Both are a long way ahead of timber
Before comparing them, worth being clear that this is a choice between two good options. Timber stumps rot at ground level and termites eat them, and in South East Queensland both of those happen faster than most places. Neither concrete nor galvanised steel has either problem. Whichever you pick, you are solving the problem that brought you here.
The comparison is about second order questions: what it costs, what happens in twenty years, and how easy it is to correct if the house moves again.
The short version
- Concrete is cheaper, immune to corrosion, extremely durable in the ground, and fixed in height once set.
- Galvanised steel costs more, has an adjustable head so the house can be re levelled without excavation, and depends on its coating staying intact.
If your ground is stable and there is no history of repeated movement, concrete is usually the sensible default. If you are on reactive clay and the floor has gone out before, adjustability is worth paying for. If you are close to the ocean or the canals, weight the corrosion question more heavily.
Cost
Steel costs more per stump. That is consistent and it is not close.
What surprises people is how much less that matters than it sounds. On any restump, the dominant costs are access, support and labour. Supporting the house on jacks and cribbing, digging holes, and working in a confined subfloor take the same time whichever material goes in. On a low set house where every hole is hand dug, the material choice can be a modest fraction of the total.
So the question is not really "which is cheaper" but "is the difference worth what it buys". On a house with 40 stumps and stable ground, probably not. On a house with 80 stumps on reactive clay that has already been levelled twice, quite possibly yes.
Adjustability
This is the real differentiator.
A concrete stump is a fixed height post. If the floor goes out later, correcting it means getting back under the house with jacks, lifting bearers and repacking. That is a proper job with a proper cost.
A steel stump has a threaded head. Correcting a floor can be a matter of turning collars. Substantially cheaper, substantially less disruptive.
One important caveat. The heads have to remain physically reachable for this to mean anything. If the subfloor is going to be enclosed, or the crawl space is 400mm and someone has to belly crawl to each stump, the practical benefit shrinks a long way. Ask specifically how a future adjustment would be carried out before paying the premium for the capability.
Corrosion, and why it matters more here
Concrete does not corrode. Its reinforcement can, but only if the stump cracks and lets moisture in, which is an installation and handling issue rather than a material one.
Steel is protected by galvanising, which is a coating. Coatings can be scratched during installation, cut ends can be left untreated, and a base sitting permanently in wet or salty soil is a harsher environment than a dry one.
On the Gold Coast that is a geographic question. A block at Currumbin or on a canal at Mermaid Waters is a more aggressive environment for steel than a block at Nerang or Canungra. It does not rule steel out anywhere, but near the coast the details matter more: the coating specification, whether the base plate is properly encased, whether cut ends were treated on site, and whether the ground drains.
Soil, which decides more than anything
Reactive clay is the reason this comparison exists on the Gold Coast at all.
Through the hinterland and the western suburbs the clay content is high, and clay expands when wet and contracts when dry. It does that forever. A house sitting on it will ride that cycle regardless of what the stumps are made of, and no material stops the ground moving.
What changes is how you deal with the consequence. On stable sandy ground, a well founded concrete stump set once may never need touching. On clay that cycles seasonally, some drift over a decade is realistic even after good work, and being able to correct it by adjustment rather than excavation is genuinely valuable.
This is also why founding depth is the question that matters most, whichever material you choose. A support founded inside the zone where the clay is still moving will move with it. Ask what depth is being used and why, and treat a cheaper quote based on shallower holes as a different job rather than a better price.
A rough decision guide
- Stable ground, no history of movement, cost sensitive. Concrete.
- Reactive clay, house has been levelled before. Steel is worth pricing.
- Coastal or canal block with salt exposure. Lean concrete unless there is a strong reason otherwise.
- House being raised, tall supports. Steel is often favoured, with engineering input on sizing.
- Very poor access, hand dug holes. Steel, because you do not want to repeat the excavation.
- Subfloor to be enclosed later. Concrete, since the adjustability you paid for will not be reachable.
Do not mix them if you can avoid it
Mixed systems under one house happen, usually where a partial replacement was done years after an earlier one. It is not a disaster but it is not ideal, because the two behave slightly differently and future levelling becomes inconsistent. Where practical, keep one system under one house.