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How to cap gap between retaining wall & fence?

Hi,

I’m looking for advice on how to cap the gap between my retaining wall and the surrounding fences. The retaining wall is U-shaped, with an old timber fence on one side, a new timber fence on the other, and a Colorbond fence at the back.

Side sections are about 3 m long
Back section is about 16 m
Depth behind the retaining wall varies from 400–900 mm
The gap between the wall and the fences ranges from 200–300 mm

I don’t want to fill or compact the whole depth — I just want to cap the top of the gap so it looks neat and prevents debris, leaves, or pests from getting in.

What is the best way to cap this long, uneven gap? (e.g., timber sleepers laid across, metal flashing). Thank You!

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Comments

EricL

Hello @ehsan_rd

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about the gap along your fence.

I propose building a medium weight timber frame along the length of the gap and covering the surface of the frame with Merbau decking panel. It is light weight, long lasting and can be cut to the correct size so that it covers the gap between the retaining wall and the fence. The timber frame can be anchored to the fence post and can act as a secondary rail for the fence. When you get to the Colorbond side the frame can be drilled into the retaining wall using concrete screws.

Let me call on our experienced members @Dave-1, @AlanM52 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

Nailbag

Hi @ehsan_rd

Thats a great idea already from @EricL worth considering and pricing up. Another possible solution which I have done previously was I had colourbond flashing made up. Manageable short lengths of 2400mm (so it doesn't twist) 50mm x gap distance to fence but on a slight 5+deg angle to allow water to flow off. Then cut out around the posts with tin snips.

We did install an agg. drain system though, previous to building the wall. Have you got any drainage sorted as there is the potential for water to pool between the fence and retaining wall?

Nailbag

Screenshot 2025-11-14 at 6.44.14 am.png

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/59387#Comment_59387

Good Morning @ehsan_rd

I was thinking along the lines of what @EricL has mentioned. An extra rail 70 *35mm H3 timber at the same height as the retaining wall on the timber fence, then screwed to this 140mm Merbau 140 x 19mm SpecRite Merbau Pre-Oiled Decking - Random Lengths.

For the colourbond, same rail same capping. Just screwed to the metal posts instead.

It should look like an lying down L shape, the gap will be 140mm + 35mm for the rail. So it should almost cover the distance.

Dave

ehsan_rd

Thank you all. very helpful and much appreciated.

@Nailbag
We installed an agg drain on the house side, not along the fence. But since the house site is higher, there should already be another drainage system behind the retaining wall. Back neighbours have retaining walls on their side on lower surface.

I think I understand what @EricL and @Dave-1 are suggesting, but could @Dave-1 please provide a sketch to show exactly how the “L-shape” looks, and where I would screw the 70×35 mm H3 rail to the 140 mm Merbau?

And one last question for @EricL: when you mentioned a light timber frame, were you referring to installing a 70×35 mm rail on the fence, similar to what @Dave-1 suggested?

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/59395#Comment_59395

Good Morning @ehsan_rd

Pink is the 140mm by 19mm Merbau

Green is 35*70mm H3 but I recomend going larger to 45*70mm H3 due to leverage when people stand on it to look over the fence or to lean down.

Install the green rail first

Then the pink "cap" goes on top. part of it overhangs the rail.

Same for the colourbond fence. If you think there is a likelyhood of children stepping on the rail/plank I would go for even larger sized rail. (90 * 50mm)

01 14-11-2025.jpg

Dave

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/59395#Comment_59395

Hi @ehsan_rd,

I believe @EricL would have been thinking of something very similar to what @Dave-1 has just put up.

The only thing I would add is a second rail on the retaining wall side so that the decking board is supported on both edges.

Let us know if you have any further questions.

Jacob

ehsan_rd

https://www.workshop.bunnings.com.au/discussion/comment/92109#Comment_92109

Thanks both @JacobZ and @Dave-1. Very clear.

Just one thing I’m a bit worried about: after the backfilling, some of the concrete sleepers developed cracks (see attached). I know there are two steel reinforcing bars inside them, but is it still okay to add a second rail on the retaining-wall side?
I should also mention that the cracks are mostly on the lower sleepers, and also not all of them.

