One of the most common questions we get asked before a shed base project starts is simply how thick the concrete actually needs to be. The honest answer changes significantly depending on your shed’s size and what you’re planning to store inside it, which is exactly why a single blanket figure doesn’t serve most homeowners well.
This guide pulls together our recommended thickness across every common shed size, all in one reference chart, so you can quickly find what applies to your specific project.
At JPD Landscaping & Building Services, we’ve built concrete shed bases across South East London for over 15 years, covering everything from small garden sheds to heavy workshop structures. This guide gives you a clear, size by size thickness reference, along with the mistakes that trip up thickness planning even when the right number is used.
Quick Answer: Concrete Shed Base Thickness by Size
| Shed Size | Recommended Thickness |
| Small (up to 6x4ft) | 100mm to 125mm |
| Medium (8x6ft to 10x8ft) | 100mm to 125mm |
| Large (12x10ft and up, general storage) | 150mm to 200mm |
| Workshop (heavy machinery or equipment) | Individual assessment, no fixed range |
Notice that small and medium sheds share the same recommended thickness range. This isn’t an oversight, it reflects that the jump in required thickness happens more at the point where a shed moves into larger, heavier territory, rather than scaling gradually with every size increase in between.
Why Small and Medium Sheds Share the Same Thickness Range
For sheds up to roughly 10x8ft, the overall weight and load distribution typically falls within a fairly similar range, meaning a 100mm to 125mm concrete base generally provides sufficient strength and stability for both categories. It’s only once you move into significantly larger structures that the additional weight and floor area genuinely start demanding a thicker slab to properly support the load.
This is useful to know if you’re planning a shed somewhere in this size range, since it means the concrete thickness question is fairly settled once you know you’re within the small to medium bracket, without needing to fine tune the figure further based on exactly where in that range your specific shed falls. For the full process of building a base at this thickness, including sub base preparation and formwork, our guide on how to build a shed base covers the complete step by step approach.
Why Large Sheds Need Noticeably More Thickness
Once you move into larger sheds, typically 12x10ft and upward for general storage purposes, the recommended thickness increases to 150mm to 200mm. This step up reflects the larger floor area distributing weight across a bigger surface, along with the generally heavier overall structure and contents that come with a bigger shed.
This range assumes general storage use rather than heavy machinery or workshop equipment. If you’re planning a large shed specifically for lighter storage purposes, this thickness range provides a solid, reliable foundation without over specifying beyond what’s actually needed.
If you’re specifically working to a 12x12ft footprint, our guide on 12×12 concrete slab cost breaks down real pricing at that exact size, which pairs directly with the thickness guidance here.
Why Workshop Sheds Don’t Get a Fixed Thickness Range
Workshop sheds, particularly those housing heavy machinery, equipment, or workbenches, genuinely need individual assessment rather than a fixed thickness range. This isn’t us avoiding giving a straight answer, it reflects a real difference in how these structures need to be evaluated compared with standard storage sheds.
The reason comes down to point loads. A workshop with heavy machinery doesn’t distribute weight evenly across the floor the way general storage does. Instead, specific points, such as machinery legs, workbench supports, or heavy equipment feet, concentrate significant weight onto small, specific areas of the base.
A generic thickness figure, even a generous one, doesn’t necessarily account for these concentrated load points properly, which is why we always recommend a proper site and equipment assessment for workshop projects rather than applying a standard range. Our guide on foundations for larger, heavier sheds goes into more depth on how depth, thickness, and reinforcement interact for this category of structure.
Understanding Point Loads vs Distributed Loads
It’s worth explaining the distinction between point loads and distributed loads a bit further, since this is genuinely the core reason workshop thickness can’t be reduced to a simple number. A distributed load, such as general stored boxes, garden equipment, or furniture spread across a shed floor, applies weight fairly evenly across the whole slab area, which a uniform thickness handles well.
A point load, by contrast, concentrates significant weight onto a very small contact area, such as the four legs of a table saw or the feet of a heavy workbench. Even a relatively modest total weight can create a surprisingly high pressure at these specific contact points, which is why a slab that’s perfectly adequate for general storage can still develop localised cracking under a workshop setup if those specific load points weren’t accounted for during planning.
How Ground Conditions Interact With Thickness Recommendations
While this guide focuses on thickness by shed size, it’s worth being clear that thickness and ground condition work together rather than in isolation. The thickness ranges given here assume reasonably firm, well prepared, and properly compacted ground beneath the slab.
On softer or less stable ground, it’s common to combine a thickness within the standard range for that shed size with additional reinforcement, or in some cases a slightly increased thickness, to compensate for the less supportive ground conditions.
This is exactly why a site visit remains valuable even when you already know roughly what thickness your shed size calls for, since the final specification often needs to reflect both the structure’s size and weight, and the specific characteristics of your garden’s ground.
Reinforcement Considerations Alongside Thickness
Thickness and reinforcement are related but separate decisions, and it’s worth understanding how they work together rather than assuming a thicker slab automatically means reinforcement is unnecessary, or vice versa. For small and medium general storage sheds on firm ground, neither additional thickness beyond the standard range nor reinforcement is typically necessary, since the load involved sits comfortably within what a standard unreinforced slab handles well.
For larger sheds, reinforcement becomes more of a case by case decision, often depending on ground conditions rather than shed size alone. For workshop sheds with genuine point loads, reinforcement, typically in the form of steel mesh positioned to address the specific load bearing areas, is frequently recommended alongside an individually assessed thickness, since this combination addresses both the overall structural demand and the specific concentrated pressure points a thickness figure alone can’t fully resolve.
Common Mistakes With Concrete Base Thickness
Based on the repair and assessment work we’ve handled, a few thickness related mistakes come up consistently, and they often overlap on the same project.
