Polycarbonate greenhouses have become a popular choice for home growers, offering high durability and beneficial growing conditions. The material, a thermoplastic polymer, is virtually shatterproof, making it significantly more impact-resistant than traditional glass and capable of withstanding severe weather conditions. Multi-wall polycarbonate sheets feature an internal cellular structure that traps air, providing superior thermal insulation compared to single-pane glass. This helps maintain a more stable internal temperature and reduces heating costs. Furthermore, the material naturally diffuses sunlight, scattering the light more evenly across the plants to prevent hot spots and ensure consistent illumination. This balance of robustness, insulation, and light distribution makes polycarbonate an excellent choice for DIY greenhouse construction.
Site Selection and Foundation Preparation
Selecting the proper location for a polycarbonate greenhouse is the foundational step that determines the structure’s long-term success. The optimal site will receive a minimum of six hours of direct sunlight, particularly during the winter months. Positioning the greenhouse ridge line to run from east to west is often recommended, as this orientation maximizes light exposure throughout the day. Proper drainage is also paramount, requiring the ground to be graded so that surface water flows away from the structure rather than pooling near the base.
Before any construction begins, the selected area must be perfectly level and square to prevent racking of the frame, which can compromise the integrity of the entire structure. Common foundation options include a perimeter of treated timber, a concrete slab, or a grade-beam footing. A simple, cost-effective choice for smaller structures is a foundation of rot-resistant wood timbers, such as 4×4 or 6×6 beams, which should be properly anchored to the ground to resist uplift from strong winds. Concrete slabs offer the most durable base but require the installation of drains to manage the heavy watering inside a greenhouse. The foundation must be robust enough to securely anchor the lightweight structure and keep the frame level.
Framing Assembly Techniques
The frame provides the skeleton for the polycarbonate panels and must be assembled to be square, plumb, and rigid before cladding begins. Common framing materials include lightweight aluminum, wood, or PVC, with the choice often dictated by the desired size and permanence. For wooden frames, pre-drill all holes for fasteners to prevent the wood from splitting when screws or bolts are driven in. Cross-bracing significantly increases the structural stability of the walls and roof by distributing lateral forces, preventing the frame from deforming under wind or snow loads.
The assembly process begins with securing the corner posts and connecting them with the base and top plates, ensuring the structure is exactly square by measuring the diagonals. Rafters and purlins are then installed to create the roof profile, which must match the dimensions of the polycarbonate panels to minimize cutting and waste. Aluminum framing offers a lightweight yet strong option that is resistant to the high humidity found inside a greenhouse. When using pressure-treated wood, a polyethylene barrier should be placed between the wood and any aluminum components to prevent the chemicals in the wood from corroding the metal over time.
Installing and Sealing the Polycarbonate Sheeting
The multi-wall polycarbonate panels must be installed with careful attention to their specific properties to ensure longevity. These sheets are manufactured with a UV-protective layer on one side, which must be oriented facing the sun to prevent the panel from yellowing and becoming brittle. The multi-wall structure creates internal channels, or flutes, that run the length of the sheet, providing the material’s insulating air pockets. Panels can be cut to size using a circular saw with a fine-tooth blade, but the resulting plastic dust must be cleared from the flutes with compressed air before installation.
Securing the panels requires specialized fasteners, such as galvanized screws with neoprene-bonded washers. These provide a watertight seal while allowing for the material’s natural expansion and contraction with temperature changes. The most crucial step is sealing the open flutes to prevent the ingress of moisture, dirt, and insects. The top, or uphill, end of the panel flutes should be sealed with a non-vented aluminum foil tape to create a watertight barrier. Conversely, the bottom, or downhill, end must be sealed with a vented anti-dust tape, which allows any accumulated condensation to drain while still filtering out debris. After taping the ends, U-channels are fitted over the taped edges to provide a final, rigid closure and a clean finish.
Essential Ventilation and Access Points
Once the polycarbonate sheeting is fully installed, the final steps involve ensuring proper airflow and accessibility. Installing a door is the first step for access, and its frame must be square and plumb to ensure it seals correctly when closed. Proper ventilation is necessary for regulating the internal environment and preventing overheating. This can be achieved through a combination of low-level intake vents and high-level exhaust vents. Roof vents are particularly effective at releasing the warmest air that rises to the peak of the structure. These can be fitted with automatic openers that use a temperature-sensitive wax to open and close without electricity.
Weather stripping and caulking around the door, window, and vent openings are important for creating a sealed environment. This final sealing process prevents drafts that can lead to energy loss and restricts the entry of pests and cold air. For any seams where panels meet the frame or each other, a high-quality silicone caulk should be applied to fully weatherproof the joints. These finishing touches ensure the polycarbonate structure is a fully enclosed and controlled environment, ready to support a healthy and productive growing season.