preventing billowing and significantly reducing wear caused by wind-induced friction.
Achieving Proper Tension and Perimeter Sealing
Achieving the correct film tension is important, as a loose cover will flap in the wind, leading to rapid material fatigue and eventual tearing. The proper technique involves securing the plastic at a central point, then pulling outward and securing the film toward the ends and sides to remove all wrinkles. This ensures the load is evenly distributed.
The perimeter must be completely sealed to prevent drafts and maintain the controlled environment. At the base, the plastic cover is often terminated by securing it to a baseboard anchored to the ground or foundation. A traditional method for ground sealing involves digging a trench around the perimeter, tucking the excess plastic into the trench, and then backfilling it with soil.
Alternatively, the Lock Channel system can be installed along the baseboard to provide a continuous, air-tight seal for the bottom edge of the film. For managing excess plastic at the gable ends and around door frames, specialized greenhouse tape (durable and UV-resistant) can be used to seal small gaps and secure cut edges, minimizing air exchange.