Concrete or Steel Stumps? What to Use When Restumping

Reactive clay above. Century-old mine workings below. In Bendigo, a house can be moving for three completely different reasons that look identical from inside the living room — and cost wildly different amounts to fix.

So no pier gets quoted here until the cause of the movement has actually been identified.

Why the original stumps failed in the first place

Understanding this makes the materials decision much easier, because the failure mode is remarkably consistent.

Timber stumps almost never rot in the middle or at the top. They fail at ground line — the narrow band where the post enters the soil. That’s where moisture and oxygen are both available, and fungal decay needs both. Below ground line there’s less oxygen. Above it there’s less moisture. At the interface, there’s plenty of each.

Which is why a stump can look perfectly sound when you shine a torch along it and be almost gone where it matters. It’s also why the durable Victorian hardwoods lasted as long as they did without lasting forever — the timber species buys you decades, not immunity.

The practical lesson for the materials decision: the thing that killed the old stumps was water at ground line. Any replacement material has to be judged on how it handles that, and the water source has to be dealt with regardless of what goes in. Drainage, ventilation and where the water comes from →

Concrete stumps

The default across Victoria, and what most quotes will assume unless you ask otherwise. Precast concrete posts, set into holes, with the bearer fixed to the top.

What’s good about them

They don’t rot. The failure mode that destroyed the original timber simply doesn’t apply.
They’re not food. No borer, no termites.
Cost. Generally the cheaper of the two options per stump.
Familiarity. Every operator installs them, so you’re not paying a premium for specialised work.
Compressive strength. Concrete carries vertical load extremely well, which is most of what a stump does.

What to know about them

They’re brittle in tension and bending. Concrete handles being squashed far better than being bent or pulled. Where ground movement applies lateral force, that matters.
Reinforcement matters. [[VERIFY — confirm what reinforcement is standard in precast stumps sold in Victoria, and whether unreinforced stumps are still permitted for structural residential use. Then state it plainly here.]] Unreinforced or poorly reinforced concrete can crack and spall under movement.
They’re not adjustable. Once set, the height is the height. If the house goes out of level again, correcting it means jacking and packing on top of them or replacing them.
Weight. Concrete stumps are heavy, which is a real factor on a gold-era block with 300mm of subfloor clearance and no machine access. It’s a labour cost.

Galvanised steel stumps

Steel posts, hot-dip galvanised, usually with an adjustable head — a threaded section at the top that allows the height to be altered after installation.

What’s good about them

Adjustability. The genuine differentiator. If the house settles again, levels can be corrected by adjusting the stumps rather than by jacking and packing or restumping a second time.
Strength in bending, not just compression. Steel tolerates lateral load and movement far better than concrete does.
Weight and handling. Much lighter than concrete, which matters more than it sounds when everything is being carried by hand through a narrow side access into a 300mm crawl space.
Not organic. No rot, no borer, no termites.

What to know about them

Cost. Generally the dearer option per stump.
Corrosion is the failure mode. Galvanising is a sacrificial coating with a finite life, and that life depends on soil conditions. Aggressive or persistently wet soils shorten it. This is the steel equivalent of ground-line rot, and the same principle applies: manage the water.
Coating damage matters. Galvanising that’s been cut, drilled or scraped during installation is compromised at that point. Installation quality has more bearing on lifespan than with concrete.
Fewer operators specialise in them, so availability and pricing vary more.

The adjustability point deserves emphasis, because it’s the one with a Bendigo-specific argument behind it — covered below.

Treated timber stumps

Still used occasionally, and worth understanding rather than dismissing.

Modern treated timber is a different product from the untreated hardwood that went into the ground in the 1880s. Structural timber for in-ground use requires treatment to a specified hazard class, which is a graded system covering everything from indoor dry use through to in-ground contact. [[VERIFY — confirm the required hazard class for in-ground structural timber under AS 1604, and state it specifically here. Do not guess the class number.]]

The case for it: generally the cheapest option, easy to work with, light to handle, and the fixings are straightforward.

