Strong Slabs Start Below the Pour: Subgrade Compaction and Vapour Barriers

Strong sheds do not begin with timber or steel. They begin at ground level. Whether you are building carports, fencing, or installing custom shed kits, what happens before the concrete pour can make or break the job. Poor prep allows movement, moisture, and air pockets to weaken the slab from the underside, leading to cracks, slope failure, or edge lifting sooner than you would like.

A reliable base requires more than just clearing weeds and tipping in some gravel. For South-East Queensland sites around Ipswich and Brisbane, we deal with reactive clay, patchy fill, and high humidity, all of which require proper control. Solid compaction and a good vapour barrier under the formwork are no-frills steps tradies rely on for a long-lasting finish.

Here is what to look for when setting the base beneath a slab, before the truck shows up with concrete in the chute.

Subgrade Basics: Why the Ground Matters More Than the Pour

It is easy to focus on what is visible once the slab is down. But the material underneath, the subgrade, does most of the work. If the ground fails, the slab shifts, and everything above it suffers.

At its simplest, the subgrade is the shaped and trimmed earth or fill that sits directly under your concrete. It spreads the slab’s load back into the ground. In places like Ipswich, common issues include:

• Soft topsoil pockets from scraped turf
• Reactive clay that swells with rain and shrinks with dry spells
• Fill zones where cut and fill meet and never compact together evenly

We always start by trimming to depth, removing loose soil, and shaping fall if needed. Then, we compact in consistent layers using proper gear, not just the back of a shovel or ute tyres. It is all about stability. The slab does not float, it relies on uniform strength below.

We provide quality steel reinforcing mesh and bar, which is ideal for creating a stable subgrade and concrete reinforcement foundation (source: haggarty.com.au/building-supplies/concreting).

Proper Compaction: Tools and Methods That Tradies Trust

When we are prepping base layers under sheds or small structures, there is no room for shortcuts. Whether you are doing a one-off slab or rolling out a slab for custom shed kits on a farm site, compaction keeps everything on track.

What gear gets used depends on the size and access:

• Hand rammers for tight spots between posts or edges
• Plate compactors, ideal for sand, crusher dust, or DGB fill under carports
• Drum rollers for bigger driveways or loads, usually with truck access

We compact in lifts. That means laying no more than 100 mm per pass, then tamping or rolling it down solid before moving to the next layer. Rushing it means soft pockets that sink once curing starts.

How do you know it is solid enough?

• It should not indent easily under boots
• You should not feel bounce when walked over
• Your levels should stay firm during screeding

Effective compaction is the bedrock of slab longevity. Solid compaction means no settling beneath the slab later, keeping custom shed kits straight, doors moving properly, and baseplates tight. It is a step that pays back long after the pour.

While it may be tempting to overlook this, a well-compacted base prevents voids and future slab issues. For homeowners and trades alike, taking the time for even compaction is worthwhile. Remember, uniform pressure and consistent layers are key to avoiding uneven settling.

Vapour Barriers That Work: Stop Moisture Before It Reaches the Slab

In warm, humid climates like ours, ground moisture rises under the slab. That is where vapour barriers come in, stopping moisture from entering the concrete over time.

A good vapour barrier keeps:

• Slab edges from powdering out
• Screed surfaces dry and ready to finish
• Mould forming under mats, tools, or stored gear in sheds

Most of what we see used locally is 200 micron black polythene. It is tough, flexible, and holds up under weight. But it is only as good as the install. Always:

• Lay it flat, trimmed to the trenches or edge forms
• Overlap joins by at least 200 mm
• Use good-quality tape to stop shifting or gaps during pouring

That moisture barrier pays off later. Cleaner working surfaces, fewer damp tools, and reduced slab repair risks a few years down the track, especially in enclosed storage like shed kits or awnings.

The right vapour barrier installation protects not just the slab but also anything you store above it. Persistent moisture can cause long-term issues, including corrosion and mould, but you can avoid these with proper preventive setup before pouring.

Checking Levels Before You Pour: The Right Way to Set String Lines and Screeds

Pour it once, or fix it forever. Before any slab is poured, final checks are crucial. Getting levels right stops water from pooling under or around sheds, patio posts, and garage entry points.

We usually do this by:

• Fixing string lines across edge boards for overall slab height
• Running laser levels or spirit levels across compacted subgrade
• Setting screed pegs or boards within the pour zone

If the slab is going under a shed, fall direction matters. You want water flowing away from finish materials, not pooling on thresholds or forming puddles near footings. Double-check slope in both directions, across and along the slab.

And keep edge forms square and firm. Trestles and timber bracing often carry splash weight during pouring, and if they move, so does the level.

We check twice, pour once, because lifting or grinding a bad slab later is no simple job. Smooth, even screeding ensures that water moves off the slab as designed, actively reducing wear and tear over time.

By maintaining correct slope away from the structure, you also lessen potential for water ingress and related damage, ensuring lasting durability for your shed kit or any structure set on the slab.

FAQ: Subgrade and Vapour Barrier Prep

Q: Do I need a vapour barrier under every shed slab?

A: Yes, especially in our climate. Even on raised sites, ground moisture can ruin floors and corrode fixings from underneath.

Q: Can I just compact with my car or ute?

A: No. It will press down the surface but leave soft zones underneath. Plate or drum compactors reach deeper and compact more evenly.

Q: What is the ideal thickness of crushed rock under a shed slab?

A: Around 100 mm on average for most soil types. More may be needed for clay or reactive soils that do not drain or support load well.

Q: How can I tell if I have compacted enough?

A: Walk it. If your boot leaves prints or the ground shifts, keep compacting. A solid base feels hard underfoot and does not shift.

Finishing Strong: Your Local Supply Partner

A quality concrete slab starts from the ground up. By using the right steel reinforcement, vapour barriers, and construction hardware sourced from The Haggarty Group Qld Pty Ltd, you can achieve a finish that stands up to the demands of South-East Queensland’s climate. We offer a convenient online ordering system and reliable delivery throughout the region, meaning you can get materials for your next project quickly and efficiently.

Every shed build starts from the base up, and selecting the correct supplies is important for durability. Whether you are tackling residential or trade work in Brisbane, Ipswich, or beyond, we have the practical products and experience to help.

Planning a new install or sizing up options for steel, roofing, and hardware supply is easy with our range of custom shed kits built for practicality and Queensland conditions. At The Haggarty Group Qld Pty Ltd, we back proper prep with quality product and family service you can count on. Give us a call and let’s get your project set right from the base up.