Expert Builder: Screw Piers in Granny Flats

July 10, 2023

What are screw piers and when are they used in Granny Flat Construction?

Screw piers, are foundation elements sometimes used in Granny Flat construction to support and stabilise the concrete that the structure bears on. Screw piers are particularly useful in areas with poor soil conditions or where traditional foundations are impractical or impossible to install, giving our clients far more options for the land that a Granny Flat can be built on.

What are screw piers?

Screw piers are sometimes known as helical piers or screw anchors. They consist of a steel shaft with helical plates or flanges attached to the end. The plates act as a screw and are driven into the soil using hydraulic or electric torque motors, creating a secure foundation for the structure.

Screw piers offer a secure foundation by using the surrounding soil to provide stability and support to the structure. When the helical plates are driven into the ground, they create a large surface area that helps distribute the weight of the structure evenly across the soil. This, in turn, helps prevent settling or shifting of the structure. As the helical plates turn, they pull the shaft down into the soil, compacting it and creating a tight grip. The torque motor provides continuous feedback on the installation torque, which is used to determine when the pier has reached the desired depth and load capacity.

The depth of the screw piers depends on the load capacity required for the structure and the soil conditions. The installation depth can vary from a few feet to several dozen feet. The depth of a screw pier is determined by the torque, measured in kilonewtons (kN), that the engineer has determined with the use of a geo tech soil test.

Once the screw piers are installed, they are attached to the structure using brackets or other connectors. The brackets are then anchored to the screw pier, creating a secure and stable foundation for the structure.

Expert Builder Content Screw Piers

What are the advantages of screw piers?

Screw piers offer several advantages over other foundation types. They can be installed relatively quickly and easily, they are also environmentally friendly, as they do not disturb the soil or surrounding landscape and can be easily removed and reused if necessary. Additionally, screw piers provide excellent load-bearing capacity and can be used to support structures of various sizes and weights.

Here are some of the main advantages in more detail:

  • Quick and Easy Installation: Screw piers can be installed quickly and easily using hydraulic or electric torque motors, without the need for excavation or heavy equipment. This can be particularly useful on Granny Flat sites with limited access or difficult terrain.
  • Versatility: Screw piers can be used to support structures of various sizes and weights and can be installed in a range of soil conditions.
  • High Load Capacity: Screw piers can support significant loads and provide a stable foundation for structures. They are designed to distribute the weight of the structure evenly across the soil, reducing the risk of settling or shifting.
  • Cost-Effective: Screw piers are a cost-effective alternative to traditional foundation systems, especially in areas with poor soil conditions or limited access that would otherwise require even more work for excavating equipment or the use of cranes. They can be installed quickly and efficiently, reducing labour and material costs.

Cubitt’s will assess your building project and make recommendations for appropriate floor structures as required by your local building regulations

What are the alternatives to screw piers?

Generally Cubitt’s will suggest a cement foundation for flat sites, however our teams may suggest screw piers if your site is sloping, is in a flood area, or has soil conditions that predicate a screw piers.

How will I know that a screw piers are best for my Granny Flat?

Cubitt’s have a team of builders, working with engineers who are knowledgeable and skilled in all aspects of construction. When we conduct a feasibility study for you, one of our builders will attend the site to confirm aspects of the construction discussed in your Design Consultation and make recommendations on the most suitable structure for your soil and land shape. Our planning team often consult engineers early on in the process and are familiar with all the local building codes.

Your geo tech reports and engineer will determined what structure will be required for your site, according to local building regulations. Whether we use a retaining wall, cut and fill, or piers, our team will ensure that the design and construction of the foundation and structure are done to the highest standards of safety and quality.

More Expert Building content from the Cubitt's Construction team:


Cubitt's are committed to making your building project stress free and simple. Responsible for your result, you can be assured that we will be aiming to complete your project with the best outcome for your long term use of the home. Building a home is a journey we are privileged to go on with our clients, and Cubitt's are committed to being completely transparent and delivering the highest standards of communication along the way.

We're working towards you enjoying your home for decades to come. We work with expert engineers who help us deliver your building plans to the most up to date local council regulations. At every step of the way we are working with the engineers guidelines to validate expected building conditions. When there are surprises, (which there mostly aren't) you can be assured that your building supervisor has the expert knowledge to guide the decisions you make next - and the team to back it up.


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About Harry Kim

Harry has worked on building sites since the young age of 16, both on commercial and multi high rise buildings, before moving into residential construction management with Cubitt’s. Harry leads a team of 10 site supervisors, 30 employee trades and a large, fluctuating group of longstanding subcontractors.