A retaining wall can support a fence or structure only if it is specifically designed and engineered to do so. Fences and structures add extra load to the wall. Without proper engineering, this additional load can cause movement or failure over time.
Why do retaining walls fail near fences and buildings?
Retaining walls fail near fences and buildings because the wall was not designed for surcharge loads, restricted drainage, or limited footing space. These issues often cause gradual movement that appears years after construction.
Should retaining walls be planned before fences or structures?
Yes. Retaining walls should always be planned and built before fences or structures. This allows the wall to be engineered correctly, prevents rework, and ensures long-term stability and compliance.
Why Retaining Walls and Structures Must Be Considered Together
Retaining walls do not exist in isolation. On most residential sites in Perth, they sit close to fences, sheds, garages, driveways, or even parts of the house itself. When these elements are treated separately, problems often emerge years later. Existing structures change how loads are applied to the soil behind a retaining wall. They also limit space for footings, drainage, and construction access. If these factors are not considered during design, the wall may perform well initially but deteriorate over time.
Fences Are Loads, Not Decorations
One of the most common misconceptions is that fences are lightweight and insignificant. In reality, fences apply continuous load along the top of a retaining wall. Wind pressure, soil movement, and the fence’s own weight all increase stress on the wall. In Perth, boundary retaining walls often support colourbond, timber, or masonry fencing. If the wall was not engineered for this surcharge load, cracking or leaning can occur slowly and unpredictably. This is why fences and retaining walls must be designed as a single system, not two separate elements.
Retaining Walls Under Existing Fences
Installing a retaining wall beneath an existing fence is particularly challenging. The fence footings may already be carrying a load, and excavation can destabilise them if not handled carefully. In many cases, the fence must be temporarily removed or supported while the retaining wall is constructed. Attempting to build around fence posts without engineering often compromises both structures. Proper sequencing and design are critical to avoid long-term damage.
Interaction with Sheds, Garages, and Outbuildings
Structures such as sheds, garages, patios, and water tanks also affect retaining wall performance. Even small buildings apply vertical and horizontal loads to the soil. When this soil is retained, the wall must resist that pressure. A wall designed only for soil pressure may fail once a structure is added nearby. This is a common cause of delayed failure, where the wall appears stable until years later, when movement becomes visible.
Driveways and Vehicle Loads
Driveways are one of the most underestimated surcharge loads. Vehicles introduce repeated dynamic forces, not just static weight. When a driveway sits above or near a retaining wall, the wall must be engineered to handle these loads safely. Without proper design, driveways can accelerate wall movement, leading to cracking, settlement, or collapse. This risk is especially high on sloping blocks and infill lots common throughout Perth.
Limited Space Creates Higher Risk
Many retaining walls in Perth are built close to boundaries or existing structures, leaving limited room for footings and drainage. This restriction increases design complexity. When space is tight, deeper footings, stronger materials, or alternative wall systems may be required. Generic designs that work on open sites often fail when applied to constrained residential blocks.
Drainage Becomes More Critical Near Structures
Existing structures reduce natural water absorption and increase runoff. Roofs, paving, and hardstand areas direct water toward retaining walls, increasing hydrostatic pressure. If drainage is not designed to account for this additional water, pressure builds slowly behind the wall. Over time, this leads to bulging, cracking, or rotation. Proper drainage design is essential when retaining walls interact with buildings or fences.
Why Timing and Sequencing Matter
One of the biggest mistakes homeowners make is installing fences or structures before retaining walls. This limits design options and increases construction risk. Building the retaining wall first allows it to be engineered correctly, with fences and structures installed later in a controlled and compliant manner. Correct sequencing reduces costs, avoids rework, and improves long-term performance.
Engineering Is Essential in These Situations
Any retaining wall supporting or located near fences, driveways, or structures should be engineered. In Perth, councils often require engineering certification under AS 4678 for these conditions. Engineering ensures that footing depth, wall strength, drainage capacity, and surcharge loads are properly addressed. This protects not only the wall but also the surrounding structures.
Common Mistakes That Lead to Failure
Many failures occur because fences are treated as an afterthought, existing structures are ignored during design, drainage is compromised due to limited space, or walls are built without engineering. These issues rarely cause immediate problems but often lead to failure years later. Understanding how retaining walls interact with nearby elements is key to avoiding these outcomes.
Planning a Retaining Wall Near Fences or Structures?
If your retaining wall will sit near a fence, driveway, or building, proper planning is essential. Early advice can prevent costly mistakes and long-term structural issues. Contact Structural Retaining Walls today to arrange a site inspection or discuss a compliant retaining wall solution designed for Perth conditions.