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Understanding Concrete Slab Thickness

One of the most critical parts of new construction inspections in Australia is the review of the initial concrete slab followed by other subsequent concrete slabs for upper floors. The majority of Australia’s building developers used concrete slab foundation because of the relatively sandy soil that covers most of Australia.  It is crucial to adhere to the standard slab thickness when constructing a concrete slab. Before we dive into how thick a concrete slab should be, let us look briefly into how concrete slabs are built.

Process Of Casting Concrete Slabs

The process of laying or casting concrete slabs begins with the preparation of the ground for construction. This stage involves the removal of grass and other vegetation and then leveling the ground. When this is completed, the next stage is the construction of a sand pan on which the concrete slab will be cast.

The next step is the installation of service penetration, including inward water, outward grey, and black water, gas, and electrical wiring. Once the ground is compacted, the next thing is to spray the termite barrier, lay waterproofing membranes, fix reinforcing steel, and then pour concrete.

Immediately after the slab has been laid, it will take a week to cure. After the slab has been cured, the drains and sewages are then connected.

Types Of Concrete Slabs And Their Thicknesses


One-way concrete slabs are made using the cast in situ method, which involves the following three steps:

  • Installation of formwork
  • Placement of reinforcement
  • Pouring of fresh concrete

The standard thickness for a one-way slab’s middle and column strips is 130 mm and 140 mm, respectively. One-way slabs are most suitable for beams spanning three to six meters and carrying a live load of three to five KN/m2.


These concrete slabs are constructed in the same manner as one-way or two-way slabs, with either walls or columns directly support them. It is a straightforward process to build flat plates slabs because the framework is quite simple.

Flat plates are ideal for spans of six to eight meters and live load between three to five KN/m2. The prestressed type can span eight to twelve meters, and it can be constructed as post-tensioned slabs.

The minimum slab thickness for flat plates is 191 mm, and this is chosen to satisfy the code requirement on two-way shear strength and deflection.

The benefits of using flat concrete slabs are as follows:

  • Low-cost framework
  • Exposed flat ceiling
  • Faster construction


These are very similar to one-way slabs on beams, but they require more supporting structures as the two-way slabs are supported on both sides. They are suitable for spans between six to nine meters and live loads of three to six KN/m2. The minimum thickness for any two-way slab without an interior beam should not be less than 125mm for slabs without drop panels and 100mm for slabs with drop panels.


These are reinforced slabs that are supported directly by capitals or columns without beams. They are easy to construct and require no framework. The loads are transferred directly to the column.

This type of concrete slab is suitable for spans of six to nine meters and for a live load of four to seven KN/m2. They make use of more framework than flat plates because of column capitals. The minimum slab thickness is about 125 mm for continuous interior panels without drops.


This type of reinforced concrete slab is made up of square grids with deep sides. To make a grid slab, you have to go through the following processes:

  • Install formwork
  • Place pods on shuttering
  • Install reinforcement between pods
  • Install pods
  • Install steel mesh on top of pods
  • Pour concrete

Waffle slabs are suitable for spans of nine to twenty-five meters and live loads of four to seven KN/m2. The framework for this type of concrete slab is expensive and includes the use of pans.

The thickness of waffle slabs is usually between 75 mm to 130 mm. As a standard, the thickness should be 1/24 of the span. The width should be around 130 mm to 150 mm, and the ribs are usually reinforced using steel rods, with the distance between the ribs kept at 915mm.


A one-way joist slab is also known as a ribbed slab and is made up of a floor slab that is 50 to 100 mm thick, supported by joists (concrete ribs). The joists are typically tapered and evenly spaced at a distance of not more than 75 mm. The beams are also supported by girders that rest on columns.

One-way joist concrete slabs are preferred for spans of six to nine meters and a live load of four to six KN/m2. The concrete and steel quantities are low because of the deep joists, but the framework that is needed is quite expensive.

