Waffle slab construction gets a lot of flak within the industry as they have been linked with severe cracking in houses in recent years. However, just like any tool, there are certain circumstances in which they are the better option.
The Rise and Fall of Waffle Slabs
Over the past 20 years, waffle slabs have become more common by new home builders as a quick and cost effective way of building a concrete slab on ground.
While the finished surface of the slab is flat, the underside of the slab has distinctive criss-crossing concrete ribs at close intervals which resembles the tasty Belgian treat from which it derives its name. The ‘ribs’ of the waffle slab are formed up by void formers which are typically polystyrene cubes.
As waffle slabs became more popular with builders, especially volume builders, because of economics, they started being specified and used in areas and sites where they were not suitable. In 2014, in Hooper v. Metricon and later in Softley v. Metricon, the Victorian Civil and Administrative Tribunal found that a defective waffle slab was the cause of severe cracking. Many believed that this was the tip of the iceburg for many developments in Melbourne’s west that were built on similar slabs.
This stain on waffle slab construction has followed it around ever since, with many builders and developers painting the waffle slab with the same brush. However, it’s probably time to set the record straight: just like any tool, there is a right way and time to use it.
Red Flags for Waffle Slabs
The underlying economics that drove the industry towards waffle slab construction are all still valid. Waffle slabs attract better energy rating and are better for insulation, they are easy to setup and build, and are a fantastic solution for piered suspended slabs on filled sites.
However, the waffle slab is not without its faults, and under specific circumstances they are unsuitable for residential construction.
Some site-specific warning signs that you should be on the look out for are:
Reactive Ground Conditions
In the instance where your site is classified as a reactive site, and is subject to high seasonal movements, a waffle slab may not be strong enough to bridge the gaps in local depressions that may occur. We generally recommend that H-class sites or worse not use waffle slabs.
Sloping Sites
Waffle slabs sit higher up in the ground than raft slabs and so they are less suitable for sloped sites where extensive levelling would need to take place first. In its final condition they may also be more prone to erosion and land slip.
Drainage Issues
Sites that are prone to drainage issues around the slab may be subject to seasonal or occasional swelling which typical waffle slabs may not be strong enough to handle. This is a similar sort of effect to having highly reactive soils (mentioned previously).
High Wind Locations
Waffle slabs use less concrete than raft slabs. While this is the reason why they are cheaper, the flip side is that they are not as good at acting as anchors for houses in high wind areas.
Should You Use Waffle Slabs?
The answer to whether you should use a waffle slab in your next townhouse or unit development is: definitely maybe. If your development is on appropriate founding soil close to the surface and is a relatively flat site, a waffle slab may end up being more energy efficient, use fewer materials, and cost less to build, in comparison to a traditional raft slab.
As with the rest of life, it’s rarely a one-size-fits-all solution, and circumstances vary from site to site. As such, we recommend that aspiring developers find a competent consulting structural engineer who can assist in making this determination for their site (we happen to know a few of them).
If you are looking for structural and drainage design engineers for your next unit, townhouse or multi-dwelling development, please get in contact with us so we can provide you with a comprehensive fee proposal for our services. We look forward to partnering with you on your next project.
