A solid shed starts with a strong foundation. Whether you’re installing a garden workshop, storage shed or outdoor office, a concrete shed base is one of the most reliable and durable solutions available.
Concrete provides excellent strength, stability and long-term resistance to weather conditions. When installed correctly, it creates a perfectly level platform that protects your shed from ground movement, moisture and structural damage.
In this guide, our team at QMix Concrete explains how to lay a concrete shed base step by step, covering everything from measuring the area to choosing the right concrete mix and allowing the base to cure properly.
Why Concrete Is the Best Base for a Shed
There are several options for shed foundations, including paving slabs or timber frames. However, a concrete base for a shed remains the most dependable solution for long-term performance.
A properly installed concrete base offers:
- Exceptional strength and load-bearing capacity
- Long lifespan with minimal maintenance
- Resistance to moisture and ground movement
- A perfectly level surface for the structure
For larger garden buildings or heavier sheds, concrete provides the stability needed to keep the structure secure for years.
Step-by-Step Guide: How to Lay a Concrete Shed Base
Below is a simple seven-step process to help you build a strong and level shed foundation.
1. Measure and Mark the Area
Start by deciding exactly where the shed will be positioned.
When marking the base area, remember that the concrete slab should be slightly larger than the shed itself. This ensures the structure is fully supported.
A good rule is to:
- Add 100mm to both the width and length of the shed
- Allow an additional 75mm for the timber formwork
Place wooden pegs at each corner and run string lines between them to outline the base. Measure diagonally between opposite corners to ensure the layout is perfectly square.
Taking the time to measure accurately will make the entire installation process much easier.
2. Excavate the Base Area
Next, dig out the marked area to a depth of approximately 150mm.
This depth provides enough room for both the sub-base layer and the concrete slab itself.
While excavating, remove all grass, roots, stones and loose material. Once the digging is complete, compact the soil at the bottom of the excavation using a tamper or roller.
A firm and level ground surface is essential for preventing future movement in the slab.
3. Install a Sub-Base
A sub-base layer prevents the concrete from sitting directly on soil and helps distribute weight evenly.
The most commonly used material is MOT Type 1 hardcore, which is widely used in construction for its strength and drainage properties.
Spread the hardcore across the base and compact it thoroughly. After compacting, add a thin layer of sand over the top to prevent sharp stones from puncturing the damp-proof membrane.
A well-compacted sub-base provides the stability needed for a long-lasting concrete shed base.
4. Build the Timber Formwork
Formwork is a timber frame that holds the wet concrete in position while it sets.
Use treated timber boards to build a frame that matches the size of the slab. Boards around 150mm x 25mm are typically used.
Position the frame within the excavation and secure it using wooden stakes placed roughly every metre around the perimeter.
Before moving forward, check that the entire frame is level using a spirit level. Any inaccuracies here will affect the final concrete surface.
5. Add a Damp-Proof Membrane
A damp-proof membrane (DPM) helps prevent moisture from rising through the concrete slab.
Lay heavy-duty plastic sheeting across the entire sub-base. The membrane should extend slightly up the inside edges of the formwork to create a tray-like barrier.
If multiple sheets are required, overlap them and tape the joints securely to maintain a continuous moisture barrier.
This step significantly improves the durability of the concrete base.
6. Pour and Level the Concrete
Once preparation is complete, you can begin pouring the concrete into the formwork.
Spread the concrete evenly across the base using a shovel or rake. Make sure the entire area is filled before leveling.
Next, use a straight piece of timber or a tamping beam to level the surface. This helps remove trapped air pockets and ensures the slab is smooth and flat.
Proper levelling is essential so the shed sits evenly on the finished base.
7. Allow the Concrete to Cure Properly
Concrete requires time to cure in order to reach full strength.
After finishing the surface, cover the slab with a raised polythene sheet to reduce moisture loss during the curing process.
Typical curing times are:
- Walkable after 24–48 hours
- Formwork removable after 48 hours
- Fully cured after around 28 days
Avoid placing the shed on the base until the concrete has fully cured.
What Concrete Mix Is Best for a Shed Base?
Choosing the right concrete mix is important for strength and durability.
For most domestic shed bases in the UK, the following grades are commonly used:
C20 / GEN3 / ST1
A standard concrete mix suitable for most garden sheds and outdoor buildings.
C30 / ST3
A stronger mix recommended for larger or heavier structures.
Both mixes provide the strength required while remaining workable for small construction projects.
If you’re unsure which mix is best, a local concrete supplier can recommend the right option based on your project requirements.
Ready Mix Concrete for Shed Bases in Kent & London
Ordering ready mix concrete is often the easiest way to ensure the correct mix and consistent quality for your shed base.
At QMix Concrete, we supply ready mix concrete to homeowners, contractors and builders across:
- Dartford
- Gravesend
- Sevenoaks
- Bromley
- Croydon
- South East London
Our team ensures the right mix is delivered directly to your site, helping you complete projects efficiently and with professional results.
Need Concrete for Your Shed Base?
Building a concrete shed base is a straightforward project when the groundwork is prepared properly and the correct materials are used.
If you’re planning a garden building or outdoor project, QMix Concrete can supply high-quality ready mix concrete delivered directly to your location across Kent and surrounding London areas.
Our experienced team can also help you choose the right concrete mix to ensure your shed foundation remains strong and durable for years to come.
FAQs
How thick should a concrete shed base be?
A typical concrete shed base thickness is around 75mm to 100mm, depending on the size and weight of the shed.
Do I need a sub-base under a concrete shed base?
Yes. A compacted MOT Type 1 sub-base provides stability and prevents the concrete slab from shifting or cracking over time.
How long should concrete cure before installing a shed?
Concrete can usually be walked on after 24 to 48 hours, but it should cure for around 28 days before placing heavy structures.
What concrete mix should I use for a shed base?
Most shed bases use C20 (GEN3) concrete, which provides the right balance of strength and workability for domestic projects.