An Owner builder slab in Australia typically use one of three approaches:

– Built off the ground with timber and joists

– Waffle Slab

– Raft Slab

Many stories exist that are concerning when it comes to slabs, and owner builders must determine which one is right for them- a waffle slab or a raft slab. Understanding their distinctions will aid in your decision. Your engineer will be important in this process; waffle and raft slabs usually involve piers or screw piles.

Both options have advantages and disadvantages, so owner builders need to understand each option before deciding. Consulting with an engineer is essential to select the appropriate slab for your project.

What is the difference between a Waffle and Raft Slab?

Waffle Slabs are quicker, simpler, and approximately 30% cheaper to set up than other builds. They can be built directly on the ground, mostly 800mm thick and have a reinforced concrete footing with a slab system. These slabs include edge beams and internal beams that run in both directions. Polystyrene is used between the beams — non-structural void formers that create a waffle-like style when viewed from underneath to allow the concrete to be poured. It’s essential to ensure good drainage around these types of structures, and an engineer should provide a plan to show the position of pods and reinforcement, as well as pier or screw pile locations.

Thanks to their reinforced steel design, Raft Slabs offer additional stability and support for loads. Thickened edges typically span 300 – 700mm, and the slab is approximately 100mm thick. This foundation style is especially beneficial in loose soil conditions as it can disperse the weight of the construction across a larger area. Some engineers recommend sloping and high wind blocks for extra strength.

Owner Builder Tips 

As an owner builder, you must work closely with your engineer to ensure that your slab, raft or waffle foundation is built correctly. Here are some tips to help you get the most out of your relationship with your contractor:

1. Be clear about what you want. When you meet with your designer, explain precisely what you’re looking for in terms of the foundation for your home. This will help them understand your vision and create a foundation that meets your needs.

2. Ask plenty of questions. Don’t be afraid to ask your contractor questions about the process of building a raft or waffle foundation. The more you know about the process, the better you’ll be at making decisions.

3. Be patient. Building a foundation can be complex and time-consuming, so patience is essential throughout the process. Trust that your contractor knows what they’re doing and allow them to do their job so that you have a high-quality foundation for your home.

Common Slab Questions from Owner Builders

These are some of the queries people often have about slabs.

At what time should the slab be laid?

In a perfect world, the drainage must first be approved by the surveyor before laying the slab. Making this timing dependent on weather conditions, dryness is key to making sure it is established.

What is the process of creating the slab?

The pad is created through drainage work, for which concreters smooth out uneven areas by spreading sand over the slab’s area. Following this, formwork and reinforcing steel are placed. Once a surveyor inspects the preparation work, and it passes, the slab is ready to be poured with concrete the following day.

Do you want the slab to be even and flat?

Most slabs are usually covered on the inside (whether with carpet, tiles etc.) and outside (using bricks or landscaping). Therefore, there’s no point in paying extra for your concrete to make the areas that will eventually be hidden smooth.

The spots that weren’t considered, such as the garage, verandah and alfresco area, would need to be finished properly by the concreter.

Are there any issues with having cracks in my slab?

Superficial cracks are common, but they shouldn’t be a cause for concern; since they don’t affect the slab’s structural integrity. The exterior of the slab dries quicker than its core, leading to tiny fissures. 

The concreters left a piece of metal protruding from the slab.

This rod serves as an earthing conductor for the electrical system. When the meter box is fixed to the frame, it will be connected to the earthing wire.

The MBA talks about shrinkage.

Conclusion

As an owner builder consulting with your engineer is mandatory to ensure all factors are considered and the design suits the specific site. This includes such factors as sloping blocks, high wind areas and geotechnical circumstances. Once all these elements have been considered, your engineer will provide the necessary advice. 

If you still have any questions about raft and waffle foundations after reading this article, feel free to contact us by either email or [email protected] or by our contact form to can provide expert advice on which option is best suited for your project.