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How to build a retaining wall under Colorbond fence?

Hi there,

I'm looking to build a small retaining wall under a section of existing 1800mm Colorbond fence - required after the earth was levelled for a new build. The section of retaining wall will be approx 19 metres in length (8 fence panels) and will rise on a slope from ground level to a max height of approx 550mm. I am thinking of using a composite sleeper solution, but am concerned that using composite fence posts in concrete piers with brackets to attach the fence might not be adequate to support the fence on top.

Wondering if anyone hads done similar and if it was successful?

Many thanks

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Comments

JacobZ

Hi @Kev72,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Is it possible to get some photos of the space so we can better understand what you are working with?

Unfortunately, as this kind of installation would be non-standard, it would not be advised without professional assistance. Any lateral loads applied to the fence would be transferred into the retaining wall, through the sleepers, into its posts and down into the ground. The loads applied to this wall would not be standard and would not fit within the standard installation methods for either the fence or the composite retaining wall.

There may also be implications in terms of council approval, as there can be rules about what structures can be built in close proximity to your boundary. It would be worthwhile checking the rules around setback in your local council area.

If instead, you offset the fence so that it is around 600mm behind the wall, you could safely install the fence using the recommended concrete piers for both the fence and the retaining wall. This would make it much simpler to install yourself.

The FITTA FRP Essential Sleepers, when combined with the FITTA FRP Essential Posts, are engineered for walls up to 800mm. You just need to ensure the concrete piers are 300mm in diameter and they are 300mm deeper than your wall. So, at a height of 550mm above ground, you would need the piers to be 850mm deep and 300mm in diameter.

I'm happy to work towards a solution, but having a fence directly in line with your retaining wall adds complexity that would require professional assistance.

Allow me to tag our experienced members @Nailbag, @Dave-1 and @AlanM52 to see what they think

Let me know what you think and if you have comments or questions, please don't hesitate to ask.

Jacob

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/55550#Comment_55550

Good morning @Kev72

I really like the 600mm setback from the 1.8m fence so as to not impact the fences supports. Otherwise as he as explained you end up in a world of pain and having to get it sorted as the fence is half yours and half your neighbours responsibility. If it falls over due to your undercutting it then the fence will probarly be more your 100% at fault issue. The setback will stop that happening.

As to the type of retaining wall, I like Gabion styled walls as they dont need a footing other then compacted earth. I would like to see some photos of the area before pushing the idea too much is all as the width of a Gabion wall would be 4usually 2/3rs of the height. Being next to the fence I would say almost 1-1 with the height and width.

Dave

Kev72

https://www.workshop.bunnings.com.au/discussion/comment/90032#Comment_90032

Thanks to you both for your replies. Unfortunately, the fence sits on the boundary line, and there is not enough space between the boundary and the property wall (1300mm) to set the fence behind the wall. the fence does really have to sit on the retaining wall. Attach are a couple of images - one showing the existing fence, illustrating the need to build a small retaiuning wall under it - and a plan of the proposed retaingimng wall with fence on top. This also includes a detail of the proposed solution - ie. a bracket which attaches the retaining wall H-Channel to the colorbond channel post (although this illustration has a wooden post and a C-Channel end-post)

I have seen a few composite sleeper solutions which propose this method - SuperSleeper and TuffPoly both suggest this as an option in their installation and engineering drawings. IMG_2204 (1).jpg

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/110857#Comment_110857

Afternoon @Kev72

Thank you for the photos, I can picture it now.

Are you going to replace the fence so the new fence will have the retaining segmants built in?

Is the fence going to be offset a little so the new retaining wall will be seperate but attached to keep the strength?

The long span between the retaining wall sections 1926mm I would halve or change to 1200 at the most. Long sections tend to bow with time.

If you line up the retaining wall posts with the fence posts and the retaining wall posts are at least 600mm deep then as a guess I would be happy and yeah I say guess as the strength that is left atm is not particuly strong, adding a post that goes deeper next to the fence post I can understand, just not 100% with the support issue 🙂

As a side question. What would it cost to redo the fence with the added depth so you can have the retaining wall as part of the fence and would your neighbour be up to you footing the costs? (yes I see the pun in footing 🙂 ) If it comes close then it may be worth it to have a solid retaining wall and fence?

Dave

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/110857#Comment_110857

Hi @Kev72,

Unfortunately, I can't see that our composite sleeper suppliers have a similar solution.

If you were to use the Ridgi Concrete Retaining Wall system, there are these Ridgi 50 x 3mm 0.58m Galvanised Steel Fence Brackets that can be used to attach your Colorbond fence.

There is more information on this product here - Galvanised Grey Ridgi 580x50x3 Bracket.

Ultimately, whatever you do, the fence would need to be taken down and then reinstalled. Would this be part of the plan?

Let me know what you think.

Jacob

Kev72

https://www.workshop.bunnings.com.au/discussion/comment/122810#Comment_122810

Hi Jacob,

Many thanks for your reply - and yes, absolutely the plan is to first take down the sections of the fence where the retaining wall is to be installed first, then build the retaining wall, and use something like the Ridgi bracket to attach the retaining wall posts to the fence posts before reinstalling the panels - exactly as per the Ridgi diagram (attached). My only concern was that the colorbond channel fence posts would not be concreted in place, rather they would be attached to the retaining wall H-Channel posts (which would themselves be embedded in concrete piers) using the brackets - and I wondered idf this would be strong enough to support the fence

Screen Shot 2025-08-05 at 3.36.03 PM.png

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/130610#Comment_130610

Hi @Kev72,

I've just noticed that it mentions this particular system can only be used for fences no more than 1.2m in height.

The Pioneer 100 x 6mm 0.58m Galvanised Steel Fence Bracket, which is much the same thing just made from thicker steel, can be used with fences up to 2.0m in height. This would be worth a look.

There's some more information here - Pioneer Sleepers and Pioneer Panels Installation & Technical Guide.

I guess in this situation, you just have to trust that the engineering has been carried out by the companies producing the products. With a large company like Austral Masonry, its brand is well-known and can be trusted. As long as you follow the installation instructions and their recommendations, you will be covered by their warranty.

Let me know what you think.

Jacob

Kev72

https://www.workshop.bunnings.com.au/discussion/comment/135424#Comment_135424

Thanks Jacob - good call on the 1.2m max - but the Pioneer 100 x 6mm 0.58m bracket should do the trick !

Many thanks for your help on this

Cheers,

Kevin.

Nailbag

Hi @Kev72

Unless you are prepared to remove the entire existing fence and have it reinstated all at your expense as @JacobZ suggests, I would build a small retaining wall say 300-450mm forward of the fence and make it independent. This could feature a raised garden bed.

Nailbag

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