The Foundation of a Good Slab: Getting the Base Right
You can't build a good concrete slab on a poor base. The foundation beneath your slab is just as important as the concrete itself – perhaps even more important. A solid, properly prepared base prevents cracking, settling, moisture problems, and structural failure.
Yet it's one of the most overlooked aspects of slab construction. Too many people focus on the concrete quality while neglecting the base preparation. This guide explains exactly what a concrete slab base needs, why it matters, and how to get it right for Brisbane conditions.
Why the Base Matters
Prevents Cracking
An unstable, poorly compacted base allows the slab to flex and move. As the soil beneath shifts, the concrete cracks. A stable, properly compacted base prevents this movement, keeping your slab solid and crack-free.
Prevents Settling and Uneven Slabs
If the base isn't level and uniform, one side of the slab will settle more than another, creating slopes, depressions, and uneven surfaces. This is particularly common in Brisbane where clay soils are reactive and inconsistent.
Manages Moisture
A proper base with moisture barriers prevents groundwater and rising damp from penetrating the slab. Without moisture control, your concrete deteriorates, reinforcement corrodes, and the slab develops surface damage (spalling).
Extends Slab Life
A well-prepared base can add 10-15 years to your slab's lifespan. Poor base preparation is the leading cause of premature concrete failure in Brisbane.
Types of Base Material
Crushed Rock (Gravel)
What it is: Quarried rock broken into roughly 20-40mm pieces with fines (dust) mixed in.
Use: General fill and base material, most common choice for shed slabs.
Advantages: Inexpensive, readily available, drains well, easy to compact.
Disadvantages: Doesn't compact as firmly as road base, can settle over time if not properly prepared.
Road Base (Recycled Asphalt or Crushed Rock Mix)
What it is: Machine-crushed gravel with careful gradation of stone sizes plus clay/fines. Compacts very firmly.
Use: Premium base material, standard for garage slabs and larger projects.
Advantages: Compacts to high density (up to 95%+ Standard Proctor compaction), very stable, resists movement, ideal for reactive soils.
Disadvantages: Slightly more expensive than crushed rock, but worth it for durability.
Recycled Concrete
What it is: Broken-up old concrete, re-crushed.
Use: Can be used as base material if properly sized and compacted.
Considerations: Works well if properly graded, but the source material matters. Quality recycled concrete performs like road base; poor-quality material doesn't compact well.
Standard Base Preparation Steps
1. Excavation
The site must be excavated to the correct depth. For a 100mm slab with 100mm base, you're typically excavating 200mm (sometimes more depending on soil conditions). The excavation must:
- Remove all unsuitable material (organic soil, topsoil, debris)
- Be deep enough for the base layer and slab thickness
- Provide a stable subgrade (the natural ground beneath the base)
2. Subgrade Assessment
The natural soil beneath the base must be assessed. In Brisbane, this is usually clay – sometimes extremely reactive clay. A poor subgrade will affect the entire slab:
- Good subgrade: Firm, stable, not prone to movement (sandy clay)
- Poor subgrade: Soft, wet, or highly reactive clay that swells/shrinks with moisture
If the subgrade is poor, additional stabilisation might be needed (thicker base, better drainage, or stabilised fill).
3. Compaction of Subgrade
Before laying the base, the subgrade should be lightly compacted to firm it up. Use a plate compactor or similar equipment. Don't over-compact – you're just removing air and firming the soil, not crushing it into concrete hardness.
4. Laying the Base Layer
Spread the road base or crushed rock in layers of approximately 100mm (or slightly less). Don't dump 200mm all at once.
5. Compaction (Critical!)
This is the most important step. Each 100mm layer must be compacted to high density using a plate compactor (vibrating compactor). Proper compaction:
- Removes air voids in the material
- Creates high density and stability
- Prevents settling after the slab is poured
- Should achieve 90-95% Standard Proctor compaction
Under-compaction is one of the most common mistakes. A base that feels firm when you walk on it might not be adequately compacted. Use a plate compactor – no shortcuts.
