If you're looking to build a shed, you may be wondering what is required for foundations. Foundations are the very base of a building, and take the weight of your structure, meaning this is something you want to get right.
What are the different types of foundations?
What type of foundation you require will often be determined by which type of shed you build. For example, foundations for a pole shed are very simple. Typically, all that is required is 1.2-1.5m deep holes to be augured and around 600mm in diameter. The pole is then placed in the hole and encased in concrete. This means the concrete floor isn't considered a structural component as the building isn't relying on the floor for strength. This makes the floor design very simple, reducing cost of materials and labour significantly.
If you are building a steel shed, then the footing design changes dramatically. The steel columns bolt onto a concrete slab which means that any concrete floor needs to be engineered to take the weight of the building. Typically, a large square footing will need to be excavated and then a channel will need to be excavated around the perimeter of the building. This is called a perimeter thickening which is required to give the floor the necessary strength. This type of foundation is considerably more expensive as they take longer to build, and they require a lot more concrete and complex reinforcing designs.
Rib raft floor
Rib style floors are typically required when ground conditions are excessively soft, and a grillage of concrete beams need to be used throughout the slab to add strength and spread the weight of the slab. This in principle makes the floor essentially "floating" as one big slab, reducing the chance of cracking compared to a normal 100mm floor with standard mesh for example. Rib floors are formed using either polystyrene cubes placed throughout the slab, or plastic pods. These floors are custom designed & engineered in relation to the ground conditions, which is stipulated by the geo tech report findings.
When might I require more extensive foundations?
If your building platform is on soft, peaty, sandy or swampy ground, standard foundations may not be sufficient. Having poor-quality ground often means that special engineered footings are required, and this can lead to significant cost increases.
In some cases, it can be as simple as having deeper holes and longer poles with more concrete. In other cases, the soft ground may have to be removed and replaced with engineered and compacted fill. Sometimes the footings and floor need extensive reinforcement to meet engineering and council regulations. The cost of these changes is directly related to the quality of the ground and the costs can vary wildly.
If you know your ground conditions are not ideal, it is advisable to get a Geotech report done. This will tell you exactly what the ground conditions are like, and your shed provider will be able to supply more accurate costs. An engineer typically charges around $1,500+GST for a standard Geotech report, however, this fee may vary depending on which company you choose to do the report.
If your property is within a flood zone, and deemed as high risk, you may be unable to build on your property. In lower-risk zones, the height of your foundations may have to increase, increasing the cost of earthworks. It’s best to investigate this by contacting your local council before you get too far ahead with the design process.
One important thing to get right is the site preparation prior to placing your concrete floor. Having a solid and well compacted gravel base under your floor is highly recommended and can actually make your slab stronger and help to prevent cracks from forming.
If you have any further questions regarding 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.
