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How to build a shed foundation?

The Situation:

I have a pool equipment shed that is roughly 1500mm x 1500mm x 1500mm in dimensions. I’m hoping to replace it with an absconding shed from Bunnings.

However, the shed requires a concrete slab as a foundation. There are existing pole equipment fixed to the ground, and pipes and wires run underneath. I’m hoping to find an alternative way to build a foundation for this shed.

The ask:

I’m hoping the solution is DIY achievable and not too expensive. I’d also like it to provide good drainage for stormwater, as the shed is located on the lower side of my property. The shed will be accessible and potentially store pool equipment and garden tools as well.

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EricL

Hello @Vincero1

You can still technically build a concrete slab around the pump system and leaving a section open to allow the wires and pipes to be accessed from underground. It would probably depend on how large the new shed will be. If you were to get a 3m x 3m shed for example, part of the slab could be kept open and still allow the shed to be anchored to the slab and keep it secure.

But if you are curious how a timber base could be built, I suggest looking at this discussion - How to build a timber shed base? by @hjt1701. Please keep in mind though that a concrete slab is the preferred base for the shed.

If you need further assistance, please let us know.

Eric

Vincero1

Hi @EricL,

Thanks for the advice.

For the section to keep open, I assume it will contain pipes entering the ground and also open the part where the pipes and wires exit the shed? Any advice on how to cover these areas? Should I cover with gravel or leave it as topsoil?

I’m building a 3m x 1.5m shed, and if I’m doing it myself, this is the first time I’ve poured a concrete slab. Is there a guide you have handy that can walk me through the process? I only have a 100L wheelbarrow, so maybe I need to pour the slab section by section?

EricL

Hello @Vincero1

I suggest having a look at this guide - How to pour a concrete slab by @tom_builds. Covering the open section with gravel will give it a much better look rather than just plain soil. I also suggest creating square concrete footings in the corners to provide an anchoring point for the corners of the shed near the pump section.

If you need further assistance, please let us know.

Eric

Vincero1

Hi @EricL,

Thanks for the guide. I noticed that the foundation preparation in the guide involves excavating ground soil and levelling it, then pouring concrete directly without laying gravel or sand.

Does this mean a gravel base layer isn’t necessary for a concrete slab? The shed will be at the back of my property, and there’s a slight dip that allows stormwater to run to the bushland at the back. Any advice on drainage? Or is this a small project and doesn’t require drainage?

EricL

Hello @Vincero1

Because the slab sample was small and the base was dug in, it was not necessary to put in a gravel base layer. However, for a larger base, I recommend putting in a gravel base and crusher dust and compacting it flat. This will help the concrete base adjust to any movement in the soil, I suggest installing an agi-pipe drain where you think it will be needed to divert any incoming water going towards your shed.

Here is a handy step-by-step guide: How to install garden drainage by @Adam_W.

If you need further assistance, please let us know.

Eric

Vincero1

Hi @EricL,

I got the specs from the shed and understand the dimensions for the concrete slab. However, I’m a bit unsure about the gravel-based foundation.

My current understanding is:

1. Dig in and make a solid, level ground.

2. Maybe weed mat? Or maybe not necessary.

3. Gravel layer.

4. Crush dust/coarse sand to level.

5. Building plastic sheet.

6. Timber frame for concrete pour.

7. Pour concrete.

Could you please let me know the size and type of gravel I should use, how thick I need to lay the gravel and crushed dust, and whether a weed mat is required? The shed manual asked for a 100mm thick concrete slab, with 75mm WIDE and 25 mm rebate.

For the rebate, do I just screw some timber at the edge of frame to make a rebate?

EricL

Hello @Vincero1

You're right on track with your plan for the slab, and it’s great that you’ve read the shed specs carefully. For an Absco 3m x 1.5m shed with a 100mm thick slab and a 75mm wide x 25mm deep rebate, here’s a clearer breakdown of your gravel base, layering sequence, and how to make the rebate properly.

Please note that you'll need to create formwork to surround the pump area so that it does get included in the concrete pour. My best advice is to make sure that it is small enough that you can still get access to the necessary pipework.

Excavation and Base Prep

Dig down around 150mm–200mm below your finished slab level to allow for your base layers. Remove any loose soil and organic matter, then compact the ground thoroughly, this is key for slab stability.
Weed Mat – Optional

A weed mat isn’t necessary under concrete, but if you're working on highly weedy soil or soft clay, some people use geotextile fabric to help prevent base contamination and improve drainage. It’s optional but not essential.
Gravel Layer (Road Base)

Use 20mm crushed rock or road base, which you can get from Bunnings. Go and see the team at the special orders desk when ordering in bulk. This acts as the main foundation. Lay it around 75mm–100mm thick, then compact it firmly using a plate compactor. If the soil underneath is soft, aim closer to 100mm.
Crusher Dust / Coarse Bedding Sand

