Shed slabs and footings in Queensland: site classes and preparation
Shed design · Updated · 6 min read · BWG Sheds
Short answer
The slab and footings are designed for the ground and the wind. Under AS 2870, sites are classified by how much the soil moves: A (sand and rock), S, M, H1, H2 and E for increasingly reactive clay, and P for problem sites. The NCC's standard footing tables only cover classes A, S and M; for H, E and P sites the design must follow AS 2870. A shed's engineering also sets the footings that hold the frame down against wind uplift.
- Site classes: A, S, M, H1, H2, E and P, by expected ground movement.
- Characteristic surface movement: S up to 20 mm, M 20–40 mm, H1 40–60 mm, H2 60–75 mm, E over 75 mm.
- Deemed-to-satisfy footing tables cover only A, S and M; H, E and P need AS 2870 design.
- AS 2870 does not cover tying the frame down for wind; the shed engineering does.
- The certifier inspects the footing or slab excavation before you pour.
Site classes in plain English
AS 2870, the Australian Standard for residential slabs and footings, covers Class 1 and Class 10a buildings, including sheds and garages. It sorts sites by how much the ground is expected to move as the soil wets and dries. Reactive clays swell and shrink, and a slab that is not designed for them can crack or move.
| Class | Ground | Characteristic surface movement |
|---|---|---|
| A | Most sand and rock | Little or no movement |
| S | Slightly reactive clay | 0–20 mm |
| M | Moderately reactive clay or silt | 20–40 mm |
| H1 | Highly reactive clay | 40–60 mm |
| H2 | Highly reactive clay | 60–75 mm |
| E | Extremely reactive | More than 75 mm |
| P | Problem site: soft soils, uncontrolled fill, landslip, mine subsidence, collapsing soils, erosion, abnormal moisture, or cannot otherwise be classified | Must be assessed |
When the footings must be engineered
The NCC Housing Provisions include standard footing and slab designs, but only for sites classified A, S or M. If your site is H, E or P, reference must be made to AS 2870, which in practice means an engineer designs the slab and footings. Many clay sites on the Darling Downs and in parts of South East Queensland are reactive, so do not assume a standard slab will do.
AS 2870 also has a gap that matters for sheds: it gives no advice on connecting the superstructure to the footings for wind or earthquake loads. A steel shed is light and has a large roof, so wind uplift is often what sizes the footings under each column. That part comes from the shed's own engineering, which is why the engineering drawings, not a generic slab plan, are the reference for your concreter.
Every BWG Sheds kit comes with engineering for your site, including the footing details your concreter builds to.
How we design and engineerPreparing the site
- Get the site classified. A soil test tells you and the engineer what you are building on.
- Pick a position with good drainage. Water ponding against a slab edge causes problems on reactive clay.
- Level the pad properly: cut and fill needs to be done and compacted correctly, and uncontrolled fill makes a site Class P.
- Keep clear of trees, which dry out clay and cause movement; CSIRO's homeowner guide covers trees and drainage.
- Locate services, easements, sewer and stormwater before you dig.
- Book the certifier's footing or slab excavation inspection before the pour.
Drainage and stormwater around the slab
A shed roof collects a lot of water, and on reactive clay that water is a threat to the slab if it is dumped next to it. Plan where the downpipes discharge before the slab is poured, and grade the ground so surface water runs away from the slab edge rather than pooling against it. CSIRO's homeowner guide on foundation maintenance and footing performance explains how drainage, garden watering and trees change the moisture in clay under a slab, and why keeping moisture even matters. If your council requires a stormwater connection, the certifier will ask how it is handled.
Think about the floor level too. A garage or workshop slab set too low on a sloping block can take in water under the roller door in heavy rain. Raising the slab, adding a set-down at the door or regrading the approach is far easier before the concrete goes in.
Slab or piers?
A garage or enclosed workshop usually sits on a full slab, because vehicles, benches and storage need a hard, level floor and the slab edge helps seal out water and vermin. An open-bay machinery shed or hay shed may sit on individual pad or pier footings under each column, with a gravel or compacted floor, depending on the design. The engineer chooses based on the use, the soil and the wind loads, and the choice changes both the cost and the approval drawings.
Requirements depend on your property, proposed structure and local planning/building controls.
Send us your site details and we will quote a shed with footings designed for it.
Request a quote