Underpinning advice

Why new homes on clay still crack

Published 24 August 2026

“But it’s only ten years old.” We hear a version of that sentence most weeks, usually from someone standing in a hallway pointing at a crack running up from a door frame in a house that, by any reasonable measure, should not be falling apart yet. It isn’t falling apart. But the objection itself rests on an assumption that turns out to be wrong: that a new house on a compliant slab shouldn’t move at all. It was never built not to move. It was built to move within limits — and working out why the movement shows up when it does, rather than on day one or never, is most of the actual answer.

The slab was designed around movement, not against it

AS 2870 is the standard that governs residential slab and footing design on Australian soils, and its whole premise is that reactive clay expands and contracts with moisture. The standard doesn’t try to stop that. It classifies how much movement a given site is likely to produce — Class M, H1, H2 and beyond across most of Melbourne’s western growth corridor — and requires the slab to be stiff enough to ride that movement out as one piece, without cracking.

That’s an important distinction most home buyers never hear explained. A compliant slab on an H2 site is engineered to tolerate up to 75mm of seasonal ground movement. It is not engineered to prevent that movement, and it is not engineered against every possible cause of uneven movement across its own footprint — only the movement the site classification anticipated when it was designed. Everything that goes wrong afterwards is something the design didn’t have to account for, because it wasn’t part of the ground’s expected behaviour. That’s the gap new-home cracking actually lives in.

Why the timing isn’t random

If reactive clay were the only variable, you’d expect cracking to appear evenly across a house’s life, or worst in year one when the ground is presumably at its most disturbed. In practice, structural cracking on estate homes clusters in a fairly specific window — roughly five to fifteen years after handover — and that clustering has three real causes, not one.

The ground itself hasn’t settled into a routine yet. Before a house is built, the lot is stripped of topsoil, cut and filled to a buildable pad, and the natural vegetation that used to manage moisture — established grass, whatever scrub or trees stood there before — is gone. For the first several years after a house is occupied, the moisture regime under and around the slab is still finding a new equilibrium: newly planted gardens are watered heavily while they establish, downpipes are freshly connected and not yet tested by a proper Melbourne winter, and any engineered fill under part of the lot is still consolidating under the load of the finished house rather than the temporary load of construction traffic. None of that shows up as a defect at the six-week handover walkthrough, because none of it has happened yet.

Landscaping catches up with the slab on its own timeline. A tree planted at handover is a metre tall and drawing next to no water from the clay beneath it. Ten years later it’s a real tree with a real root system, and if it’s within reach of the footings, it’s now drawing enough moisture out of the ground on that side of the house to matter. This is the single biggest cause we see in the more established pockets of the growth corridor — see Taylors Hill and Werribee, where twenty-five-plus years of tree growth has turned what was a non-issue at construction into the dominant cause of movement today.

And the six-year insurance window closes right in the middle of it. Victoria’s domestic building insurance covers structural defects for six years from completion. A meaningful share of the cracking we see shows up either just inside that window — which is worth pursuing properly — or just after it, which is one of the more frustrating conversations in this trade, because the cause (a marginal slab design, an undersized footing for the actual site class) was baked in at construction but the evidence took years to surface.

The three real causes, once you rule out “the slab is bad”

Once a genuinely non-compliant slab is ruled out — which an independent engineer’s report either confirms or eliminates — the movement in a young home almost always traces back to one of three things, and they need different fixes.

• Fill still consolidating. Estates built across former farmland and quarry land involved real cut-and-fill earthworks to produce flat, buildable lots. Properly compacted fill performs fine indefinitely. Fill that varies in depth across one lot, or wasn’t compacted to spec, keeps settling for years after the house is built — and a slab spanning the join between natural ground and deep fill bridges two materials moving at different rates. This is the dominant story in Tarneit and Truganina, where much of the housing sits under fifteen years old and the earthworks records matter as much as the soil report.
• A moisture source that wasn’t there at handover. A downpipe redirected during a later landscaping job, a garden bed built up against the slab edge, irrigation added when the lawn struggled in its first dry summer — any of these can concentrate water where the original design never expected it, years after the fact.
• Site-wide fill history that needs its own paperwork. Some estates carry more of this risk than others by virtue of what the land used to be. Cairnlea, built on the former Albion explosives factory site, is the clearest example in Melbourne’s west — extensive engineered fill was part of preparing the whole estate, fill depth varies from lot to lot, and establishing that history is the first real step in any assessment there, not an afterthought.

What to actually do about it

Don’t start with a quote for piers. Start with the paperwork: the original soil report, the site classification, and — where the estate involved earthworks — the fill records. Get an independent engineer, not one attached to whoever wants to sell you the repair, to establish whether the movement is within what the site classification always anticipated, or beyond it. If your home is still inside or close to the six-year structural defect window, that report is also what a warranty claim gets built on, and it needs to exist before any repair work destroys the evidence.

None of this means the crack isn’t worth acting on. It means the “but it’s only ten years old” instinct, while understandable, is pointing at the wrong question. The right one is: what changed around the slab between handover and now — and was the movement always going to happen, or is something fixable driving it?

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Cracking in a home under fifteen years old, and not sure whether it’s the ground doing what it was always going to do or something that needs fixing? Our underpinning and foundation repair page covers how we tell the difference, or book a free assessment and send us photos and, if you have it, the original soil report.

Common questions

Is my builder liable if my new home cracks?

It depends on why it cracked, and it is worth establishing rather than assuming either way. Victorian domestic building work carries a statutory warranty of ten years for structural defects, and an incorrect site classification, a footing design that did not match the site conditions, or drainage not installed as designed can all fall inside that. Seasonal cracking on a correctly designed slab where the owner's landscaping created the moisture differential generally does not. The way to find out is an independent floor level survey and engineer's report, raised with the builder in writing and early.

How much cracking is normal in a new home?

Fine hairline cracking in plaster and at cornice junctions in the first couple of years is normal and expected as the building dries out and settles — it is shrinkage rather than movement. What is not in the normal category is stair-step cracking through mortar joints in the external brickwork, cracks wider at one end than the other, cracks you can fit a coin into, cracked floor tiles along a consistent line, or doors that have started binding. Those describe the structure moving, and they deserve investigation regardless of how new the house is.

What should I do first if my new home starts cracking?

Document before you do anything else, because the record is what everything later depends on. Photograph every crack with a date and something for scale, mark and measure them, and note when you first saw them. Then check the obvious moisture causes: downpipes discharging beside the slab, garden beds or irrigation against the walls, surface levels falling toward the house rather than away. Correct what you can. Then, if the home is under ten years old, get an independent engineer's report and raise it with the builder in writing before you commission any repair yourself.

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