Steel vs concrete stumps for a Mornington Peninsula house

The short answer

Concrete for most houses. Steel where the floor is low, where the ground moves with the seasons, or where the house is likely to be relevelled again. Precast concrete stumps cost less per stump, are unaffected by salt, damp or alkaline sand, and carry a cast-in rod that bolts the bearer down. Galvanised adjustable steel stumps cost more, need a concrete pad poured under each one, and can be wound up or down with a spanner for the life of the house, which on the clay of the Western Port side is worth paying for. An industry guide lists timber, precast concrete and galvanised steel as the three options and puts steel at sometimes several times the cost of concrete per stump. Timber is not on our list for the Peninsula. Redgum stumps did their decades in dune sand and fewer in clay, and the ground has not changed.

Side by side

Precast concrete stumps

A square reinforced concrete post, cast with a threaded rod in the top, set in a dug hole on a concrete sole plate and backfilled. The bearer sits on a galvanised ant cap and is bolted down through the rod. Nothing in it rots, rusts or gets eaten, and nothing in the Peninsula's ground touches it: the calcareous dune sand of the Cups unit at pH 8 to 8.5 is alkaline rather than acid, and the sodic clays of the Bittern unit are a drainage problem for the hole, not a chemistry problem for the stump. The limits are practical. A concrete stump is fixed at the height it is set, so if the house settles again the fix is packing. And the stump has to be got under the house, which on a low fibro holiday house in Rosebud means feeding a heavy post in sideways through a gap the crew can barely crawl through. For a weatherboard in Dromana or Mornington with room to move under the floor and ground that drains, concrete is the obvious choice.

Galvanised adjustable steel stumps

A hot-dip galvanised steel post on a poured concrete pad, with a threaded adjustable head and a steel top plate fixed to the bearer. The pad does two things. It spreads the load on soft ground, and it keeps the steel out of the soil, which matters more on the Peninsula than most places because so much of the Shire's housing sits within a few streets of 192 km of coastline. Galvanising is a sacrificial coating; salt-laden air and damp shorten its life, and steel standing in wet sand or waterlogged clay goes faster still. On a pad, above the ground, with the base boards keeping sand and spray off it, hot-dip galvanised steel does well. Painted tube or pre-galvanised sheet is not the same thing and should not be in the quote.

The adjustable head is the reason to pay for them. On the Hastings plains and the Bittern clays, where a house relevelled in autumn can be out again by spring as the clay swells through a 73.5 mm June and shrinks through a 34.8 mm February, a crew can come back and wind the worst corner up without jacking or digging. For an owner who visits a holiday house a few weekends a year and would rather not organise a crew under the floor every few winters, that is a real saving. Steel also wins under a low floor, and under a raised house on the granite slopes where the stumps are on show.

Why not timber

Because the ground has already shown what it does to timber. In dune sand a redgum stump rots at the ground line, where sand and moisture meet the air, and stands sound above and below until it is hollow at the one point that matters. In the Hastings and Bittern clays it stands in water for part of every winter. The whole Shire is designated subject to termite infestation under regulation 150 of the Building Regulations 2018, and the Shire's own page says new houses and extensions need termite management to AS 3660.1. Treated pine stumps exist and the industry guide lists them, but in a subfloor that is wet for a season every year they are a shorter answer than concrete or steel at a similar installed price. Restumpers working on the Peninsula do not put timber back under a house unless the owner insists, and most will say why.

Which one where

Ground, not postcode, makes the call, and the soil test classifies the site. But the Shire's soil units give a starting point for the conversation with the builder. On the Cups dunes from Rye to Portsea and the Rosebud unit under Rosebud, Capel Sound and McCrae, concrete on a sole plate is the usual answer, with steel kept for low verandahs and the odd sleep-out. On the Dromana unit from Safety Beach to Mount Martha, where mottled clay starts at about 450 mm, either works and the drainage matters more than the stump. On the Bittern unit under Mount Eliza, Baxter and the Hastings side, and the Hastings unit across Hastings, Tyabb and Somerville, where the clay waterlogs and moves, adjustable steel on wide pads on the worst side of the house saves a second dig. On the granite slopes of the Arthurs Seat unit, where the subfloor is often open to view and the depth to rock varies, slim steel on pads is the common choice. The surveyor and the builder settle it from the first holes, and the quote should say what changes if the ground is not what the first holes suggested.

Mixing them under one house is common and sensible. Concrete under the body of the house, adjustable steel on the corner that keeps moving or under the bathroom, steel under the low front verandah where there is no room to feed a precast post in, steel under a raised deck side on a Mount Martha slope where the stumps can be seen from the garden. Each stump is set to firm ground, each bearer is packed to one laser line, and the floor does not know the difference. The building permit from the surveyor covers both, and it names the type and count of each.

What the quote should say

The stump type by name, the number of each, the sole plate or pad under them, hot-dip galvanising if steel, ant caps, the depth the holes will go to, and how the holes will be shored in sand or pumped in clay. It should say the builder's BPC registration number, because registration is required for restumping regardless of the value of the work, and whether the job is over $10,000, which requires a written major domestic building contract, and over $20,000, which from 1 July 2026 requires the builder to hold Home Warranty insurance. If the quote just says new stumps, ask which, and ask why.

Two Peninsula questions belong in the same conversation. The first is heritage: in the Sorrento Historic Precinct, East Sorrento and Back Beach Road Portsea, a change from timber to slim steel stumps under a raised limestone villa or timber cottage might be visible from the street, and any visible change can trigger a planning permit. Ask the Shire's heritage team at [email protected] before deciding; the pre-application advice is free. The second is the ground: nobody can tell you the AS 2870 site class from the suburb, the soil test classifies the site, and the depth and pad size follow from that rather than from the stump material. The soils guide explains what the test looks for.