One of the most common questions we get from homeowners planning a new shed is how thick the concrete base actually needs to be. It sounds like a simple question, but over the years we've seen plenty of shed bases fail because the thickness wasn't matched to the building that was going on top.
The truth is there isn't one thickness that suits every shed. A small timber garden shed has very different requirements compared to a large workshop, log store or heavy-duty garden building. Getting the thickness right from the beginning helps prevent cracking, sinking and costly replacement work later on.
Across Barking we regularly install concrete foundations for sheds and in most situations the thickness recommendation depends on the shed size, the weight being supported and how well the ground underneath has been prepared.
For most typical garden sheds, a concrete base thickness of around 100mm (4 inches) is generally considered suitable when installed on properly compacted ground.
This is usually enough to support standard timber sheds, storage sheds, bike stores and many common garden buildings found throughout Barking.
However, thickness alone does not determine strength. A 100mm slab placed on poorly prepared ground can fail far sooner than a properly installed slab with good sub-base preparation underneath.
In our experience, homeowners often focus entirely on the concrete itself while overlooking the preparation beneath it. Most long-term problems actually start below the concrete rather than within the concrete.
Certain situations require additional concrete thickness to handle increased loading or more demanding site conditions.
For these types of structures, we commonly see bases ranging from 125mm to 150mm thick depending on the intended use.
This is particularly important where the building will contain heavy shelving, machinery, gym equipment or workshop tools that create concentrated loads in specific areas.
The biggest mistake isn't usually making the slab too thin.
It's skipping proper excavation and sub-base preparation.
Many DIY shed bases are poured directly onto topsoil or soft ground. The concrete may initially look solid, but after a few wet winters the slab can start moving because the material underneath compresses and settles.
This often causes:
This is exactly the type of issue we regularly encounter when replacing older shed bases around Barking.
Good preparation helps distribute weight evenly across the slab and reduces movement over time.
We've seen homeowners pour 150mm of concrete onto poor ground and still experience settlement issues. We've also seen well-prepared 100mm slabs remain perfectly stable years later.
This is why experienced installers spend so much time preparing the ground before the concrete wagon even arrives.
In some situations, yes.
Steel reinforcement mesh can improve strength and help reduce the likelihood of cracking caused by movement or loading.
For larger shed bases and heavier garden buildings, reinforcement is often recommended as part of the overall installation.
That doesn't necessarily mean the slab can be made dramatically thinner, but reinforcement can improve overall durability and long-term performance.
For very small lightweight sheds it may be possible, but in most cases 100mm provides a more reliable long-term solution.
Technically yes. Excessive concrete thickness increases cost without necessarily improving performance if the ground preparation underneath is poor.
If you have a shed that needs a structure to support it you can visit our main shed base page or contact our team for guidance on your project.
Shed Base Service Page