A stable, level base is necessary for constructing a greenhouse that will withstand environmental stresses and provide a long-lasting growing environment. Building on uneven ground is a common challenge, but it can be overcome with careful planning and specific construction methods. Ignoring a slope compromises the structure, leading to issues with frame alignment, door function, and potential wind damage.
Assessing the Site and Calculating Elevation Differences
The first step involves clearing the planned greenhouse footprint of debris, rocks, and vegetation to expose the natural soil gradient. After marking the perimeter, the objective is to precisely measure the total elevation change across the area. Simple tools, such as a string line, a line level, or a laser level, provide the necessary accuracy for this assessment.
Establishing a datum point allows for consistent measurements across the entire site. By stretching a level string line from the highest point to the lowest point of the intended base, one can determine the vertical drop or rise. This measurement dictates the severity of the slope and informs the appropriate leveling strategy. For example, a difference greater than 18 inches across the length of the structure typically requires more intensive leveling techniques.
Leveling Strategies for Sloped Terrain
The cut-and-fill technique is a common method for managing a moderate slope. This involves moving soil from the high side to the low side of the footprint. The high side is “cut” by removing earth until the desired level is reached, and the removed soil is used to “fill” the lower side.
When adding soil to the low side, it is important to compact the fill material in thin layers, ideally no more than 6 to 12 inches thick, to achieve a density of 90 to 95 percent of the maximum dry density. This compaction prevents future settling and shifting that would compromise the foundation’s stability.
When the terrain is too steep for simple cut-and-fill, typically defined as a drop exceeding 24 inches, a retaining wall or stem wall system is necessary. This approach involves digging into the slope on the high side and constructing a permanent wall, usually made of poured concrete or treated lumber, to hold back the earth. The wall creates a level shelf upon which the greenhouse base can be built, providing a solid barrier against lateral soil movement. While more labor-intensive, this method offers greater stability for steeper grades.
Alternatively, a pier or post system allows the greenhouse to be leveled without moving significant amounts of earth. This involves setting adjustable footings, such as pre-cast concrete piers with metal brackets, into the ground around the perimeter. The posts are sunk and their height is individually adjusted to support a perimeter base frame at a level plane. This technique is useful for rocky or environmentally sensitive sites where extensive excavation is impractical, as it creates a raised, level platform.
Constructing the Foundation on the Newly Leveled Area
Once the site has been leveled, the next phase is constructing the foundation that will support the greenhouse frame. For sites leveled using a pier system, a perimeter base frame made of treated lumber, such as 6×6 timbers, is the preferred option. The wood must be rated for ground-contact use, or naturally rot-resistant species like cedar or redwood can be selected, to ensure a long lifespan despite moisture exposure.
This perimeter frame provides a continuous, solid surface for anchoring the greenhouse and helps keep the structure square. For larger greenhouses, using double layers of timbers or wider beams is recommended to manage the increased structural load and prevent warping. If using pressure-treated wood, all hardware, including screws and bolts, must be stainless steel or heavily galvanized to prevent corrosion from the chemical preservatives.
A gravel base is a functional foundation built on top of the newly leveled area and contained within the perimeter frame. Angular crushed stone, typically 1/2 to 3/4 inch in size, is recommended because the sharp edges interlock when compacted, providing excellent drainage and stability. This material is often placed over landscape fabric to prevent weed growth and soil migration, keeping the greenhouse floor well-drained and protecting the base from moisture exposure.
For a permanent solution, a concrete slab can be poured directly onto the leveled ground, especially when a retaining wall method was used. A concrete foundation provides maximum durability and a smooth, uninterrupted surface. It is important to reinforce the slab with wire mesh or rebar to manage tensile stresses and prevent cracking.
Securing the Structure and Ensuring Proper Drainage
The final steps involve securing the greenhouse frame to the new base and managing water runoff to protect the foundation. Anchoring the structure defends against wind uplift and lateral forces, which can damage a lightweight greenhouse frame. For a concrete slab, J-bolts or anchor bolts are embedded into the wet concrete during the pour, creating permanent attachment points.
If a wood perimeter base is used, the frame can be secured using heavy-duty straps, metal brackets, or earth anchors driven deep into the ground. Anchoring prevents the frame from shifting or being lifted by strong winds, maintaining structural integrity. The surrounding area must also be graded so that surface water runs away from the foundation on all sides.
In sloped environments, managing water runoff from the uphill side is often accomplished by installing a French drain system. This involves a trench filled with gravel and a perforated pipe, which intercepts water before it reaches the foundation and redirects it around the structure. The perforated pipe should be laid with a slight downhill slope, typically a minimum of 1/8 inch per foot, to ensure gravity efficiently carries water away to a discharge point.