- Using one thickness across the whole base regardless of weight distribution. This works fine for general storage sheds where weight is spread fairly evenly, but it can fall short for workshop setups with concentrated point loads that need extra consideration at specific spots.
- Not accounting for point loads, such as heavy machinery legs or workbenches. Even a base with an appropriate average thickness can develop localised cracking or settling under a concentrated weight point that wasn’t specifically planned for.
- Assuming thickness alone solves poor ground conditions. A thicker slab helps, but it doesn’t fully compensate for genuinely poor, unstable, or badly compacted ground underneath. Proper ground preparation still matters regardless of how thick the concrete itself is poured.
- Applying a small shed thickness to a larger structure by assumption. Since the jump in required thickness happens at a specific size threshold rather than scaling gradually, assuming a slightly larger shed needs only a slightly thicker base can lead to genuine under specification.
- Skipping a proper assessment for planned future equipment. Building a general storage thickness base with vague plans to eventually add workshop equipment later often results in a base that isn’t actually suited to that eventual, heavier use.
In our experience, these mistakes tend to show up most often on workshop and larger shed projects, precisely because these are the situations where a generic, one size fits all approach to thickness is least likely to hold up.
Planning Ahead If You Might Add Equipment Later
If there’s any realistic chance you’ll want to use your shed for workshop purposes in the future, even if your immediate plan is general storage, it’s worth discussing this possibility during your initial base assessment rather than building to the lighter specification and hoping it copes later.
Retrofitting a thicker or reinforced base beneath an existing shed is considerably more disruptive and costly than specifying appropriately from the outset, even if it means a modest additional cost upfront for a specification you don’t immediately need at full capacity.
This is particularly worth considering if you’re already budgeting for a base at a size like 12x12ft or larger, where the cost difference between a standard general storage thickness and a slightly more robust specification is often smaller in proportion to the overall project cost than people initially assume.
Thickness Reference Chart: Quick Lookup
| Shed Size | Typical Use | Recommended Thickness | Reinforcement Needed? |
| Small (up to 6x4ft) | General garden storage | 100mm to 125mm | Not typically needed |
| Medium (8x6ft to 10x8ft) | General garden storage | 100mm to 125mm | Not typically needed |
| Large (12x10ft+) | General storage, no machinery | 150mm to 200mm | Case by case |
| Workshop | Heavy machinery or equipment | Individual assessment | Often recommended |
Case Study: Getting Thickness Right Across a Mixed-Use Garden
One of our typical projects involved a customer in South East London planning two separate structures in the same garden: a medium sized general storage shed and a smaller workshop area for occasional woodworking equipment.
Project details:
| Structure | Size | Thickness Used | Reasoning |
| General storage shed | 8x6ft | 110mm | Falls within standard small or medium range |
| Workshop area | 8x6ft (same footprint size) | 200mm, with localised reinforcement | Point loads from equipment required individual assessment |
What’s notable here: Despite both structures sharing an identical footprint size, the workshop area required a significantly thicker base with localised reinforcement, purely because of the point loads its equipment would place on specific areas of the floor. The general storage shed, despite being the same size, only needed the standard thickness appropriate for its lighter, more evenly distributed load.
Result: Both structures have performed well since installation, with no cracking or settling issues on either base. This case is a clear demonstration of why shed size alone doesn’t tell the whole story when it comes to thickness, what’s actually going inside the shed matters just as much.
How to Prepare for a Thickness Assessment
If you’re arranging a professional assessment for your own shed base project, a bit of preparation on your side helps ensure the recommended thickness genuinely fits your needs. It’s worth having your shed’s exact dimensions confirmed, along with a realistic list of what you plan to store or use inside it, including any machinery, heavy shelving, or equipment, even if you’re not planning to install it immediately.
It’s also worth mentioning any known ground conditions or previous issues in that part of the garden, such as areas that have felt soft underfoot or held standing water after rain, since this information helps whoever is assessing the site give a thickness and reinforcement recommendation that reflects both your shed’s requirements and your garden’s specific characteristics, rather than a generic figure based on shed size alone.
Frequently Asked Questions
How thick should a concrete base be for a small shed?
For sheds up to around 6x4ft, we recommend a thickness of 100mm to 125mm, which provides adequate strength for typical small shed use.
Does a medium shed need a thicker base than a small shed?
Generally no. Sheds up to roughly 10x8ft typically share the same 100mm to 125mm thickness recommendation, since the difference in overall load between small and medium sheds is relatively minor.
How thick should a base be for a large shed?
For larger sheds, typically 12x10ft and upward for general storage, we recommend 150mm to 200mm to properly support the increased floor area and overall weight.
Why doesn’t a workshop shed get a standard thickness recommendation?
Workshop sheds often have concentrated point loads from machinery or equipment, which need individual assessment rather than a generic thickness figure to ensure those specific load points are properly supported.
Can a thicker concrete base compensate for poor ground conditions?
Not entirely. While additional thickness helps, proper ground preparation and compaction remain essential regardless of how thick the concrete slab itself is poured.
What’s the difference between a point load and a distributed load?
A distributed load spreads weight fairly evenly across the whole slab, such as general storage, while a point load concentrates significant weight onto a small area, such as machinery legs, which needs specific consideration beyond a standard average thickness.
Should I plan for extra thickness if I might add workshop equipment later?
Yes, it’s worth discussing this possibility during your initial assessment, since retrofitting a thicker or reinforced base beneath an existing shed is considerably more disruptive than specifying appropriately from the start.
Get a Free Quote for Your Shed Base Project
Whether you’re planning a small garden shed or a workshop with specific equipment needs, JPD Landscaping & Building Services offers free quotes across South East London to assess exactly what thickness your project needs.
