Most structures, be it a house, shed or a carport, need a solid foundation to sit on and to keep them stable, and there’s a range of options when it comes to deciding on what kind of foundation to use. One option is laying down a concrete slab. Here’s an overview of how concrete slabs work, their pros and cons, and how much it might cost you to lay one.
How much does a concrete slab cost?
The cost of a concrete slab will generally be calculated based on how big the area that needs to be covered is, and you’ll typically pay between $75 and $110 per square metre, according to tradie listing site, hipages.
However, the actual price can be influenced by many factors, including:
- What the slab is going to be used for
- The required thickness of the slab
- How easy the work area is to access
- The type of concrete and visual finish you desire
You may pay less for a concrete slab if you choose to lay it yourself, although you may need to hire or buy special equipment to complete the job, such as a cement mixer. However, you may want to consider how comfortable you are with doing so, and the extent to which you would be willing to pay that bit extra for professional expertise, particularly if it’s for a foundation of a building or structure you and your family will live in or use frequently.
Additionally, you will still generally need to pay for the services of a building certifier, who will need to inspect your building work before the concrete is poured.
How is a concrete slab made?
A concrete slab is formed by pouring a calculated amount of concrete into a pre-formed, reinforced area. The width and thickness of the slab will be determined by what you are planning to build on top of it – a slab intended to support an outdoor deck or patio will generally not need to be as thick as a slab intended to support an entire house or a slab intended for use as a driveway.
According to Cement Australia, the basic process for laying a concrete slab is as follows:
- Construct the timber barriers that will retain the concrete as it dries.
- Using the pre-constructed barriers, measure and mark out the area that needs to be dug out for the slab.
- Remove the barriers and excavate the marked area, allowing an extra 5cm of depth for a bed of material on which the concrete can sit.
- Put the barriers back in place, and put down a bed of crushed concrete 6-7cm thick – make sure to then level and compact the crushed concrete so that it reaches a final thickness of about 5cm.
- Mix your concrete in a non-porous vessel, and then pour it into the designated area, working slowly and making sure that all edges and corners are filled.
- Once all the concrete has been poured, thoroughly level it with a screed or trowel to ensure that the surface of the slab is flat and uniform.
- Once any excess water has come to the surface and evaporated, smooth over the surface with a trowel or screed.
- As soon as you’re happy with the surface of the slab, begin the process of moistening the surface and edges of the slab with a hose (set to a gentle spray) twice a day for the next seven days. This will help to ensure the surface quality and strength of the slab.
Cement Australia advises that the barriers holding a slab in place can generally be removed after 24 hours, and the slab can be walked on after three days. However, no heavy loads should be placed on the slab for at least seven days.
What are the benefits of a concrete slab?
While a concrete slab may not win any awards for looks, there are advantages to choosing one as a foundation, namely:
- Cost efficiency (if you DIY) – depending what it’s for, the cost to hire or buy special equipment and your level of DIY expertise, pouring your own concrete slab may be a viable prospect, and will generally be cheaper than paying a professional to either pour a slab for you, or an alternative foundation such as a timber one.
- Durability – concrete will generally hold up against weather, age, and general wear and tear better than a timber foundation would.
- Strength – ServiceSeeking.com.au advises that the usual thickness of a concrete slab is about 10cm, or 7.5cm for a light shed. A slab of concrete that thick will almost certainly be stronger than a wooden floor, which may bend, creak, or warp if enough weight is put on it.
- Thermal comfort – according to government sustainability resource Your Home, a concrete slab can help to regulate indoor temperatures due to the material’s capacity for storing and re-releasing heat when paired with good design. This in turn may help to reduce energy bills due to a lowered need for the use of fans and/or air-conditioning.
What are the drawbacks of a concrete slab?
Some of the possible drawback of using a concrete slab as a foundation may include:
- Aesthetics – there’s no real way to get around the fact that most people won’t find a concrete slab as visually appealing as a timber foundation. While the foundation of a structure, such as a shed, is quite a minor visual component in the scale of things, it may be quite important to some. That being said, you could potentially paint the visible parts of a concrete slab in order to make it more visually appealing.
- Slab cracks – according to Investopedia, a concrete slab can crack in certain situations, and if this happens it can be difficult and expensive to repair.
- No under-house space – houses built on timber foundations usually have space under the house, and this is often used to store air conditioning units if the house has them. A house built on a slab won’t have this space, which means that an air conditioning unit, for example, may take up space elsewhere.
- Environmental concerns – according to the not-for-profit sustainability advocacy group ‘renew.’ concrete has a not-insignificant environmental impact, including associated carbon dioxide emissions, the energy required to create it, and concerns about it causing runoff water to have an impact on aquatic life.
Is a concrete slab right for my build?
Whether a concrete slab is the right choice of foundation for your building or surface will largely come down to your budget, what exactly you’re building, the climate you live in, and your personal visual tastes. Concrete slabs can be a cheaper option than other foundations, but construction industry lobby group Cement Concrete & Aggregates Australia warns that concrete can run the risk of cracking in extreme weather, and some may not find it particularly visually appealing. And if the slab is to support your home, you may want to consult with a professional builder or structural engineer before deciding which kind of foundation is best.