Underpinning advice

Resin injection vs traditional underpinning: which one fits?

Published 24 August 2026

Ask two contractors to fix the same moving slab and you can get two completely different answers: one quotes resin injection at $8,000, the other quotes traditional underpinning at $28,000. That’s not necessarily a scam — it’s just as often two people looking at different severities of the same underlying problem and reaching for the tool that matches what they saw. The trouble is knowing, before you commit to either, which one actually applies to your house.

What each one does

Resin injection pumps an expanding structural polymer through small holes into the soil beneath a slab. The resin expands, fills voids left by settled or eroded soil, compacts the ground around it, and lifts the slab back toward level as it cures. It’s fast — often a single day on site — minimally invasive, requires no excavation, and causes very little disruption to the garden or the house above.

Traditional underpinning excavates down past the reactive or unstable layer of soil and builds new concrete piers that carry the slab’s load to genuinely stable ground below — typically $1,500 to $3,500 per pier, with most jobs needing eight to fifteen piers along the affected section. It’s slower, involves real excavation around the perimeter, and needs an engineer-designed solution and a building permit before work starts. See the full underpinning process for what that involves.

The real difference is severity, not preference

This is the part that gets sold badly. Resin injection and underpinning aren’t two competing brands of the same fix — they solve different degrees of the same problem, and choosing between them isn’t a matter of taste or budget alone.

Resin injection suits:

• Minor, localised settlement caught early, before it’s progressed far
• A void under a specific section of slab — often from a leaking pipe that’s washed soil away, or over-compacted fill settling — rather than movement across the whole footprint
• Sites where the underlying soil, once the void is filled, is genuinely capable of supporting the slab long-term
• Homeowners who need the fastest possible fix with minimal disruption, and whose engineer confirms that’s a legitimate option for the specific damage

Traditional underpinning is required when:

• Movement is driven by ongoing reactive clay behaviour — the soil itself keeps shrinking and swelling with moisture, which resin doesn’t stop, it just fills the current gap
• The affected area is extensive rather than localised — a whole wing of the house, not one corner
• Cracking is progressive and severe: wide, stair-stepped, still growing (see when wall cracks actually matter for how to tell)
• The site has a known history of deep reactive clay or engineered fill, common through Melton, Tarneit and the deeper-fill pockets of Cairnlea and Wyndham Vale

A resin fix on a problem that actually needed piers doesn’t fail dramatically — it just doesn’t hold, and the movement resumes within a year or two once the reactive clay cycles through another wet and dry season. That’s the expensive mistake: paying for the cheaper fix twice because it was never going to solve the underlying cause.

Cost and disruption, side by side

	Resin injection	Traditional underpinning
Typical cost	$5,000 – $15,000	$15,000 – $40,000
Time on site	Often 1 day	1–3 weeks, plus curing
Excavation	None	Yes, around the perimeter
Permit required	Sometimes, depending on scope	Yes, engineer-designed
Best suited to	Localised, early-stage movement	Extensive or ongoing reactive clay movement

Both figures are indicative ranges — see the full cost guide for how access, pier count and depth push a quote up or down.

How the decision actually gets made

The honest answer is: not by guessing, and not by picking the cheaper option and hoping. It starts the same way any underpinning assessment does — a floor level survey to measure the shape and extent of movement, crack mapping, and a check on whether a moisture source (a downpipe, a leak, garden watering) is driving the problem. Where the movement looks localised and early, resin is a legitimate, honest recommendation. Where it’s widespread or the soil itself is the ongoing problem, an engineer will generally specify piers, and a contractor who quotes resin instead to win the job cheaper is setting you up to pay for the real fix later anyway.

If you’re getting quotes from two different contractors offering two different methods, ask each one to show you the floor level readings and crack mapping their recommendation is based on — not just the final number.

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Not sure which one your house needs? Book a free on-site assessment — we’ll take floor levels, map the cracks, and give you a straight recommendation, including telling you when resin genuinely is enough.

Common questions

Is resin injection a cheap shortcut around proper underpinning?

Not when it is applied to the right problem. Resin injection is a legitimate engineered technique for filling voids and densifying loose soil beneath a sound slab, and where that is genuinely the problem it will outperform piers on cost, speed and disruption. It becomes a shortcut only when it is sold as a substitute for underpinning on a house whose footing is inadequate for the ground conditions, or where movement is deep-seated. The resin does not carry load down to competent ground; that is what piers do. Matching the method to the mechanism is the whole decision.

How long does resin injection last?

The polyurethane itself is inert, does not absorb water and does not biodegrade, so the material will not fail. Whether the result lasts depends entirely on whether the cause was addressed. Inject resin under a slab that is being undermined by a leaking stormwater line and leave the leak, and the ground will wash out again around it. Where the cause was a one-off — poorly compacted fill consolidating, a historical void — the correction is effectively permanent. This is why any competent operator investigates the water before quoting the resin.

Can you use both resin injection and underpinning on the same house?

Yes, and on some houses that is the correct answer. A common combination is piers to stabilise the section of the structure where the footing is genuinely inadequate, with resin used to fill voids and re-level a slab area elsewhere that is sound but has lost support. The two techniques address different problems and there is no reason a single house cannot have both. What matters is that the design comes from someone assessing the whole structure rather than from whichever contractor happens to own one type of equipment.

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