What Is the 1:3 Rule in Retaining Wall Design?

In retaining wall construction, safety starts below the surface. One principle that often comes up, especially during early design is the 1:3 rule. It’s a simple idea with serious implications: for every metre of wall above ground, a third of that height should sit below ground. But how accurate is this rule? And does it apply to your project?

This blog will unpack what the 1:3 rule actually means, when it’s valid, and where it falls short. We’ll also look at local compliance in NSW, common design mistakes, and how Murs Projects ensures every retaining wall is built for long-term performance.

Understanding the 1:3 Rule in Retaining Walls

The 1:3 rule in retaining wall design is a commonly used guideline in both residential and structural applications. It means that for every 1 metre of visible wall height above ground, approximately ⅓ of that height should be embedded below ground level.

So, if a retaining wall stands 1.2 metres high, the base or footing should extend about 400mm underground. This underground section helps stabilise the wall by countering the lateral earth pressure pushing against it, especially in sloped or high-moisture environments.

This rule is often referenced because it offers a simple, fast way to estimate footing depth, especially during preliminary planning. It’s particularly helpful when detailed soil or structural data isn’t yet available. However, while the 1:3 ratio is useful, it’s not always a replacement for proper engineering.

In areas with reactive clay, surcharge loads (like parked vehicles or nearby structures), or poor drainage, following the 1:3 rule alone may not be enough to prevent wall movement or failure. The actual footing depth may need to be deeper, reinforced, or engineered to suit site-specific conditions.

That’s why relying solely on this guideline, without a full site assessment can lead to non-compliant or underperforming walls.

Why the 1:3 Rule Matters in Structural Walling?

The 1:3 rule isn’t just about following a guideline. It’s about ensuring your retaining wall can resist the forces acting against it over time. Retaining walls hold back soil, absorb water pressure, and sometimes support structures or driveways. Without proper footing depth, a wall may crack, lean, or even collapse.

By embedding one-third of the wall’s height below ground, the structure gains more resistance against lateral pressure and shifting soil. This depth acts as a physical anchor, improving the wall’s overall stability.

For structural walling projects in NSW, especially in urban areas like Sydney or Newcastle, this becomes critical. Sloped land, clay soils, and poor drainage all increase the pressure behind the wall. The deeper the embedment, the more secure the wall becomes under these conditions.

Walls that follow the 1:3 rule are more likely to remain upright and safe long term. Walls that don’t often show early signs of failure - bulging, cracking or leaning within just a few seasons.

At Murs Projects, we’ve seen firsthand how insufficient footing depth causes major issues on site. That’s why our team assesses every wall not just against general rules, but against real-world site conditions.

Factors That Influence Retaining Wall Design

While the 1:3 rule provides a useful starting point, it doesn’t apply equally to every retaining wall. Several factors influence how deep a wall’s footing should be and whether the wall will meet structural and compliance standards.

Understanding these variables is critical in determining whether a standard rule will be sufficient or if an engineered design is needed.

1. Soil Type and Ground Conditions

The type of soil beneath and behind the wall directly affects how much force it will experience. Soft or loose soils, such as sand or fill, offer less resistance and may require deeper embedment. Reactive clays common in parts of NSW expand and contract with moisture changes, adding movement and stress.

In these cases, following the 1:3 rule may not be enough. We recommend site-specific geotechnical assessments to prevent long-term movement or failure.

2. Load Bearing and Pressure Behind the Wall

Retaining walls must withstand more than just the weight of soil. They may also need to support:

• Sloped backfill

• Driveways or vehicle traffic

• Nearby buildings or fences

These additional forces, called surcharge loads, increase the pressure behind the wall. In high-load scenarios, walls typically need deeper foundations, stronger materials, or engineered reinforcement.

3. Drainage and Water Flow

Poor drainage is one of the most common causes of retaining wall failure. If water builds up behind the wall, it increases hydrostatic pressure, pushing outward and undermining stability.

Every retaining wall design should include proper drainage planning. This could involve gravel backfill, weep holes, or ag pipe installations. The more water the wall must handle, the more important footing stability becomes.

4. Retaining Wall Material and Construction Method

The material and installation method also affect how much embedment is required. For example:

• Concrete sleeper walls may need different depth profiles compared to block or modular walls.

• Precast panel systems have engineered footing designs built into the specification.

• Timber walls often require additional bracing to prevent movement over time.

Each system comes with its own structural limits. At Murs Projects, we consider both material type and installation requirements when calculating footing depth and wall height.

When the 1:3 Rule Isn’t Enough: Engineered Solutions?

While the 1:3 rule offers a helpful starting point, it’s not always suitable, especially for walls built on unstable ground, under load, or exceeding height thresholds. Across Australia, retaining wall designs must reflect site-specific conditions and comply with national building standards, not just general rules of thumb.

Conditions That Demand Engineering Support

There are many situations across Australian sites where the 1:3 rule falls short:

• Walls over 1 metre in height

• Sites with reactive or poorly compacted soils

• Walls supporting driveways, swimming pools, or buildings

• Sloping land that increases soil pressure

• Poor drainage or high water table areas

In these cases, the wall needs to be structurally designed with deeper footings, wider bases, or reinforced systems to manage pressure safely.

What Happens When the 1:3 Rule Is Ignored?

In Australia, retaining wall construction must follow relevant codes including AS 4678 – Earth-Retaining Structures and AS 3600 – Concrete Structures. These standards outline how wall systems must be designed, reinforced, and certified.

In most states, including NSW, QLD, and VIC:

• Retaining walls over 600mm typically require engineering sign-off

• Local councils may require development or construction approval

• Compliance with drainage and stormwater impact must be demonstrated

Why Compliance Matters?

Building regulations exist for a reason. In many Australian regions, a retaining wall that fails may also void insurance or lead to legal claims if it causes damage to neighbouring properties.

That’s why following or exceeding the 1:3 rule isn’t just best practice, it’s critical to protecting both the structure and your investment.

Retaining Wall Compliance Across Australia

Across Australia, retaining wall construction is regulated by local councils and must align with national building standards. While requirements vary slightly between states and regions, most councils follow similar rules for safety, drainage, and structural integrity.

Generally, if a retaining wall exceeds 600mm in height, supports a structure, is built near a boundary, or alters natural water flow, you’ll likely need:

• Engineering certification

• Development or building approval from the local council

• Drainage and erosion control measures in line with site conditions

• Compliance with Australian Standards, such as AS 4678 for earth-retaining structures

Murs Projects works within these requirements on every project, with a strong focus on compliance throughout New South Wales. We coordinate directly with local councils and qualified engineers to deliver safe, regulation-compliant retaining walls that perform long term, whether in urban Sydney or regional NSW.

Talk to the Retaining Wall Experts

If you're planning a retaining wall, whether for a residential build, commercial project, or multi-unit development, working with the right team from the start can save time, money, and stress.

Murs Projects specialises in the supply and installation of structural walling systems that meet Australian Standards and local council regulations. Our experienced installers work closely with engineers, project managers, and builders to deliver walls that are safe, compliant, and built to last.

We service projects across New South Wales, including Sydney, Newcastle, Central Coast, and Wollongong, with the capability to support larger projects in Queensland and Victoria.

Whether you need support with council approvals, footing depths, or engineered retaining wall solutions, our team is ready to assist. Contact us tday!

FAQs: The 1:3 Rule & Retaining Walls in NSW