I had a fella ring me last week from out west of Brisbane. He’s getting quotes for a 9x6 workshop shed and three different mobs have given him three different slab specs. One quoted 75mm, one quoted 100mm, one quoted “150mm with edge thickening because of your soil.” He wanted to know who was right.

Honest answer: it depends. But there’s a lot less mystery to slab specs than the variation suggests, and most of the variation comes down to how much risk the builder is happy to carry on a job.

Here’s how I think about slabs for Brisbane sheds, what the standards actually require, and what I’d build for myself.

The soil under your shed matters more than the steel on top

Brisbane sits on a mix of soil types. Some of it’s beautiful sandy loam that drains well and supports anything you put on it. A lot of it is reactive clay - the kind that swells when wet and shrinks when dry, moving slabs around with the seasons.

Before any concrete goes in, you need to know your site classification. The categories that matter are:

• A - non-reactive (gravel, sand, stable rock). Lucky you.
• S - slightly reactive clay
• M - moderately reactive (very common in Brisbane)
• H1, H2 - highly reactive
• E - extremely reactive

If you’re in Forest Lake, Springfield, Fig Tree Pocket, or anywhere on the western suburbs, you’re probably looking at M or H. Inner-city ridge suburbs vary. The only way to know for sure is a soil test, which costs $400 to $700 and takes a few days.

A reputable shed builder will either include this in the quote or insist you get one before they finalise the slab spec. If they’re skipping the test, ask why.

Standard slab specs and what each one is good for

Here’s the rough breakdown of what you’d commonly see:

75mm with SL62 mesh, no edge thickening This is fine for a small storage shed (under 4x3) on a stable, well-drained site, where you’re storing light gear. Bikes, garden tools, the kids’ camping stuff. Don’t park a car on this.

100mm with SL72 mesh, basic edge thickening A typical residential garage spec. Works for a single-vehicle shed on stable soil. Adequate for cars and light tradie utes. Not great for trucks, machinery, or workshops with heavy loads.

125mm with SL82 mesh and proper edge beams This is what I’d default to for most Brisbane shed builds in the 6x4 to 9x6 range. The extra thickness and reinforcement copes with reactive soil movement and supports vehicles, workshop equipment and reasonable point loads.

150mm+ with engineered design For sheds bigger than about 12x6, sheds on highly reactive sites, or sheds storing heavy machinery (boat trailers, excavators, caravans), you want an engineered slab specifically designed for the load and soil. This will often include thicker edge beams, additional internal beams, and sometimes piers down to stable strata.

What the standards actually say

Concrete slab design in Australia is covered by AS2870 (Residential Slabs and Footings) and AS3600 for structural concrete. For non-residential outbuildings the requirements are slightly different but most reputable shed companies design to residential standards as a minimum.

The point is: there’s an actual standard here, not just rules of thumb. If a builder can’t tell you which standard their slab design references, that’s a flag.

The bits people skip that cause problems later

A slab is way more than just thickness. The stuff underneath matters enormously and gets scrimped on constantly.

Site preparation. Topsoil needs stripping back to firm ground. Builders sometimes pour over uncompacted fill to save time and the slab cracks within two years. Insist on proper stripping and compaction.

Sub-base. A 75 to 100mm layer of compacted road base under the slab gives you a stable, drained foundation. Skipping this is a false economy.

Damp proof membrane. A 0.2mm minimum poly membrane between the sub-base and concrete stops moisture wicking up through the slab. In Brisbane’s humid summer, this matters. Mould on the underside of stored items, rust on metal, swelling timber - all symptoms of skipping the membrane.

Reinforcement positioning. Mesh has to sit in the upper third of the slab to do its job. If it’s lying on the ground when the concrete goes in, it’s worse than useless because it’s a stress concentrator at the wrong depth. Bar chairs every metre. Walk around before the pour and check.

Curing. Concrete cured properly is twice as strong as concrete left to dry too fast. In Brisbane heat, slabs poured in summer especially need either a curing compound or a wet hessian cover for the first 7 days.

What I’d build for myself

If I were putting up a 9x6 workshop on my own block in Brisbane, I’d specify:

• Soil test first, regardless
• 125mm slab with SL82 mesh
• 300x300 edge beam, 200x200 internal beams if needed based on layout
• 100mm compacted road base sub-base
• 0.2mm damp proof membrane
• N25 concrete (32MPa would be overkill for most shed applications)
• Saw cuts at appropriate spacing for crack control
• Curing compound applied within an hour of finishing

That’s not the cheapest option. It’s the option I won’t have to think about again for 30 years.

The cost difference between cheap and proper

For a 9x6 slab the difference between a thin, basic spec and what I’d actually want is roughly $1,500 to $2,500. On a total shed build of $18,000 to $28,000, it’s a small percentage. On the lifetime of the shed, it’s the difference between a slab that lasts 30+ years and one that’s cracking and lifting at 8 years.

The other thing to remember: a cracked or moving slab affects the whole shed structure. The frame is bolted to the slab. If the slab moves, the frame goes out of square, the doors stop closing properly, the roof flashing leaks. A bad slab kills the shed eventually.

When you definitely need an engineer

Get a proper engineered design if:

• The shed is larger than about 12x6
• You’re on highly reactive (H1+) soil
• You’ll be storing heavy machinery or vehicles over 4 tonnes
• The site has any kind of slope requiring cut-and-fill
• You’re within 1m of a boundary or another structure
• The block has fill of unknown depth or origin

The engineering fee is $800 to $2,000 for a typical residential shed slab and it’s the cheapest insurance you’ll buy on the project.

What to ask any shed builder quoting your job

A few questions that separate the proper builders from the rest:

1. What’s the soil classification you’re designing for and how do you know?
2. What standard does the slab design reference?
3. What’s the sub-base specification?
4. Where exactly will the mesh sit?
5. What’s the curing process?
6. Who’s signing off on the slab before the frame goes up?

Anyone who gets impatient with these questions probably isn’t the builder you want.

If you’re at quoting stage on a Brisbane shed and want a hand interpreting what different builders are offering on the slab side, give me a ring. Happy to walk through the numbers without trying to sell you anything.