The concrete slab is the part of the shed you don’t think much about until something goes wrong with it. Cracking. Sinking on one side. Moisture coming up through the slab. Damp problems that affect everything stored on it. By the time you notice, fixing it is hard. The time to think about the slab properly is before it’s poured.

This is a working-tradie view of what’s actually worth spending on for a shed slab in 2026, drawn from years of building, buying, and re-doing sheds across Brisbane and surrounds.

Slab thickness

The shed slab thickness conversation has standard answers that work for standard sheds.

For a typical residential shed under 25 square metres, intended for general storage and light use, a 100mm slab is usually adequate. The reinforcement matters more than the thickness — a properly reinforced 100mm slab outperforms a poorly reinforced 125mm slab.

For a workshop shed where you’ll be running heavier equipment, parking vehicles, or doing serious work, 125mm is a better baseline. The extra concrete absorbs more of the dynamic loads from equipment and machinery.

For a shed that will house genuinely heavy gear — boats on trailers, large vehicles, serious workshop machinery — 150mm is realistic. Above that, you’re getting into commercial territory that probably needs engineered design rather than rules of thumb.

The variation that matters most is whether you’re building on stable ground or problematic soil. Reactive clay, poor fill, or recently disturbed ground call for thicker slabs and better engineering. The Brisbane area has plenty of reactive clay sites where the standard slab specifications need to be increased.

Reinforcement

Reinforcement is where shortcuts produce the most regret. The mesh in the slab is what stops it cracking when the ground moves slightly, when the shed contents impose loads, when the temperature cycles through summer expansion and winter contraction.

Standard SL62 mesh is the bottom of the acceptable range for residential sheds. SL72 is better for workshop use. SL82 is appropriate for slabs that will see heavier loads.

The mesh has to be properly placed. The classic mistake is to place mesh on the ground before pouring, with the assumption that the concrete will somehow lift it as it’s poured. It won’t. The mesh has to be on chairs or other supports that hold it at the right height during the pour. If you can see the mesh sitting on the ground after the pour, the reinforcement isn’t doing what it should.

The lap of mesh sheets is also worth getting right. The minimum lap on SL mesh is one full square. The mesh that’s poorly lapped develops cracks at the unreinforced gaps under load.

For slabs in problematic soil, additional reinforcement — fibre, additional mesh layers, or engineered reinforcement — can be worth the extra cost.

The membrane

A vapour barrier under the slab is the most under-appreciated piece of slab specification. The plastic membrane between the ground and the slab stops moisture wicking up through the concrete into the shed environment.

Without a membrane, the slab will be persistently damp on the surface, even in dry weather. Anything stored in contact with the slab — toolboxes, machinery, bikes, anything — will collect moisture from below. Workshop equipment will rust faster. Stored timber will warp.

With a membrane properly installed, the slab is much drier. The membrane has to be continuous, lapped at joints, and unbroken. Holes in the membrane create localised damp spots in the slab.

The cost of a proper membrane is small relative to the slab cost. Skipping it is one of the most common shortcuts and one of the most consistently regretted.

Thickening at edges

The edges of the slab are where the loads concentrate. The shed walls bear on the edges. Vehicles entering and exiting cross the edges. Equipment being moved in and out passes over the edges.

A slab that’s a uniform thickness across the whole footprint will start to fail at the edges first. Edge cracking, edge spalling, and edge sinking are common patterns.

The standard fix is to thicken the slab at the edges. The thickening can be a continuous edge beam (more concrete and reinforcement around the perimeter) or it can be specific thickening at the door openings where the loads are highest.

The cost of edge thickening is modest. The benefit shows up over years of use.

Surface finish

The surface finish on a shed slab isn’t just cosmetic. It affects how the slab performs in use.

A trowel finish — the most common — gives a smooth surface that’s easy to clean, holds dust less than rougher finishes, and is comfortable underfoot. The downside is that a trowel finish can be slippery when wet, which matters in a shed that gets wet from rain or vehicle drips.

A broom finish gives texture for grip, sacrificing some of the smoothness. The slab is less slippery when wet but harder to clean and tends to hold dust.

A polished finish takes the trowel finish further with mechanical polishing. The result is a beautiful surface that looks great in workshop and showroom contexts, but the cost is substantially higher and the durability under serious workshop use is contested.

For most workshop sheds, a clean trowel finish is the practical answer. For sheds where vehicles will be parking and moving, a broom finish in the entry/exit areas with a trowel finish elsewhere is sensible.

Drainage

The drainage of the slab matters more than people expect. A slab that’s poured perfectly level will pool any water that ends up on the surface. The slab that’s poured with a slight fall toward the door — typically about 1 in 100 — drains naturally and stays dry.

The fall can be hard to set up properly without a competent concreter. The slab that’s poured with the wrong fall (sloping into the shed rather than away from it) is much worse than no fall at all.

For sheds that will see significant water — wash bays, sheds with concrete floors that need cleaning regularly — adding a drainage point in the slab is worth the cost. Retrofitting drainage after the slab is poured is much more expensive than designing it in from the start.

The site preparation

The slab is only as good as the prep underneath it. The site has to be properly excavated, levelled, and compacted. The base material — typically crushed rock or roadbase — has to be at the right depth and properly compacted.

The compaction step is where corners get cut. A poorly compacted base settles unevenly over time, which produces cracks in the slab regardless of how good the concrete and reinforcement are. The plate compactor or roller has to actually do its job.

For sites with poor soil, the prep can extend to the import of fill material to replace problematic ground. This is expensive but unavoidable for some sites.

The honest advice is to take the prep seriously. The slab fails first at the prep stage, not at the pour stage.

When to engineer it

Most residential shed slabs follow standard specifications and don’t need engineering. The standards are well-developed and the slab performs predictably.

Engineering is worth the cost when:

The site has problematic soil that the standard specifications don’t accommodate.

The shed will house unusual loads — large boats, heavy machinery, vehicles substantially over typical residential weights.

The shed has unusual geometry that produces unusual load patterns.

Council approvals require engineered design (some councils require this above certain shed sizes regardless).

The cost of engineering is significant on small sheds — sometimes a meaningful percentage of the total slab cost. For larger or more complex sheds, the engineering cost is a smaller share of the total and more clearly worthwhile.

What I’d actually pay for

If I were specifying a serious workshop slab today on a typical Brisbane site, I’d be paying for:

A 125mm slab with SL72 mesh, properly placed.

A continuous vapour barrier, lapped and unbroken.

Edge thickening at the perimeter.

A clean trowel finish with appropriate fall to the door.

Proper site prep with adequately compacted base material.

These specifications are not exotic. They produce a slab that performs well for decades. The shortcuts that produce the regrets I’ve seen are mostly on the membrane, the reinforcement placement, and the site prep. Don’t cut those corners. The rest of the slab decisions are mostly within the band of “fine.”