Foundations and Footings
What is the difference?
As the saying goes, a solid foundation in life sets you up for success. The same is true when it comes to building a house. There are many terms builders use that aren’t entirely clear to would-be owners, especially those building for the first time. In order to decode some of the terms we use, we are going to start from the ground up… with the foundation.
A footing is what stands (literally) between the load of the house and the ground on which it is built.
Footings play several important roles including distributing loads evenly across the ground, anchoring buildings against high winds and protecting them from expansive soils and soil moisture. Footings that are well designed and constructed help ensure our buildings are safe and stable.
Foundation is the soil upon which the entire structure rests and must be able to support the weight of the building without failing. One of the first steps when looking at a site is to access the ground or soil to understand what the foundation is. Footings will then be designed to take into account the site conditions and type of foundation.
Different types of foundation soil offer different levels of stability and drainage. Generally, footings founded on bedrock will settle the least, while footings founded on sandy soil tend to settle quickly and evenly.
Clay soil is notoriously tricky as it absorbs and loses water dramatically in response to weather conditions. In wet seasons, clay soil expands by up to 50%, while during dry months, it shrinks by the same amount. This constant movement can cause mean footings move unevenly leading to cracks appearing on finished surfaces over time.
A geotechnical report provides the necessary information about the soil type, behaviour, and limitations. This allows the Structural Engineer to design an appropriate footing for any project.
What are Different Types of Footings Used in Australian Homes?
There are a range of footings being used in Australian houses. Some of the most common are:
Strip and Stumps
One of Australia’s most commonly used types of footings is the strip and stump footing system. This system consists of two main parts: a strip of linear rectangular concrete or bluestone, and isolated posts known as stumps supported on pad footings.
The strip footing supports external walls, while the stumps support the internal floors. This system is adaptable and can be built on flat and sloping sites. In addition, it creates a crawl space that can be used for storage.
A reinforced concrete raft slab is a strong and durable foundation for any building. The integral concrete beams provide added strength and support and can be spaced and positioned according to needs. This type of footing is well-suited to difficult or unstable ground. Differential movements of a raft slab can also be minimized with proper design and construction.
Two different types of raft slabs widely used in Australia are:
Waffle slabs have become a popular choice for builders in recent years due to their faster, easier and cheaper build process. Waffle slabs are typically 85 mm thick and supported by edge beams around the perimeter and internal beams (known as ribs) that run in both directions, creating a waffle-like shape when viewed from below.
Waffle slabs sit directly on the ground, either on compacted fill or natural soil. Due to their shallow-founded beams and relatively low stiffness, they have become problematic on highly reactive soils.
Waffle Rafts are the style of footing currently being installed at Prestwood Estate by Homecorp.
Once plumbing service pipes have been installed, a barrier layer is laid on the site and reinforcing is laid out. Wafffle Rafts are then placed and spaced ready for concrete to be poured on top.
Stiffened Raft typically consists of a 100mm thick steel reinforced slab with stiffened internal and external beams embedded into the Natural foundation soil for extra strength and support. The edges of the stiffened raft slabs are from 300mm to 1100mm thick, depending on the type of construction they support (e.g. clad frame, brick veneer, full masonry etc.).
Suspended slabs on piers
Some sites can be very problematic due to the significant reactive nature of the foundation soil or the presence of deep layers of unstable Fill material that are unsuitable for use as a foundation. In these cases, deep concrete piles are installed to support the concrete raft slabs and take the load down to the Natural soil, where the amount of movement, due to reactivity, is somewhat limited. The piles are rarely structurally connected to the concrete slab above, and as such, this footing system is very susceptible to soil heave.
Slab on Ground
This is usually made from reinforced concrete and is approximately 80mm thick. The perimeter footing is usually slightly thicker, which helps to increase the stability of the slab. The slab is typically built on a layer of crushed gravel and a sheet of waterproofing plastic to prevent moisture from seeping up from the ground. This type of footing is generally suitable for non-habitable areas, such as garages and driveways and is installed on flat sites.
How deep should footings be?
The depth of a footing is dictated by the type of soil beneath it and the amount of bearing capacity required. The heavier the building is, the deeper the footings should be. Also, soils that are more reactive necessitate a deeper footing in order to minimize differential movements. One of the roles of design engineers is to account for adverse factors present on site, such as trees, which could potentially impact on the footing’s performance.
The Australian Standard AS2870 Residential slabs and footings provides minimum requirements for footing design; however, this standard should be treated as a general guideline only. Every site must be thoroughly tested and analyzed by both a Geotechnical Engineer and Structural Engineer to ensure an appropriate footing type and depth is proposed.
Homecorp House and Land packages are designed so all the hard work, including things like geotech reports and engineering assessements, are all taken care of. Our team of experts look at each lot and pair it with the a plan and layout that capitalises on what that site has to offer.
To find out more about where Homecorp are developing just reach out to the Homecorp team today.