Wind uplift is the primary threat to freestanding greenhouses, acting to lift the entire assembly off its foundation. Grounding the structure is necessary, especially for hobby kits, polytunnels, and small rigid frames that lack inherent mass. Failure to anchor the frame can lead to catastrophic damage during high winds, as the light weight of typical aluminum-frame greenhouses provides minimal resistance against turbulent air movement. The greenhouse must be firmly connected to the ground to redirect these vertical uplift forces into the soil mass below. This connection ensures the frame remains stable and prevents the assembly from shifting or becoming airborne.
Securing the Frame to Soft Ground
When installing a greenhouse on substrates like topsoil, grass, or loose gravel, the anchoring method must penetrate the surface layer and engage the deeper, more stable soil. Earth anchors, such as helical screws or augers, are effective because they twist into the ground, creating resistance. These anchors are typically driven to a depth of at least 36 inches (91 cm) to engage stable soil layers, which helps them resist the tensile forces generated by wind uplift.
A simpler, less robust method involves driving lengths of rebar or metal stakes deep into the ground alongside the frame’s base rails. These stakes should be driven at a slight angle and left exposed at the top to allow the frame to be clamped or wired securely to them. While this method is quick, it relies on friction and the shear strength of the soil directly against the stake, offering less pull-out resistance than a helical design.
Another technique uses “deadmen” anchors: heavy objects buried beneath the surface and connected to the frame via tensioned cables or straps. A concrete block, a section of heavy timber, or a large flat stone can serve this purpose, typically buried 12 to 18 inches deep. The weight and surface area of the buried object provide a substantial counter-force against uplift, transferring the load to the soil above the anchor rather than relying solely on penetration friction. The tie-down strap must be kept taut and checked periodically, as soil settling can loosen the connection over time.
Attaching the Structure to Hard Surfaces
Securing a greenhouse to a hard surface, such as a concrete slab, paved patio, or timber deck, requires mechanical fasteners. This process typically involves drilling pilot holes directly into the hard surface using a hammer drill fitted with a masonry bit. The frame’s base rail is positioned, and holes are marked through the pre-drilled points in the rail and into the underlying surface.
Once drilled, the structure is secured using specialized hardware designed to withstand shear and tension forces. Wedge anchors, for instance, are driven into the concrete, and as the nut is tightened, the bottom end expands, locking the bolt securely within the masonry. Alternatively, sleeve anchors or specialized concrete screws can be used, providing a strong, permanent connection between the metal frame and the immovable foundation.
When attaching to a timber deck, lag screws or heavy-duty construction screws are used to fasten the base rails directly to the deck joists beneath the surface boards. For hard surface attachment, install an L-bracket or angle bracket at each corner and at regular intervals along the base rail (e.g., every four to six feet). These brackets ensure that the connection is distributed and that the frame cannot shift horizontally or lift vertically from the fixed surface.
Building a Permanent Perimeter Base
Constructing a permanent perimeter base provides maximum stability, integrating the structure with a heavy mass resistant to wind and soil movement. One approach involves creating a durable timber base, often using pressure-treated lumber (such as 4×4 or 6×6 beams) rated for ground contact. This lumber is assembled into a rectangular frame that matches the greenhouse footprint and is then anchored to the ground using long stakes or rebar driven through the beams and into the soil.
This timber base acts as a sill plate, distributing the load and raising the structure above damp ground, which protects the frame from premature corrosion. The greenhouse frame is then bolted directly to the top surface of this wooden perimeter using carriage bolts or lag screws. The weight of the lumber itself adds substantial dead load, significantly increasing resistance to uplift forces.
For the most robust and permanent solution, a concrete stem wall or continuous footing is constructed around the perimeter. This involves digging a trench below the frost line in colder climates, placing rebar reinforcement, and pouring concrete to create a solid, monolithic foundation. This method provides the highest level of mass and protection against shifting or settling ground.
During the concrete pour, anchor bolts (often J-bolts or L-bolts) are strategically embedded into the wet material, spaced to align with the base rail of the greenhouse frame. Once the concrete cures, the greenhouse frame is set over these protruding bolts and secured with washers and nuts, creating a mechanical lock between the structure and the massive concrete footing. A stem wall not only secures the structure but also elevates the interior floor, mitigating splash-back and improving drainage around the base of the greenhouse.