How to build a shed foundation?

[Vincero1]

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.

[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. 

1. 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.
    
2. 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.
    
3. 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.
    
4. 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.
    
5. 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.
    
6. 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.
    
7. 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