Hi everyone, I am looking for some feedback and validation on the best way to retain soil along our boundary line beneath an existing Colorbond fence. Current Situation * The Problem. The neighbour's ground level is higher than ours, creating an exposed drop of roughly 200 mm to 400 mm along a ~20-metre run. * Failing Temporary Fix. The soil is currently held back by a makeshift mix of loose bricks, timber offcuts, and reed screening that is collapsing. * Goal. Build a tidy, long-lasting retainer that manages drainage and stops soil washing through, without overloading the fence. Proposed Solution: Independent Dry-Stack Gravity Blocks Rather than attaching under-fence plinths or concrete sleepers directly to the existing Colorbond posts (which I know are not engineered to take lateral earth loads), I am leaning towards building an independent, low dry-stack block wall (using something like Brighton Masonry Eziwall or Hudson Stone) sitting just inside our boundary. My current plan: * Foundation. Dig a shallow 120–150 mm trench, pack 75–100 mm of road base (crushed aggregate with fines), and compact it thoroughly with a tamper. * Base Course. Bed the first row of blocks into the compacted base so about half of the block is buried below ground level to lock against sliding. * Wall Height. Stack 3 to 4 courses (approx. 300–400 mm high), stepping down along the slope where needed. The top course will be glued with landscape construction adhesive (e.g. SikaBond Hardscape). * Clearance. Leave a 15–20 mm air gap between the top block and the Colorbond bottom rail so the metal does not contact damp masonry. * Drainage. Run a 65 mm socked slotted ag-pipe along the back of the base course, enveloped in 20 mm blue metal aggregate wrapped in geotextile fabric, discharging to our lower lawn area / stormwater pit. Questions for the Community * Base Stability. Is a compacted road-base trench sufficient for 3 to 4 courses (300–400 mm height) along this slope, or would you recommend a concrete pad footing? * Alternative Approaches. Has anyone used an alternative modular retaining method for a similar under-fence drop that worked better or was more cost-effective? * Drainage Tips. Any extra tips for keeping the backfill aggregate clean and preventing silt from washing down the slope into the ag-pipe over time? I would love to hear your thoughts, feedback, or any potential pitfalls before I order the materials! Cheers!

nav[aria-label="Primary Navigation"] { padding: 0; & ul { list-style: none; width: 100%; display: flex; flex-direction: row; justify-content: start; align-items: start; gap: 30px; padding: 0; & li { margin: 0; } & ul li { list-style: none; } } }

How to retain a gap under existing Colorbond fence?

AlbertoC

Hi everyone,

I am looking for some feedback and validation on the best way to retain soil along our boundary line beneath an existing Colorbond fence.

Current Situation

The Problem. The neighbour's ground level is higher than ours, creating an exposed drop of roughly 200 mm to 400 mm along a ~20-metre run.
Failing Temporary Fix. The soil is currently held back by a makeshift mix of loose bricks, timber offcuts, and reed screening that is collapsing.
Goal. Build a tidy, long-lasting retainer that manages drainage and stops soil washing through, without overloading the fence.

Proposed Solution: Independent Dry-Stack Gravity Blocks

Rather than attaching under-fence plinths or concrete sleepers directly to the existing Colorbond posts (which I know are not engineered to take lateral earth loads), I am leaning towards building an independent, low dry-stack block wall (using something like Brighton Masonry Eziwall or Hudson Stone) sitting just inside our boundary.

My current plan:

Foundation. Dig a shallow 120–150 mm trench, pack 75–100 mm of road base (crushed aggregate with fines), and compact it thoroughly with a tamper.
Base Course. Bed the first row of blocks into the compacted base so about half of the block is buried below ground level to lock against sliding.
Wall Height. Stack 3 to 4 courses (approx. 300–400 mm high), stepping down along the slope where needed. The top course will be glued with landscape construction adhesive (e.g. SikaBond Hardscape).
Clearance. Leave a 15–20 mm air gap between the top block and the Colorbond bottom rail so the metal does not contact damp masonry.
Drainage. Run a 65 mm socked slotted ag-pipe along the back of the base course, enveloped in 20 mm blue metal aggregate wrapped in geotextile fabric, discharging to our lower lawn area / stormwater pit.

Questions for the Community

Base Stability. Is a compacted road-base trench sufficient for 3 to 4 courses (300–400 mm height) along this slope, or would you recommend a concrete pad footing?
Alternative Approaches. Has anyone used an alternative modular retaining method for a similar under-fence drop that worked better or was more cost-effective?
Drainage Tips. Any extra tips for keeping the backfill aggregate clean and preventing silt from washing down the slope into the ag-pipe over time?

I would love to hear your thoughts, feedback, or any potential pitfalls before I order the materials!

Cheers!

Find more posts tagged with

Tools and Materials

Building and Construction

Landscaping

Fencing and Screening

Accepted answers

All comments

EricL

Hello @AlbertoC

Thank you for sharing your question about building a retaining wall in front of your fence.

Given the conditions shown in your photo, I would recommend considering a professional contractor for this project, or at least having an experienced retaining-wall contractor inspect the site before proceeding with a DIY installation. Although a retaining wall of approximately 300 to 400 mm is relatively modest in height, the complexity comes from its relationship with the existing Colorbond fence, the sloping ground, drainage requirements, restricted working space, and the need to keep the new retaining structure independent of the fence.

