How Thick Should a Concrete Slab Be for a Shed?

Specifying the wrong concrete slab thickness for a shed is one of the most common and most avoidable concreting mistakes in residential construction. Too thin and the slab cracks under load, settles unevenly and deteriorates within a few years. Too thick on a simple garden shed and you are spending money the project does not need. The right answer is not a single number – it is a combination of four factors: what the shed will be used for, what sits beneath it, whether vehicles or machinery will access it, and how the sub-base is prepared.

What the Shed Is Used For Determines the Starting Point

The intended use of the shed is the primary driver of how thick a concrete slab should be for a shed. Different applications generate entirely different load profiles and the slab must be specified to handle the actual loads it will carry, not a generic estimate.

Small garden sheds and light storage sit in the 75mm to 100mm range. These slabs carry foot traffic and light storage only. A 75mm slab with a properly compacted sub-base is structurally adequate for a garden shed that will never carry a vehicle or anything heavier than a ride-on mower. Increasing to 100mm adds a meaningful margin without a significant cost increase and is worth doing on any shed that may see heavier use over its life.

Standard residential sheds and hobby workshops are typically poured at 100mm. This is the most common shed slab specification across Sydney – adequate for foot traffic, light machinery, workbenches and the day-to-day loads of a general-purpose home shed. Steel mesh reinforcement is standard at this thickness.

Sheds with vehicle access or heavier equipment move into the 100mm to 125mm range. A domestic garage used by passenger vehicles sits comfortably at 100mm. A workshop housing a car hoist, a heavy lathe or regular trailer movements warrants 125mm to handle the point loads those items generate. The distinction between these two scenarios is not always obvious at the planning stage – when in doubt, go to 125mm.

Commercial, agricultural or industrial sheds handling sustained heavy vehicle movements, forklift traffic or significant point loads from racking systems should be specified by a structural engineer. The general guide of 125mm to 150mm is a starting point, not a final answer, for these applications.

Under-Specifying the Thickness – What Actually Happens

A slab poured too thin for its intended use does not fail dramatically on day one. It deteriorates quietly. Surface cracking appears first – often within the first year or two as the slab flexes under loads it was not designed to carry. The cracks allow moisture penetration, the sub-base erodes beneath the weak points, and sections of the slab begin to move independently of each other.

By the time the failure is obvious, the slab is beyond repair. Rectification means demolition, debris removal, sub-base re-preparation and a full new pour. The cost is multiples of what a correctly specified slab would have been in the first place. Getting how thick a concrete slab should be for a shed right at the quoting stage is always cheaper than learning it later.

Sub-Base Preparation: As Important as the Slab Itself

A 100mm slab on a well-prepared compacted sub-base will consistently outperform a 125mm slab poured onto soft, uncompacted ground. The sub-base is not secondary to the slab – it is the foundation of the foundation.

The sub-base for a shed slab typically consists of a minimum 50mm compacted sand or crushed aggregate layer on stable ground. This layer provides uniform support across the full footprint of the slab, prevents moisture wicking up from below and gives the formwork a consistent surface to work from. On sites with soft or expansive soil – common on older clay-heavy residential blocks across western Sydney – additional sub-base depth or soil stabilisation may be required before the slab is poured.

On these sites, improving the sub-base preparation often has more structural impact than simply increasing the slab thickness. KMA assesses the ground conditions before quoting on every shed slab to confirm what the sub-base requires before a thickness is recommended.

Reinforcement: Mesh for Most, Bar for Heavy Applications

For residential shed slabs at 75mm to 125mm, SL82 or SL92 steel mesh reinforcement is the standard inclusion. The mesh sits in the central third of the slab depth and provides crack control – it holds cracks tightly together so they do not widen, propagate and allow moisture ingress. It does not prevent cracking entirely, but it keeps surface cracks manageable under normal loads.

For thicker commercial or heavy-use slabs, or slabs carrying sustained point loads from vehicle hoists or racking systems, bar reinforcement may be specified in addition to or instead of mesh. Bar reinforcement provides greater tensile strength and is the typical specification for vehicle access slabs at 125mm to 150mm. If your shed will carry anything beyond domestic vehicle access or light machinery, ask the concreter specifically about reinforcement rather than assuming mesh is sufficient.

Drainage Fall: The Detail That Determines Long-Term Performance

No competitor blog on shed slab thickness covers this – which is why sheds with correctly specified concrete slab thickness still end up with performance problems when the drainage fall is wrong.

A shed slab needs a slight gradient – typically 1 in 100 toward the door or a drainage point – built into the pour. Without it, water from rain entering through the door, vehicle washing or cleaning pools on the surface. Pooling water accelerates surface deterioration and, over time, creates moisture problems at the base of the shed structure itself.

KMA designs drainage fall into every shed slab before formwork is set. Where the fall should run, how much fall is needed and where the water ends up after it leaves the slab are assessed as part of the site visit – not assumed during the pour.

Council Requirements and Cost in Sydney

In NSW, the shed structure typically triggers any permit requirement rather than the slab itself, but the slab specification forms part of what is submitted with the application. Most residential shed slabs within standard size limits and on unremarkable sites do not require a structural engineer’s certificate. Commercial sheds, larger agricultural applications or sites with difficult ground conditions will require one. Check with your local council or a licensed building certifier before pouring.

On cost: shed slab pricing in Sydney is driven by slab area, specified thickness, sub-base preparation requirements and site access. A 3m x 3m garden shed slab at 75mm on a straightforward flat site is a very different cost to a 12m x 9m workshop slab at 125mm with sub-base work and drainage. KMA provides clear, itemised quotes based on the actual job.

Call 02 9054 3040 to discuss your shed and get a straight number before any work begins.

The One Question Worth Asking Before You Commit to a Thickness

Before specifying how thick a concrete slab should be for your shed, ask yourself one question: what is the heaviest thing this slab will ever carry, and how often will it carry it?

The answer to that question – not the size of the shed or the cost of the pour – is what drives the correct specification. A slab that is right for the actual use will still be performing in 30 years. One that is not will be showing its problems in three.

KMA Concrete Constructions has been pouring shed slabs across greater Sydney since 1999. Learn more about our concreting services.