Foundations are the very base of a building and are responsible for supporting the weight of the structure. Because of this, getting the foundation design right is an important part of building a shed.
The type of foundation required will depend on the shed design, ground conditions, and construction method.
In this article, we look at the different types of shed foundations, what can affect your foundation requirements, and what you need to consider before building.
Most sheds require some form of foundation to safely support the structure and transfer its loads into the ground. However, the type of foundation you need will depend on the shed design, construction method, and ground conditions.
For example, a pole shed transfers its structural loads through timber poles set into the ground, while a steel shed may rely on engineered concrete footings and the floor slab for structural support.
The type of foundation you require will often be determined by the type of shed you build.
Foundations for pole sheds in NZ are relatively simple. Typically, all that is required is holes around 1.2–1.5m deep and approximately 600mm in diameter to be augured into the ground.
The timber pole is then placed into the hole and encased in concrete.
Because the poles transfer the building loads directly into the ground, the concrete floor isn't considered a structural component. The building isn't relying on the floor for its structural strength.
This makes the floor design for many pole shed plans relatively simple and can significantly reduce the cost of both materials and labour.
If you are building a steel shed, the footing design changes considerably.
Steel columns typically bolt onto a concrete slab, which means the concrete floor needs to be engineered to support the weight and structural loads of the building.
Typically, a large square footing will need to be excavated beneath each column, along with a channel around the perimeter of the building. This is called a perimeter thickening and is required to give the floor the necessary strength.
These types of concrete footings for a shed are considerably more expensive because they take longer to construct and require more concrete and more complex reinforcing designs.
Rib-style floors are typically required when ground conditions are excessively soft and a grillage of concrete beams needs to be used throughout the slab to add strength and spread the weight of the building.
In principle, this makes the floor essentially "float" as one large slab, helping reduce the chance of cracking compared with a standard 100mm concrete floor using standard mesh, for example.
Rib floors are formed using either polystyrene blocks or plastic pods placed throughout the slab.
These floors are custom-designed and engineered based on the specific ground conditions identified in the geotechnical report.
The right shed foundation will depend on several factors, including:
The type and construction method of your shed
The size and intended use of the building
Ground and soil conditions
Site location and flood risk
Engineering and local council requirements
Understanding these factors early in the design process can help determine the foundation requirements and provide a clearer picture of the overall project costs.
If your building platform is on soft, peaty, sandy or swampy ground, standard foundations may not be sufficient.
Poor-quality ground can mean special engineered footings are required, which can lead to significant increases in the overall cost of the project.
In some cases, the solution may be relatively simple, such as using deeper holes, longer poles and more concrete.
In other situations, the soft ground may need to be removed and replaced with engineered and compacted fill. Sometimes the footings and floor will also require extensive reinforcement to meet engineering and council requirements.
The cost of these changes is directly related to the quality of the ground, so costs can vary significantly from one site to another.
If you know your ground conditions are not ideal, it is advisable to have a Geotech report completed.
This will tell you exactly what the ground conditions are like and allow your shed provider to provide more accurate foundation and construction costs.
An engineer typically charges around $1,500–$3,000 + GST for a standard Geotech report, although this may vary depending on the company you choose.
If your property is within a flood zone and is deemed high risk, you may be unable to build on the property.
In lower-risk flood zones, the required height of the foundations may need to increase, which can also increase earthworks costs.
It’s best to investigate this by contacting your local council before getting too far through the shed design process.
One important thing to get right is the site preparation before placing your concrete floor.
Having a solid, well-compacted gravel base underneath the floor is highly recommended. A properly prepared base can make the slab stronger and help prevent cracks from forming over time.
Getting the site preparation right from the outset can help provide a solid base for your shed foundation and reduce the risk of problems developing later.
Whether you're comparing different foundation systems or reviewing pole shed plans, understanding the ground conditions early can help prevent unexpected engineering requirements and additional costs later in the project.
If you have any further questions about shed foundations or want to know more about building a shed, you can download our free Buyer's Guide.
Otherwise, feel free to contact the team.
What type of concrete footing does a shed need?
The type of concrete footing for a shed will depend on the shed design, ground conditions and structural requirements.
For pole sheds, all that is typically required is holes around 1.2–1.5m deep and approximately 600mm in diameter to be augured into the ground. The timber pole is then placed into the hole and encased in concrete.
For steel sheds, engineered footings may be required beneath each column, along with perimeter thickening to provide the necessary strength.
Do I need a Geotech report before building a shed?
Not always. However, if your site has soft, peaty, sandy or swampy ground, a Geotech report can identify the ground conditions and help determine whether special engineered foundations are required.
Does a pole shed need a concrete floor?
A pole shed does not typically rely on the concrete floor for its structural strength. Instead, the poles transfer the building loads directly into the ground, which can make the floor design simpler than for some other types of sheds.