6. Levelling
The base surface must be level (or to the correct grade if the slab needs to slope for drainage). Use a laser level or straightedge to check. The formwork (the edge guides for the concrete) will be set on this base, so accuracy matters. Bumps and hollows = uneven slab.
7. Moisture Control (Vapour Barrier)
If required, a moisture barrier is laid on top of the compacted base. See section below on moisture barriers.
Recommended Base Thickness
| Project Type | Slab Thickness | Recommended Base | Total Depth |
|---|---|---|---|
| Small garden pad | 75mm | 75mm road base | 150mm |
| Standard shed slab | 100mm | 100mm road base | 200mm |
| Larger shed/garage | 100mm | 150mm road base | 250mm |
| Garage slab | 100-150mm | 150mm road base | 250-300mm |
| Reactive soil areas | 100mm | 150-200mm stabilised base | 250-300mm |
Key point: 100mm of compacted road base is the standard for typical shed and garage slabs. In areas with reactive clay or poor drainage, thicker base (150mm) or stabilised material is often recommended.
Brisbane Soil Types and Base Requirements
Reactive Clay (Common in Brisbane)
Brisbane's predominant soil is clay, much of it highly reactive. This clay expands when wet and shrinks when dry – exactly what you don't want beneath a slab.
Impact on base: A thicker base (150mm vs 100mm) provides a buffer between the slab and the reactive clay. A well-compacted, stable base prevents the slab from moving as the clay moves beneath it.
Best approach: Use good-quality road base (not just crushed rock), compact it thoroughly, and consider a moisture barrier to control the moisture cycling in the clay.
Sandy Soils (Some Brisbane Suburbs)
Areas like the bayside suburbs sometimes have sandy subgrades. Sand can be unstable because it shifts easily.
Impact on base: Sandy bases need firm compaction and might benefit from a finer material (road base with more fines) that compacts more securely than coarse gravel.
Fill/Disturbed Ground
Many Brisbane properties have been filled or had previous structures. Fill ground can be very unstable if not properly compacted when it was originally placed.
Impact on base: If the subgrade is fill, it must be compacted before the slab base is laid. Sometimes additional base thickness is needed.
Compaction Requirements
What Is Compaction?
Compaction is the process of using machinery (a vibrating plate compactor) to force particles together, removing air voids and creating high density. A compacted base is stable and resists settling and movement.
How Much Compaction Is Needed?
Standard practice calls for 90-95% Standard Proctor compaction. This is a measure of density – the ratio of the material's actual density to its maximum theoretical density.
For practical purposes: the base should feel absolutely solid and immovable when you walk on it. You should not be able to sink into it or kick rocks loose. A properly compacted base will not shift under foot traffic before the concrete is poured.
Why Under-Compaction Fails
If the base is under-compacted, the material continues to settle after the slab is poured. This causes:
- The slab to crack as it settles unevenly
- Depressions in the slab surface
- Structural problems (the slab is no longer properly supported)
Vapour Barrier / Moisture Barrier: When and Why
What Is a Vapour Barrier?
A vapour barrier (also called a moisture barrier) is typically a sheet of plastic film laid on top of the compacted base, just before the concrete is poured. It prevents moisture from rising through the slab.
When Is It Needed?
Definitely needed for:
- Garage slabs (where vehicles and people spend time)
- Areas with high water tables
- Brisbane clay soils with poor drainage
- Any slab where moisture protection is critical
Often recommended for:
- Large shed slabs where storage includes moisture-sensitive items
- Slabs in low-lying or poorly draining sites
Sometimes omitted for:
- Small garden pads in well-draining areas
- Open-sided structures (carports) where air circulation is high
Why It Matters
Moisture rising through concrete (rising damp) causes:
- Spalling (surface deterioration)
- Rust in embedded reinforcement
- Interior dampness if the slab is covered
- Shortened slab lifespan
In Brisbane's humid climate with clay soils and reactive subgrades, a moisture barrier is a small investment (cost is minimal) that significantly extends slab life.
