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How to build footing for raised concrete planter?

It is a 15m x 5m backyard where I want to erect a concrete planter made of limestone bricks with walls on all 4 sides. I want a long narrow planter of 15m x 0.5m x 1m. The base is sandy soil (class C), so please suggest how should I construct the footing as it is a DIY. Please also suggest what material should I use for the footing so that it doesn't crack due to expansion and contraction.

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EricL

Hello @sujoy_kb

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about footing for a raised concrete garden.

Can I please ask for a little bit more information in regards to the materials you'll be using for your project? What are the dimensions of the limestone bricks you are planning to use? How heavy are the limestone bricks (this is important when digging for the foundation). Please confirm the size of the planter 15m long x 0.5m wide and 1m tall are these measurements correct? Due to the size and weight of the planter it is almost like a concrete fence wall. Have you considered making it smaller down to 0.5m tall?

As soon as you provide us with more information, we'll be able to offer recommendations in regards to the foundation.

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

Eric

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/53829#Comment_53829

Hi @EricL

Thank you so much for your response.

I have planned to use limestone bricks 400 x 200 x 100 for the garden bed.
Yes, you are right about the height. I am happy to reduce the height of the planter to 0.5m and the width to 0.25m. So, the new dimensions are 15m (length) x 0.25m (width) x 0.5m (height). Please suggest how should I build a footing for it.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/89142#Comment_89142

Hello @sujoy_kb

To build a stable and durable footing for your revised garden planter measuring 15 metres long, 0.25 metres wide, and 0.5 metres high on Class C sandy soil, I suggest following this step-by-step approach that balances structural integrity with practicality for a DIY project. Since the soil is sandy and thus prone to shifting, it's crucial to establish a well-compacted base and use materials that can accommodate some degree of ground movement without cracking.

Start by marking out the trench for the footing along the entire 15-metre length and around 350 mm wide, this gives a little room on each side of your 250 mm wall. Excavate the trench to a depth of approximately 300 mm. This depth is suitable for a structure of your planter's size on Class C soil and ensures the footing is deep enough to resist minor ground movement. Compact the base of the trench firmly using a hand tamper or mechanical plate compactor to reduce the risk of settling.

Next, lay a 100 mm deep layer of crushed rock or road base at the bottom of the trench and compact it thoroughly. This forms a solid sub-base that helps with drainage and further stabilises the foundation. Then, construct a formwork along the edges of the trench using timber boards to shape the concrete pour. The formwork should match your footing’s final dimensions, typically 300 mm wide and 200 mm deep above the crushed rock, giving a total of about 300 mm depth when combined.

Use a reinforced concrete mix for the footing to prevent cracking and ensure long-term stability. You should incorporate steel reinforcement, specifically, a single N12 or N16 rebar along the length of the trench with small stirrups or corner bars every 600 mm to help resist any soil movement or expansion stresses. Pour standard concrete mix into the formwork, filling to the top, and use a shovel to agitate the mix and remove any air pockets. Level the surface with a screed board and trowel it smooth.

Let the concrete cure for at least 48 hours before removing the formwork, and wait at least a week before starting bricklaying to allow it to reach sufficient strength. For added protection against moisture and expansion, consider placing a damp-proof membrane (such as plastic sheeting) between the footing and the first course of bricks. Also, use a flexible, high-quality mortar mix that includes a plasticiser to allow some movement and reduce cracking over time.

This process ensures that your limestone brick planter has a strong and flexible foundation that can accommodate minor soil shifts, avoiding cracking and providing lasting support for your garden structure.

I also suggest having a look at this discussion - Solved: How to lay bricks for raised flower bed? by @Brunnie_Rich.

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

Eric

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/110349#Comment_110349

Hi @EricL,

Thank you so much for detailed instruction above. Really appreciate you help.

However, just a modification to the planter box - 15 metres long, 0.30 metres wide, and 0.90 metres high. Do you recommend the same footing instructions with the new dimensions of the planter box? If not, please suggest how to construct the footing for the same.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/122565#Comment_122565

Hello @sujoy_kb

Given the updated size of your planter box 15 metres long, 0.30 metres wide, and 0.90 metres high, I propose consulting a concrete or structural specialist, especially if this is your first time working with reinforced concrete or a structure of this scale. While the construction is still within DIY reach for someone with moderate experience, the height and length do introduce higher loads and stress points that make proper footing and reinforcement important to long-term stability.

A specialist can assess the specific conditions of your site, such as actual soil compaction, drainage patterns, and any nearby structures that might influence ground movement. They can confirm the appropriate depth, width, and reinforcement details for the footing, which is particularly important for sandy Class C soil that may behave unpredictably if not compacted or supported properly. Additionally, a specialist can advise on expansion joints, drainage considerations, and ensure your design complies with any local building codes or restrictions even if council approval isn’t technically required for a planter box.

