Galvanised steel stumps

What makes steel different

A galvanised steel stump is a hollow section post with a base plate that sits in a concrete footing, and a head that adjusts. The adjustment is usually a threaded collar or a screw jack arrangement, and it is the entire reason people choose steel.

With concrete, the height of each support is fixed the moment it is set. Bringing a floor back to level later means jacking the bearer and repacking, which means getting under the house with equipment again. With adjustable steel, correcting a floor can be a matter of turning collars.

That difference matters most on ground that keeps moving, which describes a lot of the Gold Coast hinterland.

Where steel earns its cost

• Reactive clay. Through Nerang, Mudgeeraba, Worongary, Canungra and up toward Tamborine the clay content is high, and clay never stops cycling between wet and dry. A house levelled today may drift over the next decade regardless of how well the work was done.
• A history of repeated movement. If the floor has been packed or levelled before and gone out again, adjustability is worth pricing.
• Difficult access. On a low set house where every hole was hand dug, nobody wants to repeat that. Being able to adjust from a crawl space without excavation is a real advantage.
• Raised houses. Taller supports carry more slenderness and often more load, and steel sections can be engineered for it more readily than long precast stumps.

Where steel is the weaker choice

Corrosion is the honest downside. Steel in the ground is protected by its galvanising, and galvanising is a coating. Cut it, scratch it, weld it after the fact or leave it sitting in permanently wet or salty soil and the protection degrades.

That matters on the Gold Coast more than it does inland. Properties close to the ocean, on the canals, and anywhere with salt in the ground or the air are a harsher environment for steel than a block twenty kilometres west. It does not rule steel out, but it makes the details matter: the coating specification, whether the base plate is properly encased in concrete, whether cut ends have been treated, and whether drainage keeps the base from sitting wet.

The second consideration is cost. Steel is more expensive per stump. On a house with 70 stumps that difference adds up, and it has to be weighed against how likely you are to actually use the adjustability.

Installation details worth asking about

• Footing. The base plate needs a properly formed and compacted concrete footing sized for the load and the soil. This is where settlement problems come from, not from the steel.
• Encasement. How much of the base is encased in concrete, and whether the concrete sheds water away from the shaft rather than holding it against it.
• Coating. What the galvanising specification is, and how cut ends and drilled holes are treated on site.
• Head fixing. How the bearer is fixed down to the head, since tie down matters as much as bearing in a high wind area.
• Adjustment access. Whether the heads will actually be reachable later. An adjustable stump you cannot get to is just an expensive fixed stump.

That last one is worth saying twice. The whole value proposition of steel is future adjustment. If the subfloor is going to be enclosed, or the crawl space is 400mm, ask specifically how a future adjustment would be carried out before paying the premium.

Maintenance

Steel stumps are not a maintenance free product in the way concrete effectively is. A sensible owner checks them occasionally.

• Look for rust staining or blistering at the base, which is where it starts
• Keep soil, mulch and paving away from the base so it is not sitting permanently damp
• Keep subfloor vents clear so the space dries out between rain events
• Check that adjustment collars have not been packed with dirt or seized
• If the floor starts drifting again, get it looked at while the adjustment is still small

Steel does not exempt you from the paperwork

Whichever material you choose, the work is licensed structural work. Look the contractor up on the Queensland Building and Construction Commission register, and confirm home warranty insurance where the value of the work requires it. Neither the material nor the method changes those obligations.