Shed Slab & Footing Design
Your shed is only as good as what it sits on. Here's how slab and footing design works, why soil matters, and what your engineer is calculating beneath the shed.
Why the Slab Is as Important as the Frame
The shed frame can be perfectly engineered, but if the slab and footings are inadequate, the shed will still fail. The foundation system must resist:
- Bearing loads — the weight of the shed, roof, and any stored contents pushing down on the soil
- Uplift forces — wind trying to pull the columns out of the ground (often the critical design case)
- Sliding forces — horizontal wind pushing the shed sideways along the ground
- Overturning moments — wind trying to tip the shed over, creating high bearing pressure on one side and uplift on the other
- Soil movement — reactive clay shrink/swell creating differential movement that cracks the slab and distorts the frame
Common Shed Slab Types
Thickened-Edge Slab with Pier Footings
The most common solution for structural sheds. The floor slab (100–150mm reinforced concrete) sits on prepared ground, with thickened edges at the perimeter (300–450mm deep) and deeper pier footings (600–1200mm+ deep) at each column location. The piers resist the concentrated uplift and bearing forces from the shed frame.
Waffle Raft Slab
A stiffened slab system using a grid of concrete beams with void formers between them. Suitable for reactive clay sites where the slab needs to resist soil movement without cracking. More expensive than a simple thickened-edge slab but may be necessary for Class H/E soils.
Strip Footing with Separate Floor
Continuous strip footings under the wall lines, with a separately poured internal floor slab. The strip footings carry the structural loads while the floor slab sits on prepared ground and is not structurally connected to the frame footings. This allows for differential movement between the frame and floor without structural consequences.
Soil Classification (AS 2870)
AS 2870 classifies soil based on its reactivity — how much it shrinks and swells with moisture changes:
| Class | Description | Typical Footing Depth |
|---|---|---|
| A | Stable — sand, rock, gravel | 300–450mm |
| S | Slightly reactive clay | 450–600mm |
| M | Moderately reactive clay | 600–900mm |
| H1 | Highly reactive clay | 900–1200mm |
| H2 | Very highly reactive clay | 1200–1500mm |
| E | Extremely reactive clay | 1500–2500mm |
| P | Problem sites (fill, soft, mine subsidence) | Special assessment required |
Column Loads — The Concentration Problem
Unlike a house where loads are spread along continuous walls, a shed concentrates all its forces at discrete column locations. Each column footing must resist:
- Downward loads (dead + live + stored) — typically 2–10 tonnes per column
- Uplift forces (wind suction) — typically 3–8 tonnes per column in non-cyclonic areas, much more in cyclonic
- Horizontal forces (wind shear) — transferred through bracing to specific column locations
This is why a shed slab must have engineered pier footings at every column, not just a flat slab. The piers provide the depth and mass to resist uplift and the bearing area to distribute compression loads into the soil.