The Complete Guide to Concrete Slabs in Australia
Pouring a concrete slab is one of the most common DIY projects for Australian homeowners. Whether you need a solid base for a garden shed, a new driveway, or an outdoor entertainment area, getting the concrete quantity right saves you money and avoids the frustration of running short mid-pour. This guide covers everything you need to know about planning, sizing, and costing a concrete slab.
How to Size Your Concrete Slab
Before calculating concrete volume, you need to determine the right slab dimensions for your project. Shed slabs should extend 100-200mm beyond the shed walls on each side to provide a stable edge. Driveway slabs need to be wide enough for comfortable vehicle access - a minimum of 3 metres for a single car, or 5.5-6 metres for a double driveway. Patios should be large enough for your furniture layout with room to move, typically starting at 3m × 3m for a small setting.
Site Preparation for Slab Pouring
Proper site prep is just as important as the concrete itself. Strip all topsoil and organic material from the area, digging down to stable natural ground. Compact the sub-grade with a plate compactor, then add 100-150mm of road base (crushed rock) as a sub-base. Compact this in layers of no more than 100mm at a time. Level the surface accurately using a string line and spirit level - any unevenness in the base translates directly to uneven concrete thickness and wasted material.
Setting Up Formwork
Formwork creates the mould for your slab. Use straight, sturdy timber (typically 90mm × 45mm or 100mm × 50mm for a 100mm slab) pegged firmly into the ground. The top of the formwork sets your finished slab height. Include a slight fall of 1-2% away from buildings for drainage - that's a 10-20mm drop per metre of length. For driveways, the fall should direct water toward the street or a drainage point. Oil the inside of formwork with mould release or even diesel to make stripping easier.
Understanding Concrete Volume for Slabs
Concrete volume for a rectangular slab is calculated as Length × Width × Thickness, with all measurements in metres. The result gives you cubic metres (m³). For example, a standard 3m × 3m shed slab at 100mm (0.1m) thickness needs 3 × 3 × 0.1 = 0.9 m³ of concrete. Always add 10% for wastage to account for spillage, uneven ground, and mixing losses, bringing the total to 0.99 m³ or approximately 1.0 m³.
Control Joints and Crack Prevention
Concrete shrinks as it cures, and without control joints it will crack randomly. Cut control joints (or tool them into the wet concrete) at intervals of no more than 25-30 times the slab thickness. For a 100mm slab, that means joints every 2.5-3 metres in each direction. Joints should be cut to a depth of one-quarter to one-third of the slab thickness. For a shed slab, this might mean a single cross-shaped joint dividing it into four panels.
Finishing Options for Your Slab
The surface finish you choose affects both appearance and cost. A basic broom finish provides good grip and is the cheapest option, ideal for sheds and driveways. Exposed aggregate involves washing away the top layer of cement paste to reveal the stones underneath, giving a decorative textured surface popular for driveways and paths. Coloured concrete uses oxide pigments mixed through the batch or applied as a surface hardener. Stamped concrete can mimic brick, stone, or tile patterns but adds significant cost.
Weather Considerations for Slab Pouring
In Australia's varied climate, timing your pour matters. Avoid pouring in temperatures above 35°C as rapid drying causes surface cracking and reduces strength. Similarly, avoid pouring if overnight temperatures will drop below 5°C, as cold slows curing dramatically. Ideal pouring conditions are 15-25°C with overcast skies. If pouring in summer, consider starting early morning and have enough help to pour and finish quickly before the concrete sets too fast.
Pre-Calculated Slab Guides
View detailed calculations for popular Australian slab projects: