Concrete Shed Slab Guide: Planning, Thickness & Sizing

A shed is only as good as the slab it sits on. Get the slab right — the correct thickness, proper reinforcement, a well-prepared base and the right falls — and your shed will sit level, stay dry and last for decades. Get it wrong and you’ll deal with cracking, moisture problems and a shed that never quite sits true. On the Fraser Coast, with our reactive soils and wet seasons, the slab matters even more.

This guide walks you through planning a shed slab from the ground up: thickness, reinforcement, sizing to common sheds, site preparation, drainage falls and siting basics. The figures are a guide only — your exact spec and price should come from a concreter who has seen your site and knows your shed plans.

Start with how you’ll use the shed

Before any numbers, ask what’s going inside. The use determines the slab spec:

• Garden shed, storage, light workshop → standard slab
• Garage, car or boat storage → thicker slab for vehicle loads
• Heavy machinery, hoists, big trailers → heavier-duty slab, possibly engineered

A slab built for a few tools and a mower is very different from one that needs to carry a boat, a car hoist or a loaded trailer. Decide the use first, then spec to it.

Shed slab thickness

For a typical shed, the benchmark is 100mm of concrete with steel reinforcing mesh — the same standard we’d use for a residential driveway carrying cars. That suits most storage sheds and light workshops.

Step up the thickness when the loads are heavier:

Shed use	Recommended thickness	Reinforcement
Storage / garden / light workshop	100mm	Steel mesh
Garage, car or boat storage	125mm	Heavier mesh
Heavy machinery, hoists, trucks	150mm+	Engineered (mesh/bars)

These are indicative. If you’ll fit a car hoist, store heavy machinery or run a serious workshop, the slab may need to be engineered to a structural design. We cover the load-vs-thickness logic in more depth in our guide on how thick a concrete driveway should be — the same principles apply to shed slabs.

Reinforcement: don’t skip the steel

Reinforcing mesh inside the slab is what holds it together when the ground moves and loads flex it. Concrete is strong in compression but weak in tension; steel handles the tension. On the Fraser Coast’s reactive soils, that reinforcement is doing real work every wet-then-dry season.

For heavier-duty slabs we may step up to a heavier mesh grade or add reinforcing bars in high-stress areas. Thickened edges (an edge beam) are also common to stiffen the slab perimeter, especially for larger sheds and garages.

Sizing the slab to your shed

Match the slab to the shed footprint, with a few practical considerations:

• Footprint match. Most shed slabs are poured to the shed’s footprint dimensions so the walls sit on the edge.
• An apron. Many people add a small concrete apron in front of the doors (an extra strip outside the shed) so you’re not stepping off concrete onto dirt or mud — handy in our wet season.
• Door access. Make sure the slab and any apron suit how the doors open and how vehicles or mowers will enter.
• Check the shed plans. Shed kits often specify how the slab should relate to the building. Have the shed details before pouring.

Here are indicative areas for common shed sizes to help with planning:

Shed size	Slab area
3 x 3 m	9 m²
6 x 3 m	18 m²
6 x 6 m	36 m²
6 x 9 m	54 m²
7 x 9 m	63 m²
9 x 9 m	81 m²

For indicative pricing on these sizes, see our concrete slab cost guide for QLD, which breaks down shed slab costs by size.

Site preparation: the part that makes or breaks it

A slab is only as solid as the ground beneath it. Good site prep is non-negotiable:

1. Clear and strip. Remove vegetation, topsoil and any organic material — slabs poured on topsoil are asking for trouble.
2. Level the area. Cut and fill as needed to get a workable, even pad.
3. Build and compact a sub-base. Bring in road base and compact it properly. This gives the slab a stable, even surface to sit on.
4. Form up accurately. Set the formwork to the right dimensions and levels.
5. Place reinforcement correctly. Mesh needs to sit at the right height within the slab on bar chairs — not lying on the ground.
6. Address moisture. A vapour barrier (plastic membrane) under the slab is common, especially where the shed will be enclosed or used as a workshop, to keep ground moisture out.

On much of the Fraser Coast you’re building on reactive clay that swells and shrinks. Proper compaction and drainage reduce how much that movement affects your slab — which is exactly what keeps it from cracking.