My other question is whether I need to install a weed mat along the gap to prevent grass from growing through?

Ehsan

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/112093#Comment_112093

Hi @ehsan_rd,

I would be a little concerned about those cracks. As you said, there will be reinforcing in the sleepers, but multiple cracks running the full width of the sleeper are still not good.

Did you have a contractor install the retaining wall? Have you mentioned the cracks to that contractor?

I would be asking them to come back and replace the damaged sleepers, as they aren't in the condition you should reasonably expect.

What are the retaining wall posts like?

You might be able to screw the second rail to them, or even just raise the fence post rail so that the decking board sits on top of the retaining wall.

For the weeds, it would definitely be worthwhile putting down some weedmat and a layer of drainage gravel to suppress the weed growth.

Let me know if you have any further questions.

Jacob

ehsan_rd

https://www.workshop.bunnings.com.au/discussion/comment/123550#Comment_123550

Hi @JacobZ,

Thanks again much appreciated.

They came back and inspected the sleepers. They said "the sleepers were pushed hard during backfilling, but they are not going to break from that. They also mentioned that if any sleeper ever fails to retain the soil, they will come back and fix the affected area."

The posts are fine, they were dug out with an excavator and filled using a concrete pumping machine.

Sorry for asking so many questions; hopefully this is the last one.
Is it possible to install premium pine posts at the height of the retaining wall and then screw the decking boards onto them instead of screwing the second rail to sleepers?

Thank You.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/131075#Comment_131075

Hi @ehsan_rd,

Honestly, installing separate posts would be the best option as it would mean you don't have to drill into the sleepers in any way. If you are happy to put in the work, then I would definitely encourage this solution.

You could install posts at the midpoint of the sleepers, as there will be no concrete there to deal with, then run the rails between those posts.

Please don't hesitate to ask questions; that is why we are here.

Let me know if there is anything else I can assist with.

Jacob

ehsan_rd

https://www.workshop.bunnings.com.au/discussion/comment/135700#Comment_135700

Hi @JacobZ,

Can you suggest what type of posts can I purchase from Bunnings for this job?

And, we still need to add rail to support decking board on both edges. Am I right?

Thank You.

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/135700#Comment_135700

Good Afternoon @ehsan_rd

Instead of the extra posts, how about a short timber piece that sits on top of the concrete sleeper and then screwed to the new timber rail. Then the "capping" 140mm merbau can get screwed to that?

And there is "never" too many questions 🙂 We all ask questions and its the only way to learn 🙂

Dave

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/138844#Comment_138844

Hi @ehsan_rd,

You can use anything that is H4-treated. These 90 x 90mm 2.4m Post F7 H4 Treated Pine Premium KD would work.

Yes, you would still need a rail on both edges unless you simply sit the edge on top of the retaining wall side and don't screw it down.

Jacob

ehsan_rd

https://www.workshop.bunnings.com.au/discussion/comment/138845#Comment_138845

Hi @Dave-1,

May I ask you to draw sketch? I'm not sure if I fully understand. Appreciated.

Thank You.

Ehsan

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/140952#Comment_140952

Good afternoon @ehsan_rd

Give me a sec, will do when I finish eating my lunch 🙂

Dave

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/142514#Comment_142514

Good Afternoon @ehsan_rd

So the Dark blue crossbeams are the extra bits I was refering to. One end rests on the concrete sleeper (its not going anywhere) and the other is screwed to the new rail you still need to install. You will be able to cover the whole gap this way as a bonus. Using 2 of 140mm Merbau it should fit easily across the width of the gap.

You could also paint the underside of the short crossbeams with a tar substance to help keep them in resonable condition.

02 14-11-2025.jpg

Would love to see a project created if you go the route of these timber caps. There are a lot of instance that people have similar to yours and it would help future questions.

Dave

ehsan_rd

https://www.workshop.bunnings.com.au/discussion/comment/143622#Comment_143622

Hi @Dave-1,

I like this idea! I’ll probably need to screw on another thin piece of timber to fill the small gap between the Merbau and the concrete sleeper on the grass side. Once I finish the project, I’ll take a photo and post it here.

Thank You.

Ehsan

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