Building a terraced garden bed on a slope transforms unusable, erosion-prone ground into productive, level planting space. This construction method uses a series of small retaining walls that step up the hillside, creating flat, usable tiers. Terracing effectively manages water runoff on an incline, slowing its descent and allowing it to soak into the soil, which prevents the washing away of topsoil and nutrients. Careful planning and structural integrity are required to ensure the walls can withstand the lateral pressure exerted by the retained soil and water.
Assessing the Slope and Choosing Materials
The initial planning phase requires accurately measuring the hillside’s grade to determine the height and number of terraces needed. The slope’s steepness is measured by calculating the rise (vertical distance) over the run (horizontal distance), often using stakes, a string, and a line level. This calculation helps determine the total elevation change that must be accommodated by the tiered walls. This measurement directly dictates the required height of each retaining wall and the necessary step-back distance between them.
The choice of construction material affects both the aesthetic and the structural longevity of the terracing system. Pressure-treated lumber is often the most affordable and easiest for a do-it-yourself installation, though its lifespan is shorter than other materials, even with chemical treatment. Modular concrete blocks offer superior durability, require minimal maintenance, and are manufactured with interlocking features for stability, but they carry a higher initial cost. Natural stone provides a classic look and exceptional longevity, but the installation is more labor-intensive and may require professional masonry skills. Regardless of the material chosen, the wall height for residential terracing is kept under four feet to avoid the need for complex engineering and permits.
Preparing the Base and Marking the Layout
Site preparation begins with marking the layout of the lowest terrace, which will bear the greatest soil load. The lines for the first wall should be defined with stakes and string, establishing the final footprint of the terracing system. The foundation trench, or footing, for this wall must then be excavated, typically to a depth of about 10% of the wall’s planned height plus space for the base material. It is critical that this trench is perfectly level along its length, which may require removing or adding soil and then compacting the base of the trench with a plate compactor.
Proper placement of successive walls is determined by the required “setback,” which is the horizontal distance between the face of one wall and the toe of the wall directly above it. A common guideline suggests the setback should be at least equal to the height of the lower wall to maintain stability and prevent the upper wall’s weight from stressing the lower structure. For example, a three-foot-tall wall should have a minimum of three feet of level planting area before the next wall begins. This step-back is a structural necessity that accounts for the angle of repose of the retained soil and the forces of gravity acting on the entire slope.
Building and Anchoring the Retaining Walls
The construction process begins by placing the first course of material into the trench. For concrete block systems, the blocks are typically dry-stacked, relying on gravity and their weight for stability, often with the first course partially buried for foundational support. Timber walls are secured by driving half-inch steel rebar pins through pre-drilled holes in the wood and into the subsoil below, effectively tying the lowest course to the ground. As the walls rise, a slight tilt, or batter, into the slope is often incorporated, increasing the wall’s resistance to the lateral soil pressure.
For taller timber walls, or those retaining a significant soil mass, internal anchors known as “deadmen” are necessary to prevent the wall from bowing outward. A deadman is a perpendicular timber buried deep within the hillside and connected to the wall face with a tie-rod or another timber, using the weight of the retained soil itself to stabilize the structure. This system of internal bracing counteracts the immense horizontal force exerted by the soil. Successive courses are built, stepping back the appropriate distance up the slope, with each tier’s structural integrity relying on the firm foundation of the one below it.
Implementing Drainage and Backfilling
Effective drainage is essential for the longevity of a terraced wall, as water accumulation behind the structure is the primary cause of retaining wall failure. Saturated soil significantly increases its weight and introduces hydrostatic pressure, which can push even a well-built wall outward. To mitigate this, a drainage layer of coarse gravel or crushed stone should be placed immediately behind the entire length and height of the wall. This gravel layer acts as a filter and a conduit, allowing water to pass through freely instead of building up pressure against the wall face.
A perforated drainpipe is often installed at the base of the gravel layer, running horizontally along the wall’s footing. This pipe collects the water channeled through the gravel and directs it away from the wall to an outlet at the side of the terrace or down the slope. For walls made of solid material, small openings called weep holes can be incorporated into the bottom course to provide an exit for the water. Once the drainage layer is complete, the terrace is backfilled with a soil mixture suitable for planting, taking care to avoid compacting the soil immediately behind the wall.