On top of the gravel, add a 20mm–25mm thick layer of crusher dust or coarse bedding sand. This helps you get the final level right before laying down your moisture barrier and formwork. Screed it level and give it a light tamp to firm it up.
Plastic Sheeting

Use 200 micron builder’s plastic, available at Bunnings, as a moisture barrier to prevent rising damp. Overlap the sheets by 200mm and tape the joins with cloth tape or builder’s tape. Let it run up inside your formwork edges slightly.
Timber Formwork and Rebate

For your 100mm slab height, use 90x45mm H3-treated pine for the formwork. To make the 25mm deep x 75mm wide rebate, screw a 25x75mm timber strip flat along the inside top edge of the formwork where the shed wall panels will sit. Remove this rebate strip once the concrete starts to set, before it hardens fully.
Pouring and Finishing Concrete

Use a general-purpose concrete mix like Australian Builders Concrete Mix 20kg or rent a cement mixer if you're planning on mixing your own concrete. Make sure to:

Pour on a dry day.
Use a screed to level it off.
Use a float or trowel for a smooth finish.
Let the slab cure for at least 24–48 hours before removing the formwork, and 7 days minimum before placing the shed (full cure takes 28 days).
To prevent rain damage, cover the slab with builder's plastic so that the concrete does not get soaked in rainwater.

Extra Tips:

Ensure your slab extends at least 100mm beyond the shed footprint on all sides (so frame timber should be 3.2m x 1.7m).
Slightly slope the slab away from the center to help water runoff (not the rebate area, just the outer edges if desired).
Reinforcing mesh is highly recommended for strength. Sit it on bar chairs so it stays suspended in the slab during the pour.

If you need further assistance, please let us know.

Eric

Vincero1

Hi @EricL,

Thank you for the detailed guide. I really appreciate your time on my first home project.

I have a couple of follow-up questions. Firstly, I have some pines available, but I’m not sure if they’re treated. Can I use them for formwork? If so, do I need to paint some oil on top of them before the concrete pour to make it easier to remove?

Secondly, I got some shelving units from a garage sale and some metal mesh available. Could I use these for the reinforcing mesh? Lastly, for the bar chairs, would bricks or concrete pavers be suitable?

JacobZ

Hi @Vincero1,

You will be fine using the timbers for formwork. It is not really that important for them to be treated as they will likely be damaged in the process and won't really be usable afterwards. Oiling your formwork is also a good idea to help release it once the concrete has set.

While the metal mesh from your shelving units could offer some reinforcement, it would be better if you used reinforcing mesh as it was designed for this purpose.

I would not advise using bricks or pavers as bar chairs, as being masonry, they will allow water to rise through them and come into contact with the reinforcing mesh, which can cause rust and premature weakening of the slab. It is best to just use reinforcing bar chairs.

Let me know if you have any further questions.

Jacob

Vincero1

Hi @JacobZ, @EricL.

It took me a while to remove all three tree stumps in the area, and I started to understand more about the underlying pipe. There are four pipes going underground, plus electrical wire, and they seem to be heading in different directions. Since the location of the pipes is at the corner of an existing 1.5m x 1.5m pool pump box, there’s not much room for me to put in formwork and pour concrete.

If that’s the case, should I use concrete foundation blocks from Bunnings for the two corner areas that will be in the existing pool pump box? This would mean I’d have a 1.5m x 1.5m open area with two foundation blocks on one side filled with gravel, and a 1.5m x 1.5m concrete slab on the other side. The 3m*1.5m shed would then sit on the concrete slab, and the two foundation blocks.

Would that work? Here is the illustration below.

For the existing pump area, I’m thinking of using 100mm 10-20mm drainage gravel (https://www.bunnings.com.au/ki-carma-20kg-10-20mm-drainage-gravel_p3460052). I’d dig around the existing pipe and equipment, then use a hand tamper to compact as much as possible before filling with gravel. Do I need Geofabric, weed matt, or building plastic?

I’m also wondering what the ideal level for the shed should be. Since it’s between the boundary of my property and the pool, I assume it should be slightly lower than the paver around the pool, but still the same level as the rest of the land.

Screenshot 2025-07-06 at 10.39.02 am.png

JacobZ

Hi @Vincero1,

As long as the shed is bolted to the concrete slab and the foundation blocks, and the crushed rock is compacted to support the bottom of the frame at the same height as the slab, I think this is a great solution.

I can't see any issue with your plan.

Jacob

Vincero1

Hi @JacobZ

Do I need anything other than gravel for the existing pump section? Like weed mat layer? How thick the gravel needs to be?

JacobZ

Hi @Vincero1,

With how thick the slab is, you would need to create a border around the outside of the shed to keep the stone in place. This could be done with timber sleepers and sleeper pegs, or something like garden edging with stakes.

It would be worth adding a layer of geotextile fabric or weedmat beneath the gravel. This will help suppress weeds growing through while still allowing any water that gets into the gravel to drain freely.

I'd aim for the gravel layer to be as thick as the concrete slab. Around 100mm at a minimum.

Let me know if you have any further questions.

Jacob

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