Building a retaining wall is a common landscaping project for Australian homeowners, but doing it correctly requires careful planning, proper materials, and an understanding of local regulations. This step-by-step guide walks you through the process of building a retaining wall, from initial planning to final backfilling, with specific guidance for Adelaide and South Australian conditions. While small walls under 600mm can be a DIY project, walls above this height are best left to licensed professionals.
Step 1: Planning and Council Requirements
Before any construction begins, you need to understand the legal requirements for retaining walls in your area. In South Australia, retaining walls over one metre in height generally require development approval from your local council and must be designed by a structural engineer. Even walls under one metre may need approval if they support a surcharge load (such as a driveway or building), are near a boundary, or affect stormwater drainage. Contact your local council, whether that is City of Adelaide, City of Marion, City of Onkaparinga, or any other SA council, to confirm requirements for your specific situation. During planning, determine the wall’s exact location, height, and length. Check for underground services by calling Dial Before You Dig (1100) to have electricity, gas, water, and telecommunications services marked on your property. Consider drainage requirements, as every retaining wall needs a system to manage water pressure behind it. For walls requiring engineering, engage a structural engineer before selecting materials, as the engineer’s design will specify the construction method and materials required.
Step 2: Choosing Materials and Design
Your material choice depends on wall height, budget, aesthetic preference, and site conditions. For low DIY walls under 600mm, interlocking blocks and timber sleepers are the most accessible options. For walls between 600mm and one metre, concrete sleeper systems and engineered block walls offer better structural performance. For walls above one metre, professional construction with engineer-specified materials is required. In Adelaide, concrete sleeper walls are the most popular choice for their balance of strength, appearance, and cost. Consider your soil type: Adelaide’s clay soils require more robust drainage and deeper footings than sandy soils. Check the retaining wall types guide for a comprehensive comparison of all materials to help make your decision. Calculate the materials needed based on your wall dimensions, adding 10 percent extra for waste and cuts. Order all materials before starting construction to avoid delays mid-project.
Step 3: Site Preparation and Excavation
Mark the wall line using string lines and stakes, ensuring it follows the planned alignment. Excavate the foundation trench to the required depth: typically 200mm below finished ground level for block walls and 600 to 900mm deep for post-and-panel systems. The trench width should be at least twice the wall thickness for block walls or large enough to accommodate post holes for sleeper systems. In Adelaide, excavation in clay soils requires awareness of the soil’s moisture condition: very dry clay can be extremely hard to dig, while wet clay becomes sticky and unstable. For small walls, hand excavation with a shovel and mattock is feasible. For larger walls, hire a mini excavator, which is available from equipment hire companies across Adelaide. Remove all organic material, tree roots, and loose soil from the trench base. Compact the trench base with a plate compactor or hand tamper. If the excavation reveals unstable soil or unexpected conditions (rock, saturated ground, old fill), stop work and seek professional advice before proceeding.
Step 4: Building the Wall
The construction method depends on your chosen material. For interlocking blocks: lay a 100-150mm compacted gravel base, set the first course level, and stack subsequent courses with the interlocking mechanism engaged. For timber sleeper walls: set posts in concrete at specified intervals, allow concrete to cure for 24-48 hours, then fix sleepers horizontally behind the posts using coach bolts. For concrete sleeper walls: set steel H-beams in concrete footings, cure, then slide concrete sleepers into the H-beam channels. Regardless of material, install drainage behind the wall as you build: lay perforated 100mm agricultural pipe wrapped in geotextile fabric at the base of the wall, angled to fall towards a discharge point. Place 20mm gravel around and above the pipe to at least 300mm behind the wall face. Cover the gravel with geotextile to prevent soil migration into the drainage layer. Backfill behind the wall in 200mm layers, compacting each layer to prevent settlement. Use free-draining material for backfill directly behind the wall, transitioning to excavated material further back.
Step 5: Finishing and When to Call a Professional
Complete the wall by installing capping, finishing the ground levels, and connecting the drainage pipe to a suitable discharge point such as the stormwater system, a soakaway pit, or natural drainage path. Check the wall for plumb and alignment, making any adjustments before the backfill is fully compacted. For block walls, apply capping adhesive and set the top course or coping stones. For timber walls, apply a UV-protective wood oil or stain to exposed surfaces. Landscape around the wall with appropriate retaining wall plants that do not have aggressive root systems that could undermine the wall over time. While building a small retaining wall can be a satisfying DIY project, know when to call a professional. Engage a licensed builder if your wall is above 600mm, retains a driveway or structure, is on a boundary line, is on a slope steeper than 1:4, has complex drainage requirements, or if you encounter unexpected ground conditions during excavation. The cost of professional construction is always less than the cost of repairing a failed DIY wall.
Need professional help with your retaining wall project? Connect with licensed Adelaide retaining wall builders through our free matching service. Whether you need a full build or just advice on your DIY project, we connect you with qualified local professionals.