Most houses in Australia are typically built with either a timber floor built off the ground with the required clearance or a concrete slab on ground or “Raft Slab”. Concrete slabs on ground are becoming the more common type of construction especially in the new sub-divisions which are flat sites. Although sloping sites can be accommodated for with slab on ground construction. There are two types of raft slabs, a traditional raft and a waffle raft. This gives a stiff base to build the walls off, with extra stiffening under brick walls to prevent damage as a result of the contraction and expansion of reactive clay foundations which are prevalent in Australia. Articulation joints should always be provided in any brick work to limit the effects of any movement which may still occur as result of seasonal wetting and drying of reactive clay soils.
A traditional raft has edge beams and internal beams that are excavated into the ground with an excavator.
A waffle raft uses polystyrene void formers or “waffle pods” to form up the edge beams and internal beams and give the slab the required stiffness, without using excessive amounts of concrete. Note that the polystyrene is not structural, it is simply a void former.
Note that both of the above examples show brick veneer construction but other types of construction such as clad timber frame and full brick can be done with either type of raft slab. Alternative types of construction such as straw bale and rammed earth can also be done with the above slab types with special consideration by a structural engineer.
The slabs are typically piered with either mass concrete augured piers or screw piles. However, it may be possible to not have piers, in particular if the building is to be constructed with a light weight clad frame, rather than brick veneer or full brick construction. Pier spacings are typically 2.4m centre to centre under brickwork and 3.6m internally.
There have been some negative reports in the media about waffle slabs, especially with regards to some new houses built on the outskirts of Melbourne that have problems with foundation movement and subsequent cracking and damage to walls and floors. It is our opinion that these reports have misguidedly blamed waffle slab construction for the issues when these issues are related to Geotechnical factors which would have caused problems with any type of construction unless the Geotechnical issues were known about before the design stage and could have been considered and appropriate measures taken to prevent or allow for the movement to occur without damaging the structure.
Waffle slabs are the preferred method of construction for many project builders and prestige home builders alike. This is due to the simplicity of the construction and the potential to save money in concrete volume. Rather than excavating beams, which can expand and require a far greater volume of concrete than estimated, the pods are placed on a flat bench and the required concrete can be estimated very precisely and will usually be much less than that required for a traditional stiffened raft.
Sloping sites can be catered for with waffle raft construction. We provide all of the relevant details for dropped edge beams, shallow and deep steps, wet area set downs and attached water tanks.
We provide a plan which shows exactly how the pods and the reinforcement is to be placed and location of any piers or screw piles. We also provide all of the specifications for slab including concrete strength. We tank into consideration any special geotechnical factor associated with the site.
Contact Gilcon to have your waffle slab designed and drafted. For a slab alone we typically charge $550 + GST. If there is a second storey and upper level framing is required and/or building over sewer details are required, there would be extra charges for these.