The case against it: it’s still timber in soil. Treatment substantially extends life but doesn’t make it inert, and the failure mode remains the same ground-line interface that took out the originals. If the reason you’re restumping is that timber stumps failed after a century, choosing timber again is a decision that should be made deliberately rather than by default.

Where it can make sense: smaller structures, verandah and deck supports, and situations where budget genuinely determines the outcome and the alternative is deferring necessary work.

The three compared

	Concrete	Galvanised steel	Treated timber
Relative cost	Middle	Highest	Lowest
Adjustable later	No	Yes	No
Rot risk	None	None	Reduced, not eliminated
Insect risk	None	None	Reduced, not eliminated
Main failure mode	Cracking or spalling under movement	Corrosion where the coating fails	Decay at ground line
Handles lateral movement	Poorly — brittle	Well	Moderately
Weight to handle	Heavy	Light	Light
Installer availability	Universal	More limited	Universal

[[VERIFY — a lifespan row would strengthen this table considerably, but only with sourced figures. Manufacturer durability data or a published industry source, not estimates. Leave the row out rather than guess.]]

Which makes more sense on Bendigo ground

There’s a specific local argument here, and it isn’t a sales pitch for the dearer option — it’s a consequence of what’s under the city.

Bendigo’s reactive clay carries deep-movement classifications, meaning seasonal moisture change reaches further down the soil profile here than it does south of the Great Dividing Range. On top of that, the goldfield means some sites sit above backfilled shafts and uncompacted mullock that continues consolidating. The full explanation of Bendigo’s ground →

Both of those mean the same thing for a stump: there’s a meaningful chance the ground will keep moving after the job is finished. Not necessarily much, and not necessarily soon, but more than in a suburb on stable sand.

That’s the argument for adjustable steel. If the house needs its levels touched up in fifteen years, adjustable stumps mean adjusting them. Concrete stumps mean another jacking job.

The honest counter-argument: that advantage only pays off if the movement actually happens, and on plenty of Bendigo sites it won’t. Concrete on a well-drained block on stable ground will do the job for a very long time at lower cost. Paying a premium for adjustability you never use is a waste, and the site classification and floor level survey are what tell you which situation you’re in.

Which is the point: the materials question can’t be answered before the ground question. Anyone recommending a material before they’ve measured levels and looked at the site is recommending what they stock.

The mistake to avoid: mixing materials across one house

This turns up constantly under Bendigo houses and it causes ongoing problems.

Someone part-restumps the worst side of a cottage — often the wet side, often decades ago — and installs concrete stumps there while the original timber stays under the rest. The house now sits on two different structures with different stiffness, different settlement behaviour and different remaining life.

The result is a house that keeps going out of level no matter how many times it’s corrected, because the two halves are moving independently. Levelling it treats the symptom. The cause is that it’s structurally two houses.

So if a partial restump is being recommended, two questions are worth asking: what’s staying, and how long has it got? Replacing twelve stumps today and the remaining thirty in five years costs more in total than doing the lot once, and produces a worse outcome in between.

That doesn’t make partial restumping wrong — sometimes it’s clearly correct and the remaining stumps are genuinely sound. It does mean the decision should be explicit rather than a way of getting the quote down. How to compare quotes properly →

The thing that matters more than the material

Worth saying plainly, because it undercuts the whole materials debate: water management will affect how long your new stumps last more than the material choice will.

Timber rots at ground line because water is there. Steel corrodes where the coating fails and the soil is aggressive and wet. Concrete cracks where ground movement is driven by moisture cycling. All three failure modes trace back to the same variable.

So the work that protects a restumping job is the cheap work:

Downpipes discharging into stormwater, not beside the house
Cracked or disconnected stormwater lines repaired
Ground falling away from the walls
Subfloor vents clear and cross-flow working
Air conditioner and hot water outflow directed away from the perimeter
Watering and planting reasonably even around the house

Spending an extra several thousand dollars on premium stumps while a downpipe still empties beside the house is the wrong order of operations. Subfloor, drainage and ventilation →

Work out the ground question first

Five questions — what you’re seeing, what the house sits on, roughly when it was built, why you’re looking into it now, and how to reach you. The materials decision follows from the diagnosis, not the other way round.