Plus, engaging a professional for consultation can give you peace of mind that your footing won’t shift, crack, or cause problems over time. It’s a small investment that can prevent costly fixes later on.

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

Eric

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/130444#Comment_130444

Thanks @EricL for your guidance.

I have spent time after reading your previous response in analysing the right dimensions I am looking for.

I have modified my dimensions to below.

Length - 15m

Width - 0.30m

Height - 0.60m

I know that I have flipped the dimensions before and apologies for modifying these again. Could you please suggest the footing for the new dimensions. Thanks again.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/135337#Comment_135337

Hi @sujoy_kb,

With the very minor change in shape, nothing would really change from the instructions @EricL has given above.

You just need to ensure you build a strip footing that is 300mm wide along that perimeter.

You can follow the instructions Eric gave above, just modify them so they sit 50mm wider to accommodate the change in shape.

Let me know if you have any further questions.

Jacob

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/138601#Comment_138601

Thank you so much @JacobZ

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/138601#Comment_138601

Hi @JacobZ and @EricL ,

Thank you for your suggestions. I’ve finalised the planter box designs with the following dimensions:

Length: 4.3 m
Width: 0.3 m
Height: 0.7 m

There will be three such planters, each positioned 0.53 m apart from one another.

Would these dimensions require any changes to the foundation?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/140808#Comment_140808

Hi @sujoy_kb,

No, this would not change the fundamental design of your footing. Simply follow the same advice Eric gave above and adjust it to suit the shape of your garden beds.

Jacob

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/140808#Comment_140808

Hi @JacobZ @EricL ,

How do I take care of water drainage and adding a sprinklers through the foundation slab. I am aware of setting up an irrigation control, but I just need guidance of using it through the concrete planter slab.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/142411#Comment_142411

Hi @sujoy_kb,

In order to allow your planter to drain, you'll need to create ports at the rear. You can use these ports to route your irrigation pipework too. When creating your formwork for the foundations, you should use blocks of timber to prevent concrete from filling some areas, as per my rendering for you below. Once the formwork is removed, there will be small gaps for drainage and irrigation.

Please let me know if you have any questions.

Mitchell

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/143553#Comment_143553

Thanks so much @MitchellMc . I really can’t appreciate enough how helpful the above image is!

Just for my curiosity, do you think the below diagram with pipes through the concrete slab help in drainage or will it introduce slab instability?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/144418#Comment_144418

That would be fine, @sujoy_kb, and I do not think it would cause the slab to become unstable. However, the water still needs to flow somewhere freely. If it can't freely flow out, it will remain in the planter soil, which will become sodden. I don't think it's sufficient to have a pipe with the end blocked by the soil underneath the planter.

Mitchell

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/144418#Comment_144418

Thanks again @MitchellMc . I’m convinced to go ahead with your first suggestion of introducing gaps while pouring the slab. However, the rear side of the planter would face the fence. Does this mean the water coming out of the gaps will flood my neighbours on the other side of the fence?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/145047#Comment_145047

Flood is not the word I would use, @sujoy_kb. Water will slowly weep out of the back, most of which would likely evaporate rather quickly under warm temperatures. Ideally, no water will come out of the rear ports unless there is an excessive amount in the planter, like when it's pouring with rain.

You could move the ports to the sides or front if you wished. You'd then make a small one at the back to route your irrigation pipe through.

Mitchell

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/145544#Comment_145544

This totally makes sense. Thanks a lot @MitchellMc

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/110349#Comment_110349

Hi @EricL

Although I had initially planned to use 400 x 200x 100 limestone bricks, could you suggest other alternatives that match my project’s requirements? It’d be great if you could suggest a budget alternative.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/122870#Comment_122870

In terms of block options @sujoy_kb, there aren’t a huge number of budget alternatives to limestone that will still give a solid, durable footing for your raised planter. A coloured concrete block is usually a more affordable option than natural limestone and can still provide a neat, sturdy finish. It won’t have the same natural texture or variation as limestone, but it will perform well structurally and can be painted or finished if you want to customise the look.

Mitchell

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/130659#Comment_130659

Thanks @MitchellMc .

I’ll use stone clad on them so should be okay with concrete blocks. However, do you think the block dimensions are okay? 400x200x100 is what I’m planning to use.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/135444#Comment_135444

You could use Brighton Masonry 390 x 190 x 90mm Besser Blocks, and then the cladding will bring them close to your original size.

Mitchell

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/138678#Comment_138678

That’s a great suggestion as always @MitchellMc Thanks so much

sujoy_kb

https://www.workshop.bunnings.com.au/discussion/comment/138678#Comment_138678

@MitchellMc @Do I stack the bricks on each other by 390x90 side?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/140827#Comment_140827

Yes, the blocks will sit on their 90mm edge @sujoy_kb.

Mitchell

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