Site & slab
Concrete slabs and footings for sheds: what decides the design
By Lockyer Sheds, Crowley Vale QLD ·Comparing options · 6 min read · Published
Short answer
A shed slab is designed, not guessed. The engineer sets the slab and footings from the shed's loads, especially wind uplift at the columns, and the site's soil classification under AS 2870. Reactive clay and problem sites need more. The slab has to be set out to the shed's drawings so hold-down bolts land in the right place, and the footing excavation is inspected before concrete is poured.
Why the slab matters more than it looks
For a steel shed, the footings do two jobs. They carry the weight of the building and whatever is inside it, and they hold the building down. A light shed with large doors can see strong uplift and sideways forces in a storm, and those forces end up at the column footings. That is why a shed's engineering covers the footings as well as the frame, and why the slab is not something to leave to guesswork.
Site classification
AS 2870 sets how sites are classified for footing design and covers Class 1 and Class 10a buildings. Its site classes describe how much the ground moves as it wets and dries:
| Class | Ground | Practical meaning |
|---|---|---|
| A | Most sand and rock | Little or no ground movement |
| S | Slightly reactive clay | Slight movement |
| M | Moderately reactive clay or silt | Moderate movement |
| H1 / H2 | Highly reactive clay | High movement |
| E | Extremely reactive clay | Very high movement |
| P | Problem sites: soft soils, uncontrolled fill, landslip, erosion or abnormal moisture | Needs specific engineering |
The Housing Provisions' deemed-to-satisfy footing details only cover classes A, S and M; for H, E and P sites the design must refer to AS 2870. That is why a soil test is often the first step before a larger shed or a garage attached to the house.
What the engineer decides
- Slab thickness, reinforcement and any thickened edges or internal beams.
- Pad or pier footings under each column, and their depth and size.
- Hold-down bolts or cast-in brackets that connect the columns to the footings.
- Any extra requirements for heavy loads, such as vehicle hoists, racking or machinery.
The certified engineering drawings, not a 3D model or a brochure, are the authority for the slab and footings. If the concreter wants to change something, the change goes back to the engineer.
Set-out: getting the bolts in the right place
The most common slab problem on a shed job is not strength but position. Hold-down bolts or brackets have to match the column positions on the drawings, and the slab must be the right size and square, or the frame will not stand true. Set out from the approved site plan so boundary distances are what the approval assumed.
Drainage and levels
Water should run away from the slab, not towards it. CSIRO's homeowner guide on footing performance explains how poor drainage and trees near footings can make reactive clay move, and why keeping moisture conditions steady around a building matters. Plan where the roof water from the shed's gutters goes, and set the slab height with the finished ground levels and any flood level the council has set.
Full slab or column footings only
Not every shed needs a full slab. An open hay shed or machinery bay may only need pad or pier footings under the columns, with a gravel or compacted floor between them, while a garage, workshop or anything with roller doors usually has a slab so the doors seal and the floor can be swept. The choice changes the engineering, the approval drawings and the cost, so decide it before the design is finalised.
Things to settle before the concreter books in
- The finished floor level, including any step down at the roller doors or a set-down for a wash bay.
- Where plumbing and electrical conduits come up through the slab, so they are cast in rather than cut in later.
- Any heavy point loads, such as a vehicle hoist, heavy racking or a mezzanine, which the engineer needs to know about.
- Access for the concrete truck or pump, and where excavated soil will go.
- Curing time before the frame goes up, which the concreter and engineer confirm.
Inspection before the pour
Under Queensland's building rules, the footing and slab excavation stage is inspected against the building approval before concrete is placed. Book the inspection in time and do not pour until it is passed.
Our concrete service
Lockyer Sheds can prepare the site and pour the slab for sheds we supply and build in Queensland, working to the shed's engineering. If you prefer your own concreter, we supply the slab and footing drawings with the kit and answer their questions about set-out.