Footings and slabs: why site classification decides everything above it

The slab is the first thing built and the last thing anyone thinks about. Once it is poured, framed over and tiled, it disappears from view for the life of the house. But nearly every crack that worries an owner later, in a wall, a cornice or a tiled floor, has the same starting point: the ground moved, and the footings were not designed, built or looked after to handle it. This article covers what the standards require of residential footings and slabs, why site classification sits at the centre of it, where things go wrong, and how we check this work before it is buried.

What the standards require

For houses, the structural performance requirement is in the National Construction Code (NCC) 2022, Volume Two, Part H1 Structure: the building must resist the loads and actions it will reasonably see, including movement of the ground it sits on. The Deemed-to-Satisfy detail for footings and slabs is in the ABCB Housing Provisions, Section 4 Footings and slabs, which in turn relies on AS 2870, Residential slabs and footings (the 2011 edition is the one referenced by NCC 2022).

The first step is always the same. The foundation must be classified in accordance with AS 2870 before the footings are designed. The site classes are:

• Class A: most sand and rock sites, with little or no ground movement from moisture changes.
• Class S: slightly reactive clay sites.
• Class M: moderately reactive clay or silt sites. Where movement is deep-seated, typical of drier climates, the class is further divided (M-D).
• Class H: highly reactive clay sites, split by AS 2870 into H1 and H2 as expected movement increases.
• Class E: extremely reactive clay sites.
• Class P: problem sites, including soft soils, uncontrolled fill, landslip, mine subsidence, or sites affected by trees, abnormal moisture or other conditions that rule out a standard classification.

That classification then drives everything else: the depth and spacing of edge and internal beams, the reinforcement, and whether a standard design can be used at all. Under NCC 2022 the Housing Provisions' own footing and slab details are limited to Class A, S and M sites within set geometric limits. If a site is classed H, E or P, or M-D, the design has to come from AS 2870 itself, which in practice means a structural engineer.

The Housing Provisions also set out the "associated elements" that make a slab perform, not just stand up. Among them:

• A vapour barrier under the slab: 0.2 mm polyethylene film of the impact resistance AS 2870 specifies, lapped at joints, taped or sleeved at every service penetration, repaired wherever it is punctured, and carried under the edge beams to ground level.
• Reinforcement of the specified size, positioned on bar chairs so it ends up with the right concrete cover once poured.
• Termite management at the slab edge and at every pipe or conduit that passes through it (see our timber frame article for why this matters whatever the walls are made of).

Why the standard is built around the soil

Clay moves, and Sydney has a lot of it. Reactive clay swells when it gets wet and shrinks when it dries out. Much of the Sydney basin sits on clay derived from shale, and those soils can be anywhere from slightly to highly reactive depending on the site. The ground under a house is not static. It rises and falls with the seasons, with drought and heavy rain, and with anything that changes the moisture around the building. A slab designed for a Class S site sitting on Class H clay is undersized before the first wall is built.

The movement is uneven, and that is what cracks buildings. If a whole slab rose and fell evenly, very little would crack. Damage happens when one part of the footing system moves more than another: the edge dries out faster than the middle, a leaking pipe wets one corner, a tree draws moisture from one side. AS 2870's beam depths and stiffness requirements exist to resist exactly this kind of differential movement. That is why the classification has to be done properly for the actual site, not guessed from the suburb.

You only get to check it once. A slab cannot be inspected after it is poured. Whether the membrane was punctured and not repaired, whether the reinforcement was sitting on the ground instead of on chairs, whether the beams were dug to the right depth: none of it can be seen once the concrete is down. Reinforcement without enough cover corrodes over time, and rust expands and cracks the concrete around it. A torn membrane lets ground moisture wick into floor finishes. These are slow failures, and they start on the day of the pour.

The building keeps depending on the site long after handover. CSIRO's homeowner guide, Foundation Maintenance and Footing Performance, makes the point bluntly: footings are designed on the assumption that moisture around the house stays reasonably stable. Poor drainage, ponding against the slab, leaking plumbing, or trees planted too close can push the ground well outside the conditions the design allowed for.

Where footings and slabs commonly go wrong

• The wrong class. A site classified without a proper geotechnical investigation, or a classification carried over from a neighbouring lot.
• Trees and fill not accounted for. Existing or removed trees and uncontrolled fill can change a site to Class P. They need to be identified before design, not discovered during excavation.
• Membrane damage. Tears from foot traffic and reinforcement, and unsealed penetrations around plumbing.
• Inadequate cover. Reinforcement sitting low or out of position because chairs were missing, crushed or too few.
• Poor curing. Concrete that dries too quickly, especially in hot or windy weather, is weaker at the surface and more prone to cracking.
• Drainage after handover. Garden beds and paving that direct water toward the slab, downpipes discharging at the footing, or leaking stormwater and sewer lines.

Questions worth asking your builder

1. What is the site classification, who classified it, and has a geotechnical report been done for this lot?
2. Is the slab a standard design within the Housing Provisions, or engineered to AS 2870? Who is the engineer?
3. Were any trees, old footings or fill found on the site, and how has the design dealt with them?
4. Who inspects the excavation, membrane, reinforcement and cover before the pour, and is that inspection recorded?
5. How will the concrete be cured, particularly if the pour is in hot weather?
6. What should I do after handover to keep the ground around the house stable?

How we build it so it lasts

• We start with the ground. We confirm the site classification against a geotechnical investigation for the actual lot, and flag trees, fill or slope that could push a site outside a standard class.
• We build to the engineer's design, not a near enough version. Beam depths, reinforcement and any design outside the Housing Provisions' scope go to a structural engineer under AS 2870, and changes during the build go back to them.
• The pre-pour inspection is a hold point. Excavation, membrane laps and penetrations, reinforcement size and position, chairs and cover, and termite management are checked and photographed before the concrete is ordered. Nothing gets poured over work that has not been checked. Our hold-point article explains how this works across a whole job.
• We cure the concrete properly, and adjust the method for hot, dry or windy conditions.
• We get drainage right around the building so surface water runs away from the slab, and we hand over plain guidance on keeping it that way.
• We hand over the record. Site classification, engineering, inspection photos and certificates go to the owner, so the footing history of the house is on file, not lost.

The slab is the first hold point on a new build, and the framing above it depends on it. See our article on carpentry and structural integrity for the next stage, and our Quality & Building Compliance policy for how inspections work on our projects.

Why this protects the building and your investment

Footing repairs are among the most expensive and disruptive repairs a house can need: underpinning, re-levelling, and then fixing every crack the movement caused above. Almost all of that is avoidable with the right classification, a properly built slab and a site that stays well drained. It is cheap to get right at the start, and very expensive to get wrong.

If you are planning a new build or an extension and want to talk through the site, get in touch.

Sources

• NCC 2022, Volume Two, Part H1 Structure, Australian Building Codes Board
• NCC 2022 Housing Provisions, Part 4.2 Footings, slabs and associated elements, Australian Building Codes Board
• Foundation Maintenance and Footing Performance: A Homeowner's Guide, CSIRO Building Technology Resources

The illustration is simplified, not a construction detail. This article is general information, not geotechnical or structural engineering advice for your site. Site classification and footing design depend on the investigation for your lot, so confirm them with your geotechnical engineer, structural engineer and certifier. Full website disclaimer