Building a tiered garden on a slope, known as terracing, transforms unusable hillside into flat, accessible planting areas. This process maximizes growing space and prevents soil erosion by slowing water runoff. Properly constructed tiers stabilize the slope, allowing cultivation of plants otherwise impossible to grow on an incline. The project requires careful planning and a methodical approach to construction.
Assessing Your Slope and Planning the Layout
Accurately measuring the slope’s grade determines the height and number of tiers required. Grade is calculated as the “rise over run” (vertical distance divided by horizontal distance), typically expressed as a percentage. To measure this, pound stakes at the top and bottom of the slope. Run a level string line between them, then measure the vertical drop from the string to the ground at the lower stake.
The grade measurement determines the height of the retaining walls, which relates directly to local building codes. Many municipalities require a building permit and professional engineering plans for walls exceeding four feet (1.2 meters) in height. To avoid these complex requirements, divide the total change in elevation into multiple, smaller tiers. This ensures each individual wall remains below the four-foot threshold.
Planning the layout must account for the surcharge, which is the pressure exerted by the upper tier’s soil on the lower wall. To manage this force, the horizontal offset between the face of the upper wall and the lower wall should be at least two times the exposed height of the lower wall. This spacing ensures each wall acts independently, preventing the system from being treated as a single, large structure.
A robust drainage strategy is essential, as hydrostatic pressure from saturated soil is the primary cause of retaining wall failure. Water must be managed behind and between the tiers to prevent buildup. This involves planning a drainage zone of gravel backfill directly behind the wall face. Incorporate a perforated pipe at the base of each tier to collect and direct water away from the wall structure and the slope.
Choosing Materials for Your Retaining Walls
The choice of material impacts the cost, aesthetics, durability, and required skill level. Segmental Concrete Blocks (SRWs) are popular due to their uniform size, interlocking mechanisms, and durability, often lasting over fifty years. SRWs offer excellent drainage and a consistent, finished look. They are a moderate-cost solution manageable for experienced builders.
Pressure-treated lumber is the most cost-effective and easiest material to work with for a budget-friendly option. Lumber has the shortest lifespan, typically lasting fifteen to twenty years, and is best reserved for walls under four feet due to lower load-bearing capacity and rot susceptibility. Dry-stacked natural stone or large boulders offer a premium aesthetic and the highest durability, potentially lasting over a century. However, stone requires higher material and labor costs due to weight and specialized placement skill.
Gabion walls are wire mesh cages filled with rocks, offering a unique combination of strength and natural appearance. These structures are flexible, adapting well to soil movement and curved layouts, while providing superior drainage. Gabions are a durable, long-lasting solution. They can be cost-effective if locally sourced stone or recycled concrete is available for the internal fill.
Constructing the Tiered System
Construction begins by marking the footprint of the lowest tier and digging a trench for the base course. The trench must be deep enough to bury at least ten percent of the wall’s exposed height, plus the depth required for the base material. Fill the trench base with a six-inch layer of compacted crushed rock, such as three-quarter inch clean gravel. This serves as a level, stable, and free-draining footing for the wall.
The first course of wall material must be perfectly level, as any error will be magnified in subsequent layers. For stability, set the blocks or timbers with a slight inward tilt, known as the “setback” or “batter.” This lean into the slope counteracts the forward pressure of the soil. This inward slope is achieved either through the design of interlocking block units or by slightly recessing each successive timber course.
Drainage incorporation occurs as the wall is built up in successive courses. Place a drainage zone, consisting of a minimum of twelve inches of clean, crushed stone, directly behind the wall face. Lay a four-inch perforated drainpipe, wrapped in filter fabric, at the base of this stone layer and angle it to daylight at the side of the wall. This system collects water from the retained soil and directs it out, preventing the buildup of hydrostatic pressure.
Timber walls require reinforcement using galvanized steel rebar driven through pre-drilled holes into the ground below. Taller timber walls require “deadmen” anchors, which are perpendicular timber ties buried in the slope and connected to the wall face. Deadmen provide internal resistance against outward movement. After each course is set, fill the drainage zone. Add the main infill soil in six to eight-inch lifts, which must be compacted to provide a solid mass behind the structure.
Preparing the Beds and Final Planting
Once the retaining walls are structurally complete, the new flat beds must be properly filled to support healthy plant growth. Avoid using only the excavated native soil, which is often dense and low in organic matter. Instead, fill the beds in layers to promote drainage and long-term soil fertility.
The bottom layer should consist of coarse organic materials, such as untreated wood chips or sticks, which promote drainage and slowly decompose. Fill the bulk of the tier with a combination of excavated native soil and high-quality topsoil, mixed with compost and organic amendments. The final six inches should be a premium blend of topsoil and compost, providing a nutrient-rich medium for root growth.
Plant selection should focus on species offering functional benefits and aesthetic appeal. Deep-rooted groundcovers and shrubs are recommended as they help bind the soil within the tiers, preventing erosion. Consider the microclimates created by the terracing. The top tier will be drier and warmer due to increased sun and wind exposure, while lower tiers retain more moisture. Choose drought-tolerant plants for the upper tiers and moisture-loving varieties for the base.