Sand Blinding Layer
A sand blinding layer is a thin layer (20-50mm) of sand or fine material laid on top of the compacted base, just before the concrete. Its purposes are:
- Fills voids: Creates a smoother surface for the concrete to sit on
- Reduces concrete loss: Prevents the concrete mix from being absorbed into the base during pouring
- Smooths the surface: Makes levelling easier
A sand blinding layer is often used, though not always mandatory. On a well-prepared base, it's a nice refinement that improves the final slab quality.
What Happens With Poor Base Preparation
Cracked Slabs
Cracking is often blamed on concrete quality, but it's usually caused by base problems. If the base settles unevenly, the concrete above cracks from differential movement.
Uneven Slabs
Depressions and high spots indicate an uneven or poorly compacted base. Water pools in depressions, accelerating deterioration. High spots are structural weak points.
Sinking and Settling
Over time (months to years), the slab sinks as the base compresses further. If extreme, this can affect door frames, wall alignment, and the structural support for buildings.
Moisture Damage
Without a moisture barrier and good drainage base, water rises through the slab, causing spalling, reinforcement rust, and interior dampness.
Premature Deterioration
All of the above lead to a slab failing in 15-20 years instead of lasting 30-40 years.
Common Questions About Slab Base
You can, but road base is better. Road base compacts more firmly due to its finer gradation and clay content. Crushed rock is cheaper but doesn't achieve the same density, making it more prone to settling. For small sheds, crushed rock with proper compaction might be acceptable. For garages or larger slabs, road base is strongly recommended. The cost difference is modest; the durability improvement is significant.
A properly compacted base should feel absolutely solid underfoot. You should not be able to press down on it with your heel, sink into it, or kick loose rocks. A professional compactor will test compaction using a nuclear density gauge or similar equipment, achieving 90-95% Standard Proctor compaction. If doing this yourself, use a vibrating plate compactor and make multiple passes (at least 3-4) over each layer until the material no longer visibly shifts.
For garage slabs and most Brisbane locations, yes. Brisbane's clay soils and humid climate mean moisture will rise through concrete without a barrier. For small open-sided sheds in well-draining areas, it might be optional. But for any slab where moisture control matters, it's worth the modest extra cost. A moisture barrier adds only a small percentage to total slab cost but can extend slab life by 10-15 years.
Use a thicker base (150-200mm instead of 100mm), use quality road base instead of crushed rock, ensure excellent compaction, and install a moisture barrier. Some engineers recommend soil stabilisation (adding lime or cement to the fill) for highly reactive subgrades. Your concreter can assess your specific soil and recommend the best approach. Don't skip base preparation in Brisbane clay – it's where most slab problems originate.
Yes, but compaction quality matters enormously. Any contractor using proper equipment and techniques can do good base prep. The risk is if they under-compact or don't understand Brisbane clay soils. Use someone experienced with concrete slab bases in your area. Ideally, the same team handling the concrete pour should oversee the base prep – they'll ensure it's done correctly for the slab they're about to pour.
Important Disclaimer
Not a licensed builder. This guide covers general base preparation principles. Specific requirements for your project depend on local soil conditions, building codes, and site assessment. Always have a professional assess your site before finalising base specification.
We specialize in small concrete jobs only – shed slabs, garage slabs, concrete footpaths, and small pads. For house slabs or commercial projects requiring structural engineering, consult a qualified structural engineer.
Final Thoughts
The concrete slab is only as good as the base beneath it. A proper base – compacted road base, correct thickness, good drainage, and moisture control – is the foundation of a long-lasting, crack-free slab.
In Brisbane, where clay soils are reactive and moisture control is critical, base preparation is not something to economise on. The difference between a 20-year slab and a 40-year slab often comes down to how carefully the base was prepared.
Ready to build your slab? Make sure the base is done right. Contact us today to ensure your base preparation meets Brisbane standards.