What a concrete stump is
A precast concrete stump is a square section post, reinforced with steel bar, cast in a factory and cured before it comes to site. It sits in a hole, is concreted at the base, and carries a galvanised steel ant cap on its head. The bearer sits on the ant cap and is packed and fixed down to it.
They are not glamorous and that is the point. They do not rot, termites have no interest in them, they do not corrode in the way steel can, and they have decades of track record under Queensland houses. When a contractor quotes a restump without specifying otherwise, this is usually what they mean.
Sizes and what they carry
Precast stumps come in a range of lengths so they can be matched to the height of the subfloor and the required founding depth. The section size is chosen for the load and for how much of the stump is above ground, since a taller stump is more slender and behaves differently under load than a short one.
For a standard single storey timber home on a level block this is routine and the contractor will size them from experience and from the relevant standard. Where it stops being routine is on a steeply sloping site with long stumps at the downhill end, under a two storey addition, or where a house is being raised. Long or heavily loaded stumps are a design question and engineering input is appropriate.
Founding depth, which is the part that matters
A concrete stump is only as good as what it is standing on. Set it too shallow and it sits inside the zone where the soil is still swelling and shrinking with the seasons, and the stump moves with the soil. You have then replaced a rotted timber post with a concrete post that goes up and down.
This is a live issue on the Gold Coast. Through Nerang, Mudgeeraba, Worongary and out into the hinterland there is a lot of reactive clay, and reactive clay has a zone near the surface where seasonal moisture change causes real movement. Founding below that zone is what stops the house riding the cycle.
So when you compare quotes, ask what depth they are going to and why. A quote that is cheaper because the holes are shallower is not cheaper, it is a different job with a worse outcome. On any site with significant clay, a soil classification is worth having rather than guessing.
Ant caps
The galvanised plate between the stump head and the bearer is not decorative and it is not a termite barrier in the sense most people assume. Termites can build over anything. What the ant cap does is force them to build around the outside of the turned down edge, in the open, where an inspector with a torch can see the mud track.
It is an early warning device. Which is why a missing ant cap, a corroded one, or one that has been bridged by a garden bed built up against the stump, is a genuine problem rather than a cosmetic one. Replacing them is cheap while the house is already up on jacks and expensive as a standalone job later.
What goes wrong with concrete stumps
- Cracking. Usually from impact, from being installed with an uneven bearing at the base, or from being loaded off centre. A cracked stump with exposed reinforcement will corrode from the inside and needs replacing.
- Settlement. Almost always a founding depth problem or a hole that was not properly compacted at the base.
- Corroded ant caps. Common on older installs, and more common near the coast.
- Packing failure. Timber packers between the stump head and the bearer crush and rot over time. A stack of loose packers under one bearer is a sign someone has been chasing a level.
- Ground built up over them. Garden beds, concrete paths and paving raised against stumps trap moisture and bridge the ant cap. Very common and entirely preventable.
Concrete or steel
The short version. Concrete is generally cheaper, extremely durable in the ground, and fixed in height. Galvanised steel costs more, has an adjustable threaded head so the house can be re levelled later without excavation, and depends on the galvanising staying intact.
Concrete tends to be the right answer on a straightforward job where the ground is stable and there is no history of repeated movement. Steel earns its extra cost on reactive clay where the house has moved before and may move again, or where future adjustment is a priority.
Neither is a premium product and neither is a compromise. They are different tools. The full comparison, including cost, corrosion and adjustability, is on the concrete versus steel page.
What a good quote specifies
- Number of stumps and whether it is full or partial
- Section size and the founding depth being used, with the reason
- Whether a soil classification has been done or is being relied on
- New galvanised ant caps on every stump
- How bearers are being packed and fixed down
- Any bearer or joist replacement, or a clear statement that it is excluded and will be priced if found
- Who disconnects and reconnects services under the house
- Whether making good plaster and cornice cracking is included
- Confirmation of QBCC licence and home warranty insurance where the value requires it