The slab’s the foundation for everything else. Get it wrong and you’re dealing with problems for the life of the shed. Get it right and it’s the most boring part of the build, which is exactly what you want.

Most people approach the slab decision the same way. Ask a concreter “what do I need”, get told “we’ll do you a 100mm with F62 mesh”, pay the invoice, done. That works most of the time, but it doesn’t tell you why, what you’re really getting, or when you actually need more.

Here’s what to know before you book the concreter, including the bits that catch people out.

Why You Need a Proper Slab

A shed needs a stable, level, dry base to sit on. Concrete delivers all three. It also gives you a solid floor you can drive on, store things on, and work on without dealing with dust, mud, or moisture coming up from below.

Some people try to skip the slab and put the shed on bare ground or gravel. It can work for very small basic storage sheds, but for anything you’ll actually use, a proper slab pays for itself quickly. Tools last longer. The shed stays cleaner. Heavy items don’t sink. The floor doesn’t go soft when it rains.

The slab also locks the shed down. Steel sheds are designed to be anchored to a concrete base. Without that anchor point, wind loading becomes a real problem in any decent breeze.

Standard Thickness vs Heavy Duty

The standard for residential shed slabs is 100mm thick. That handles cars, motorbikes, light workshop use, general storage, and anything you’d normally have in a backyard shed.

When you go thicker, you’re usually accommodating heavier loads:

• 125mm to 150mm for commercial sheds, larger machinery, or workshop use with bigger equipment
• 200mm or thicker for industrial sheds storing heavy plant or stock

A 4WD or caravan won’t damage a properly poured 100mm slab. A tractor, forklift, or loaded trailer might mark it up over time. If you’re storing or working on something seriously heavy, mention it to your concreter and your shed supplier so the slab gets engineered accordingly.

Going thicker just to be safe isn’t a bad idea if budget allows. The extra cost is mostly more concrete, and a 125mm slab is significantly stronger than 100mm.

Reinforcement: Mesh, Bar and Why It Matters

Concrete handles compression brilliantly. It handles tension terribly. The mesh inside the slab is what holds it together when it wants to crack.

The standard residential spec is F62 mesh, sometimes F72. It’s a square grid of steel bars welded together, laid into the slab before the pour. The mesh sits in the middle of the slab depth, held up off the ground by bar chairs.

Without proper reinforcement, a slab will crack. Not might. Will. Concrete shrinks as it cures, and that shrinkage creates tension. The mesh holds the slab together through that.

For heavier loads or larger slabs, you might step up to F82 mesh or add steel bar around the perimeter. Industrial slabs often use a combination of mesh and bar to handle specific load points.

What you don’t want is mesh sitting on the ground at the bottom of the slab, which is what happens if it’s not properly chaired. That gives you essentially unreinforced concrete with some metal at the bottom doing nothing useful.

Sizing the Slab to Your Shed

This trips a lot of people up. The slab needs to match the shed’s footprint, but how it matches matters.

Most shed manufacturers, including us, design for the slab to sit flush with the outside of the wall sheets. The wall sheets come down to slab level, and the base channel sits on the slab edge.

Make the slab too big and your wall sheets land mid-slab with concrete extending past them. Rain hits that exposed concrete and runs back under your wall sheets. Not what you want.

Make the slab too small and your wall sheets hang past the slab edge. The base channel doesn’t sit properly, the seal at the bottom is compromised, and you’ve got problems on day one.

The right size is the exact footprint of the shed, no more, no less. Your shed plans will give you the slab dimensions. Hand them to your concreter and have them confirm before pouring.

Edge Thickening and Cyclone Tie-Downs

For most residential sheds in standard wind zones, a flat 100mm slab does the job. The shed gets bolted down with chemical anchors or expansion bolts into the slab edge.

In cyclone-prone areas like Region C and D, tie-down requirements get serious. The slab edge often needs to be thickened to 200mm or more around the perimeter, with additional reinforcement, to handle the uplift forces a cyclone can generate.

You might also need pier footings under specific columns, or longer anchor bolts that extend deep into the slab. None of this is optional. It’s what stops your shed leaving the property in a cyclone.

If you’re in northern Queensland, the top end, or any coastal area subject to cyclones, your shed quote should specify what the slab needs to handle. Don’t let the concreter improvise it. Get the engineer’s spec and follow it exactly.

Falls, Drainage and Vapour Barrier

A perfectly flat slab sounds great but isn’t actually what you want. You need a slight fall to let any water that gets in drain out, rather than pooling.