The main challenge is establishing the correct position for the new wall. Adequate space is required behind the blocks for drainage aggregate and an Agi pipe, yet moving the wall further away from the fence creates a larger gap and reduces usable space. Positioning it closer to the fence could interfere with the existing Colorbond post footings. Excavating the base could also expose or undermine those footings if their dimensions are unknown. These conditions are much easier for an experienced contractor to assess on site.

I would therefore obtain two or three quotations from contractors experienced with low segmental retaining walls. Rather than specifying exactly how the wall should be built, I would show them the existing situation and ask how they would retain the soil without transferring lateral loads to the Colorbond fence. Their proposals should address the fence-post footings, foundation, drainage, separation from the fence and appropriate discharge of the subsoil water. It would also be worth confirming that their chosen block system is rated for the required retaining height.

Professional involvement does not necessarily mean paying someone to complete the entire project. A practical compromise could be having a contractor undertake the excavation, establish the levels, install the drainage and construct the foundation and first course. These are the parts where accuracy is particularly important. Depending on the system selected, you could potentially complete the remaining blockwork and landscaping yourself.

The deteriorated timber, loose bricks and irregular ground visible beneath the fence are another reason an on-site assessment would be valuable. Once these materials are carefully removed, the available foundation width, underlying soil and size of the existing post footings should become clearer.

Overall, your independent gravity-wall concept remains a sensible approach. The main question is whether there is sufficient space to construct the foundation and drainage correctly without affecting the existing fence. A professional site assessment should resolve those uncertainties and give you confidence that the finished solution will be durable, properly drained and appropriate for the site.

Let me call on our experienced members @Dave-1, @Nailbag and @DIY4Knuckleheds for their recommendations.

If you have any other questions we can help with, please let us know.

Eric

AlbertoC

Thanks Eric, appreciate the cautious perspective and the valid point regarding the existing post concrete footings.

I plan to clear out the loose bricks, offcuts, and debris first to map out exactly where the concrete bulbs sit before digging the trench. If the footings are tight, I can offset the base course slightly to maintain clearance.

Keen to hear what @Dave-1, @Nailbag, and @DIY4Knuckleheds suggest regarding working around those post bulbs with dry-stack blocks!

Nailbag

Hi @AlbertoC

As the ground is significantly higher on your neighbours side, then thats where the work need to be started with the soil pulled away from the fence line and all the old retaining materials pulled out. Basically, the weight needs to be taken off the fence because you have already noted, it's not structurally engineered.

I would then consider an off-set retaining wall that would allow you to box out around each of the fence posts. Then pour in concrete thats tied down into the existing post footings to increase their strength being so far out of the ground. Once set, you can fill the retaining wall to the bottom of the fence line with soil and plant it out, avoiding trees with large root systems. The further out from the fence the wall is would allow for larger species.

Nailbag

Noyade

Just a casual observer @AlbertoC - but I take @Nailbag's view that your neighbour's retaining wall (was it ever a retaining wall?) has failed with pressure points now against the fence. To a layman that needs addressing first?

Have you spoken to them (neighbour)?

Dave-1

Good Morning @AlbertoC

Im putting my hand up for a low gabion styled wall :) It is weighted and allows free drainage through it. Plus the compacted base would work as that is all it needs. Please note that it can be a bit of a pain for stitching the gabions together to get an ultra straight line. Its doable but will take some effort at least at first.

I agree with the other members that the retaining section really should be started from the other side of the fence, Otherwise it will still want to push the fence out of true.

Gabion wall bench seats Is the size that I am thinking of for your wall. As a guide the base should be two thirds the height to prevent toppling and movement. If you look at my bookmarks there are a few more gabions to see how they can be put together,

Bunnings also sell Jack 80 x 40 x 40cm Rectangle Landscaper's Plant Support Garden Gabion Note that the mesh is thinner in width as it has shrunk over the years. Making them yourself is not hard and would look straighter with longer boxes.

Dave

AlbertoC

Thanks everyone for jumping in, really valuable food for thought here.

@Nailbag & @Noyade . Great point on the source of the pressure. The neighbour is very easy to get along with, so getting access to pull the soil back 300 to 400 mm from their side and clear out the failing makeshift rubbish will be straightforward. That will give us a clean, unweighted boundary to work with. Nailbag, boxing out around the post bulbs to pour an enlarged collar is a smart way to lock them in if they are sitting high out of the ground.

@Dave-1 . A low gabion setup is an interesting lateral idea, especially with zero hydrostatic buildup to worry about. My only hesitation is the footprint width along that run, but I will check out your bookmarks to see how tidy people have managed to get them along narrow boundaries.

I will arrange to clear the dirt and old timber back from both sides first so we can see the bare posts and ground line clearly.

Dave-1

Morning @AlbertoC

for the height you are after a 400mm by 400mm wall would work perfectly. All that is really needed is to scratch back the top soil to the hard under layer, if its loose compact it. I have clay so it works really well for me.

Dave

AlbertoC

@Dave-1 Thanks, do you have a picture?

Dave-1

Morning @AlbertoC

Picture… Whoops, sorry I had posted the tops of the cages for bench seats section. Here is the actual cage being built

Gabion wall for front pathway for constructing the cages for the benchseats.

and

Large gabion retaining wall Shows how to make the spirals :)

Dave