PERTHFixing footing movement: underpinning, trades and standards
Rectifying footing movement in Perth: stabilising moisture first, underpinning methods, the structural engineer and registered builder required, and AS 2870
You have a report that says the footings are moving. The next three questions are always the same: how is it fixed, who is legally allowed to fix it, and what standard governs the work. Here are the answers, and the honest news that not every case needs the dramatic fix.
First, find the cause — often that is the fix
Reactive-clay movement is driven by moisture change, so the first rectification is frequently to stabilise the moisture rather than to touch the footing at all. That means fixing the things putting water where it should not be: redirecting stormwater away from the footings, repairing leaking drains and pipes, correcting ground that falls toward the house, and managing large trees whose roots draw moisture unevenly from under one corner.
AS 2870's own maintenance guidance (Appendix B) is built around this — keeping the moisture conditions around a footing as stable and consistent as possible. In a genuine number of cases, stabilising the water is enough to arrest the movement, and no structural underpinning is needed. A good engineer tries the cheap fix before the expensive one.
When the footing itself needs work: underpinning
Where movement is progressive and the footing genuinely inadequate for the site, the structural fix is underpinning — extending the footing down to more stable ground or transferring the load to it. The common methods are mass-concrete underpinning, and screw piles or bored piers taken down to competent founding material below the reactive zone.
Re-levelling, using controlled resin injection or jacking to lift a settled section back toward level, is sometimes used alongside underpinning. Which method suits depends on the soil profile, the depth of the reactive layer, and the structure, which is precisely why it is designed rather than chosen from a menu.
Who is legally required
Two parties, and neither is optional.
A structural engineer designs the remediation. Footing and underpinning design sits outside the deemed-to-satisfy provisions of the NCC's housing provisions for reactive sites — the Code itself directs that Class H, E and P sites be referred to AS 2870 for design, and remediation of a failed footing is engineering, not a standard detail. The engineer classifies the site, often with geotechnical input under AS 1726, and specifies the fix. Any structural fill involved is controlled fill under AS 3798.
A registered building contractor carries out the work. In WA, building work over $20,000 must be contracted by a builder registered with the Building Services Board, and structural underpinning is squarely building work. An unregistered operator doing underpinning is a problem stacked on top of a problem — see our guide on checking builder registration.
The standards that govern the repair
The completed footing has to satisfy AS 2870 for the site's classification, the same standard that governed it when the house was built. The concrete work is governed by AS 3600, the concrete structures standard. And the whole thing has to comply with the National Construction Code, published by the Australian Building Codes Board, which calls up AS 2870 for residential footings. A remediation that cannot demonstrate compliance with those has not fixed the problem in any way you could rely on at resale.
What it means for a buyer
If you are buying a house with evidence of footing movement, the sequence matters. Get the building inspection first, then a structural engineer's assessment if the inspection flags it, and price the remediation before you are committed rather than after. If underpinning has already been done, ask for the engineer's design, the compliance documentation, and evidence the moisture cause was addressed — underpinning without fixing the drainage that caused the movement is a repair with a countdown on it.
Where a builder's original footing was inadequate for the site classification and the house is still young, this can become a building service complaint or an expert witness matter. The WA disputes process sets out the path.
Why the cause matters more than the crack
The instinct when a house shows footing movement is to want the cracks fixed, but the cracks are the symptom, and fixing them without addressing what caused the movement is close to pointless, because the movement will simply crack the repair. The first and most important step in dealing with footing movement is therefore diagnosis: working out what is driving the movement, because in a great many cases dealing with the cause stabilises the situation and the expensive structural intervention is not needed at all. Footing movement on Perth's reactive clay soils is usually driven by uneven moisture in the ground, and the moisture is usually being disturbed by something identifiable: a large tree drawing water unevenly from under one part of the house, a plumbing leak keeping one area permanently wet, poor stormwater drainage soaking the ground next to a footing, or a combination.
Where that is the case, the fix often starts not with the footing but with the cause: managing or removing the offending tree, repairing the leak, correcting the drainage so water is directed away from the building and the ground moisture is allowed to stabilise. Address the moisture cause, and a house that has been moving with the seasons often settles into a new equilibrium and stops. This is why a rush to underpin before the cause is understood can be both unnecessary and ineffective, because underpinning a footing while leaving a leaking drain or a moisture-hungry tree in place is treating the symptom while the cause keeps working.
When the footing itself needs work
Sometimes, though, the movement is beyond what managing the cause will resolve, and the footing itself needs structural intervention. Underpinning is the technique for this: strengthening or extending the existing footing downward to found it on more stable ground or to increase its capacity, so that it no longer moves with the reactive soil above. It is significant structural work, and it is not a first resort; it is what is warranted when the movement is serious, progressive, and not resolvable by dealing with the moisture cause alone, or where the damage is severe enough that stabilising the footing is necessary regardless.
Because underpinning is major structural work, it is properly the domain of a structural engineer to design and specify, working from an understanding of the soil, the existing footing, and the movement. This is not a job to be scoped by a general builder or, worse, a contractor who leads with underpinning as the answer before the cause has been investigated. The right sequence is diagnosis first, by someone qualified to do it, then a decision about whether managing the cause is sufficient or whether structural work is genuinely needed, and only then, if it is, an engineered underpinning solution. Skipping the diagnosis and going straight to underpinning risks spending a great deal of money on an intervention that may not have been necessary and may not work if the cause is left running.
What it means for a buyer
For a buyer looking at a house with signs of footing movement, or evidence of past underpinning, the important thing is to understand the situation rather than to react to the cracks alone. Signs of footing movement warrant a structural engineer's assessment before committing, because the questions that matter, what is driving it, whether it is active or historic and stable, and what if anything needs to be done, are engineering questions that a visual inspection can flag but not answer definitively. A house that has moved historically and stabilised, with the cause addressed, can be a perfectly sound buy; a house that is actively moving with an unaddressed cause is a different and more serious proposition.
Evidence of previous underpinning is worth understanding rather than fearing: it may mean a past problem was properly diagnosed and fixed, which is reassuring, or it may mean an intervention was made without addressing the cause, in which case movement may continue. Either way, the history of what was found, what caused it, and what was done is the information a buyer needs, and where footing movement is in the picture, a structural engineer's involvement before purchase is money well spent against the potential cost of the problem. A building inspection identifies the signs and flags the need for that assessment; it is the engineer who answers the questions that decide whether the house is sound.
Common questions
Who can fix footing movement?
A structural engineer designs the remediation and a registered building contractor carries it out. In WA, building work over $20,000 must be contracted by a builder registered with the Building Services Board.
What standard governs footing repair?
AS 2870 for the residential footing and site classification, AS 3600 for the concrete, AS 3798 for controlled fill, all under the National Construction Code published by the ABCB.
Does footing movement always need underpinning?
No. Because the movement is moisture-driven, stabilising drainage and managing trees often arrests it. AS 2870's maintenance guidance is built around keeping moisture conditions stable, and a good engineer tries that before underpinning.
This comes up on every pre-purchase building inspection we carry out.
Read next: the name on the sign may not be the business building your house and reactive-clay footing movement.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.