The standard fall is 1 in 100, which is 10mm over a metre. Imperceptible to walk on, enough to drain water out the door or to a designated low point.

Where the fall goes depends on the shed. If you’ve got roller doors at one end and a back wall at the other, fall toward the doors. If the shed sits on a sloping site, fall with the natural slope. Your concreter will work this out, but it’s worth asking so you understand what’s planned.

Under the slab, a polythene vapour barrier (the orange or black plastic sheeting) stops moisture rising up through the concrete. Concrete is porous, and without a vapour barrier, ground moisture wicks through and shows up as damp patches on the floor or condensation on stored items.

The vapour barrier is cheap. Skipping it isn’t worth the saving.

How Level Is Level Enough?

A shed slab doesn’t need to be glass-flat, but it can’t be wonky either. The shed kit is engineered to specific dimensions, and a slab that’s significantly out of level will cause problems lining up the frame and panels.

Reasonable tolerance is around 5mm to 10mm over the full length of the slab. Any more than that and you’ll be packing under base plates or fighting alignment issues during the build.

This is one of the reasons we recommend hiring a concreter for any slab that isn’t tiny. Levelling concrete properly is harder than it looks, and a few hundred dollars saved on a dodgy slab can cost you triple that in build complications.

DIY or Hire a Concreter?

For most people, hiring a concreter is the right call. Slabs are one of those things that look simple but reward experience.

A good concreter does this every day. They’ve got the screeds, floats, edgers, and broom finishes dialled. They know how to manage the pour timing, finish the surface, and cut the control joints. They get the levels right.

DIY can work for very small slabs. A 3×3 metre garden shed slab is doable for someone reasonably handy. Beyond that, the risk of getting it wrong outweighs the savings.

Concreter costs typically run between $80 to $150 per square metre for a standard residential shed slab, including labour, concrete, mesh, and finishing. That varies by region, site access, and slab spec. Get a few quotes and ask what’s included before comparing prices.

Timing the Pour Around Delivery

Shed kits typically take 15 to 20 working days to manufacture and deliver after order. That’s your window to organise the slab.

You want the slab poured and cured before the shed arrives. A standard 100mm slab needs about a week of cure time before you can start the build. Heavier slabs might need longer.

The sequence usually runs:

• Order the shed and confirm slab dimensions from the engineered plans
• Book the concreter for a date that gives you cure time before delivery
• Pour and finish the slab
• Let it cure for 7 to 14 days minimum
• Shed arrives, build starts

Trying to compress this timing causes problems. A slab that hasn’t cured properly when you start drilling for anchors can crack around the bolts. Worth waiting the extra few days.

Alternatives When a Full Slab Isn’t Right

Not every shed needs a full concrete slab. There are situations where alternatives make more sense.

Pad footings work for some larger rural sheds where you want a dirt or gravel floor inside, like machinery sheds or hay storage. Concrete pads go under each column, the shed gets anchored to those, and the floor stays earthen.

This saves significant money on concrete and works well when you don’t need a sealed floor. Not suitable for vehicles or anything that requires a clean dry surface, but for storing implements or farm machinery, it’s perfectly fine.

Strip footings are another option, mostly used for very large industrial sheds. A continuous strip of concrete runs under the wall lines, with the floor either left as compacted earth or poured separately later.

For very small garden sheds, under about 10 square metres, a properly prepared compacted gravel pad can work. Not ideal, but acceptable for low-use storage of garden tools or similar.

Common Mistakes That Cost You

The patterns we see repeatedly:

Wrong size slab. Either too big or too small. Always pour to the shed’s specified dimensions, not what looks about right.

No vapour barrier. Cheap component, big consequence. Always include it.

Insufficient cure time. Building on green concrete creates problems that don’t show up until later.

Mesh sitting on dirt. Steel chair bars exist for a reason. Mesh that isn’t properly chaired does nothing structural.

No fall. A perfectly flat slab traps water. Tiny fall, big difference.

Wrong slab for the use. Standard 100mm for what’s actually going to be a heavy workshop. Reinforce and thicken if you’re parking serious equipment or running heavy machinery.

Pouring too early or too late. Slab needs cure time before the build starts, but you don’t want a finished slab sitting weathering for months waiting for the shed.

If you’re planning a shed and want help working out exactly what slab you need, talk to us before you book the concreter. Call us on 1300 887 433 or request a quote. We’ll give you the engineered slab spec for your shed and your site conditions, so the slab gets built properly the first time.