A retaining wall is one of the most useful structures you can add to a sloping block, holding back soil, creating level ground, and turning an awkward slope into usable space. Knowing how to build a block retaining wall helps whether you're tackling it yourself or hiring a tradie, because it lets you plan the materials, budget properly, and avoid the mistakes that cause walls to fail. This guide walks through the full process from the ground up.
Before You Start: Council Rules and Engineering
Before any digging, sort out the paperwork. As a general rule, retaining walls over about one metre high usually need council approval and an engineer's design, though the exact trigger varies by state and council, so check with your local council before you commit. Knowing the council requirements for block walls in your area is the first job, not an afterthought.
An engineer is more likely to be required for taller walls, walls carrying extra load, such as a driveway or structure above, and tricky sloped sites. Lower garden walls are often fine without approval, but don't assume.
Getting this wrong can mean fines, tearing down finished work, or worst of all, a wall that isn't safe. A quick call to the council early saves a lot of grief later. If you want the bigger picture on block types and uses first, our guide to blocks covers it.
Choosing the Right Blocks
The block you choose shapes the whole build. There are a few common options when it comes to concrete blocks for retaining walls in Australia:
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Interlocking systems like Adbri Versaloc. These mortarless blocks lock together and stack quickly, which makes them a popular, DIY-friendly choice for lower walls.
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Hollow-core besser blocks, the standard grey blocks. These are laid, then reinforced with steel and core-filled with concrete. Core-filled blocks are the go-to for engineered and taller walls.
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Purpose-made retaining blocks like Adbri Versawall, designed specifically for retaining and finished to look good when left on show.
Which one suits you comes down to wall height, the load behind it, the look you want, and your budget. Taller or load-bearing walls lean toward core filled besser blocks with an engineer's design, while a low garden wall might be a simple interlocking job.
The size of the block matters too, and for a full rundown, you can see our guide to concrete block sizes. On the other hand, if you're still weighing blocks against bricks for the project, our besser blocks vs. bricks comparison helps.
The Materials and Tools You'll Need
Now on to the most practical aspects. First and foremost, get everything on site before you start. That includes:
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Materials
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Retaining wall blocks in your chosen system
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Steel reinforcement (rebar) for core-filled walls
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Concrete for the footing and core fill
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Gravel or blue metal for drainage
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Ag pipe (slotted drainage pipe)
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Geofabric to keep soil out of the drainage
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Mortar or block adhesive, if your system needs it
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Tools
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Shovel, or an excavator for bigger digs
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String line and pegs
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Spirit level
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Plate compactor
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Wheelbarrow
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Concrete mixer, or order pre-mixed concrete delivered
Step-by-Step: How to Build a Block Retaining Wall
Here's the full process. Knowing how to build a block retaining wall the right way is mostly about getting the base and the drainage right, so don't rush the early steps.
Step 1: Excavate and prepare the base. Dig a level trench along the wall line, deep and wide enough for your footing and a compacted base layer. A solid, level base is what everything else sits on.
Step 2: Create and level the footing. Lay and compact a road base layer, or pour a concrete footing for heavier walls. Get it dead level and stable before you lay a single block.
Step 3: Lay the first course. This is the most important step in the whole build. Set your string line, lay the first row of blocks, and check the level constantly in every direction. If the first course is true, the rest follows. If it's off, every course above magnifies the error.
Step 4: Install reinforcement and core fill. For besser block walls, stand steel reinforcement in the cores and fill them with concrete as specified by your engineer. This is what turns a stack of blocks into a structural wall.
Step 5: Continue laying courses. Stack the following courses, staggering the joints, and keep checking level and alignment as you go. Don't build too high in one day if you're using mortar or wet core fill, since it needs time to set.
Step 6: Install the drainage system. This is where most failed walls go wrong. Lay ag pipe along the base behind the wall, cover it with gravel, and wrap or cap it with geofabric so soil can't clog the drainage. Water building up behind a wall is the single most common cause of failure.
Step 7: Backfill behind the wall. Fill behind the wall with free-draining material, compacting in layers as you go rather than all at once. This supports the wall without trapping water against it.
Step 8: Install capping. Finish with capping blocks or caps to suit your system. Capping protects the top of the wall from water and gives it a clean, finished look.
Common Mistakes to Avoid
A concrete block retaining wall fails for predictable reasons. Watch out for:
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Poor or missing drainage is the most common cause of wall failure
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Not compacting the base properly, which leads to settling and bulging
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Skipping reinforcement or core fill on walls that need it
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Ignoring council requirements and engineering
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Choosing the wrong block or wall type for the height
How Much Does a Block Retaining Wall Cost?
Cost varies a lot, so it's hard to give a single figure. The main drivers are the height and length of the wall, the block system you choose, the amount of reinforcement and drainage involved, and labour.
A low interlocking garden wall you build yourself sits at the cheaper end, while a tall engineered wall built by a contractor costs considerably more once design, materials, and labour are added up. Doing the build yourself saves on labour, but only take it on if the wall is low and simple enough to do safely.
When to DIY vs. Hire a Professional
Be honest about the job in front of you. DIY makes sense for low walls well under a metre, simple straight layouts, and interlocking systems built on stable ground. Bring in a professional when the wall is over about a metre, when it's structural or engineered, when the site is steep or unstable, or when there's a load such as a driveway or building above it.
A retaining wall that fails can be dangerous and expensive to fix, so there's no shame in handing the bigger jobs to someone qualified.
Shop Retaining Wall Blocks at BBPO
Once you know how the build works, getting the right materials is the easy part. BricksBlocksPaversOnline carries a full retaining wall range with delivery across the country, including interlocking and purpose-made systems from Adbri, National Masonry, Austral, and QPro to suit everything from a low garden wall to an engineered build.
For core-filled walls, you'll also find the grey concrete blocks you need. Whether you're a DIYer planning a weekend project or a builder ordering for a job, it's an easy way to get quality retaining wall blocks delivered to your site