Falls and drainage

Water is the enemy of any slab and anything stored on it. A shed slab should have a slight fall so water runs off rather than pooling:

• Set a gentle fall away from the shed and toward where water can safely drain.
• Keep the slab above surrounding ground where possible so water doesn’t run back onto it.
• Plan where water goes — don’t direct it onto a neighbour’s property or against the shed walls.
• An apron with a fall helps keep the doorway dry.

In our wet summers, getting the falls right is what keeps the shed floor dry and stops water sitting around the slab edges and softening the ground.

Siting basics

Where you put the shed matters as much as how the slab is built:

• Check boundaries and setbacks. Sheds usually need to sit a certain distance from boundaries — confirm local requirements.
• Avoid easements and services. Don’t build over easements or buried services. Always locate utilities before digging.
• Mind the trees. Large trees nearby can affect soil moisture and send roots toward the slab.
• Think about access. You’ll want to get vehicles, mowers or trailers to the shed easily — and the concrete truck needs access to pour.
• Drainage and ground. Choose a spot that drains well rather than a low, boggy corner.
• Approvals. Sheds above certain sizes, or in certain locations, may need approval. Check before you commit.

A bit of planning here saves a lot of regret later. The slab is permanent — get the location right the first time.

Curing the slab

Once poured, the slab needs to cure properly before it carries loads. In our heat, that means protecting it from drying too fast so it reaches full strength. Don’t rush to put the shed up or drive on it — see our guide on how long before you can drive on new concrete for the timeline. As a rule, give it the full cure before heavy loads.

Common questions

Is 100mm enough for a shed slab? For storage and light use on a well-prepared base, yes. Step up to 125–150mm if you’re storing vehicles, machinery or heavy loads.

Do I need a vapour barrier? For an enclosed shed or workshop, a membrane under the slab is well worth it to keep ground moisture out. Discuss it with your concreter based on how you’ll use the shed.

Can the slab be poured before I buy the shed? It’s best to have the shed details first so the slab dimensions, edge details and any hold-down requirements match the building. Check the shed plans before pouring.

Should the slab be bigger than the shed? Many people add an apron in front of the doors, but the main slab usually matches the shed footprint. It depends on your shed and how you’ll use the space.

The most common shed slab mistakes

Over the years, the same few mistakes turn up again and again on shed slabs that go wrong. Knowing them upfront helps you avoid every one:

• Pouring on topsoil. Skipping the strip-and-compact step is the number one cause of slabs that crack and settle. The slab needs a firm, compacted sub-base, not soft organic ground.
• Going too thin for the use. A 100mm slab is fine for storage, but ask it to carry a boat, a car hoist or heavy machinery and it will struggle. Match the thickness to the real load.
• Forgetting the falls. A perfectly flat slab lets water pool. A slight, deliberate fall keeps the floor and doorway dry through our wet season.
• No vapour barrier in a workshop. Ground moisture wicking up through an enclosed slab causes damp problems. A membrane underneath solves it cheaply.
• Poor siting. Building in a low, boggy corner, over services, or too close to a boundary creates problems that the best slab in the world can’t fix.
• Rushing the cure. Loading a green slab before it has cured is an easy way to crack a brand-new floor.

Every one of these is avoidable with a bit of planning and a concreter who knows the local ground.

Why the base and drainage matter most here

If you take one thing from this guide, make it this: on the Fraser Coast, the ground and the water are what test your slab the hardest. Our reactive clay soils swell when wet and shrink when dry, and our wet summers followed by dry spells put the slab through that cycle every single year.

A slab with a properly compacted sub-base and good drainage rides out that movement far better than one thrown onto bare, uncompacted ground. Keeping water away from under and around the slab — through correct falls, keeping it above the surrounding ground, and directing run-off sensibly — reduces how much the soil moves in the first place. That is why we treat base prep and drainage as the heart of a good shed slab, not an afterthought. The concrete and steel get the headlines, but the ground underneath decides how long it all lasts.

Plan your shed slab with local experts

A shed slab is a long-term foundation, and on the Fraser Coast it pays to build it for our soils and climate from day one — the right thickness, proper reinforcement, solid base prep and good drainage.

We pour concrete slabs for sheds, garages and workshops across Hervey Bay, Maryborough, Rainbow Beach and Tin Can Bay. If you’re planning a new shed and want a slab that sits level and lasts, get in touch for a written quote — bring your shed size and intended use, and we’ll spec the slab to suit.