Standard Concrete Slab Thickness In Australia

In Australia, concrete slabs must be fixed in adherence with the Australian Standards AS 2870 and AS 3600. The most common slab thickness in Australia is between 85mm and 100 mm.

The thickness of concrete slabs is usually stated in specification documents. These documents are agreed upon and signed between the builder and the client.

For building sites which are generally sandy and flat, an 85 mm slab is sufficient for a single story residential building construction. For those developing a two-story building or a building on a hill, engineering requirements stipulate that a 100 mm concrete slab should be used with thickening under the walls.

For those using long concrete slabs with lengths greater than 20 meters, any reduction in 85 mm for the length should be compensated with more steel reinforcement. The essence of the steel reinforcement is to enhance the tensile strength of the concrete and manage the width of shrinkage cracks.

Minimum Concrete Slab Thickness You Can Use

The minimum thickness for concrete slabs should be 50 mm. Anything less than this will be a disaster. This thickness is for the concrete slab alone and does not include that of the gravel base if you are using gravel.

Again, the thickness of the concrete slab will depend on what you intend to do with the slab. It is essential to adhere to thickness requirements for concrete slabs because it influences the quantity of concrete and cement that needs to be poured and, by extension, the project’s total cost.

Ideal Depth Of A Concrete Slab


The minimum requirement for a light shed is 75 mm of concrete. However, a 100 mm concrete slab is recommended. You need to check the council regulations when constructing a shed because you cannot construct your shed anywhere in all Local Government areas. When you have checked, then you can mark the soil where you want to dig and prepare the framework for concreting.


A garage slab should at least be able to hold the weight of your car and other car-related items that you keep in your garage. This means the weight on the garage slab is quite extensive. The thickness of a concrete slab for a garage should be at least 150 mm.

If you are a truck driver or you want to have a double garage for two small cars, you need to increase the thickness to 200 mm. If you use your garage for large loaded trucks, you will also need to seal your concrete and protect it from shrinkage cracking.


The patio should have a slab thickness of at least 150 mm for it to be solid, but you can reduce this thickness to 100 mm if you know that the patio will not expect heavyweight. Other factors that determine patio concrete slab size include the type of soil on which you are building your deck and the weight you expect the patio to carry.

For instance, if all that will be on the patio is just a small table and two chairs on well-drained soil, a patio of 75 mm will do. However, if you expect to have concrete benches, tables, BBQ, and an outdoor kitchen, then the minimum should be 150 mm.

You also need to carry out a soil test to determine the composition of your soil. If the soil is not well-drained, you must construct a base for the concrete slab, and this is done by:

  • Digging up a hole
  • Placing framework around the hole
  • Filling up the hole with gravel
  • Pouring your concrete over it


Ideally, the concrete slab for a driveway should be about 100 mm thick, and there is no need for a gravel base. All you need to do is get the soil underneath prepared by removing all the big rocks while making sure that the soil is well compacted.

If the driveway is expected to witness a heavy load, such as heavy-duty vehicles, you will need to put this into consideration when preparing the slab. In this situation, the slab should be 120 mm instead of 100 mm, and you should put a sealer on the  concrete so that it doesn’t get damaged easily.

Another thing you need to do is to ensure that your driveway has a slope; this is to ensure that the slab is adequately drained.


Constructing a concrete path is one of the easiest projects. The slab for a path does not need to be more than 75 mm thick. It doesn’t require a gravel base, but be sure to provide an expansion joint every two meters. You also don’t need to drain the pathways in any way since they are not wide enough to cause any issue.

The 75 mm specified above is the minimum requirement for a concrete path. However, most people use 100 mm of thickness and also add a minimal gravel base to enhance the quality and durability of the project.

Call A Building Inspector To Check Your Slab

Slab assessment is a vital part of new building inspections. When constructing a house, it is indispensable to hire a qualified building inspector to properly inspect the slab and ensure that its dimensions (thickness, width, and depth) comply with the Australian Standards for concrete slab thickness.

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