A waffle slab home can provide cost savings, but you need to do your homework. Photo: Cornell Engineering.
A Wagga-based structural engineer has urged people considering a ‘waffle-pod’ home to properly research their land and soil conditions before building.
The warning comes after Region’s exclusive report on Adrian and Letticia Di Lullo, who were awarded $1.25 million after the NSW District Court found their newly built Jerilderie home had suffered extensive damage after its concrete slab moved.
The court found poorly compacted sandy fill had allowed water to reach the edge of the slab, causing it to lift and resulting in extensive damage to the house. The court ultimately found the best solution was to demolish the home and rebuild it – though only after the Di Lullos endured a costly nine-year legal battle.
A waffle slab is a type of reinforced concrete footing and slab system built on the ground. Plastic or polystyrene ‘pods’ create voids between a series of internal concrete beams.
When viewed from underneath, the beams resemble a waffle, giving the system its name.
The waffle pod is a generally cheaper alternative to the more conventional raft footing system, which tends to be dug into the ground.
Nicholas Gleeson, a structural engineer and director of Wagga-based Xeros Piccolo Consulting Engineers, provided expert evidence in the court proceedings on the Jerilderie home.
He said he could not comment on the specifics of the judgment because the matter remained ongoing, but wanted to provide general advice to people considering a waffle-pod slab. Mr Gleeson answered questions from Region.
In your professional opinion, what is the fundamental problem with waffle-pod slabs that homeowners need to understand?
There is no fundamental problem with a waffle pod. They can provide a cost-efficient solution on the right block of land. Problems arise when they are used incorrectly or on a block which is not suitable for them. As the cost of construction is increasing, everyone is looking at ways to cut costs. One way is using a waffle pod. This can come at the cost of performance if misused.
Are there particular soil types or site conditions around the Riverina where people should be particularly cautious about using waffle pods?
Residential blocks are classified by how much the soil will expand when wet and shrink when dry, called the soil reactivity. The Riverina contains a wide range of soil classifications from non-reactive sites to highly reactive blocks. Xeros Piccolo would generally avoid waffle pods on highly reactive blocks.
If you were building your own house in the Riverina, would you personally choose a waffle-pod slab? Why or why not?
If I was building on a flat block which is moderately to non-reactive area, with a local reputable and experienced builder I would consider a waffle pod. However, a raft slab is generally more reliable.
What is the one piece of advice you would tell every person who is about to buy or build a house with a waffle-pod slab?
It requires considerable skill and experience to design residential slabs therefore we recommend to use an engineer that is experienced with residential design in the Riverina. Ensure that the plans are produced specifically for your house and block. Problems can arise when the plans are drawn up interstate or overseas. This can result in plans and details which are not specific to your house and site and therefore may not be the correct details to use.
If you are going to buy a house, ask an engineer to conduct a structural inspection of the house and plans. They will be able to guide you on the risks associated with your new house.
