Basalt Clay and Your Stumps: Why Northern Suburbs Floors Keep Moving

If you dig a hole in a back yard in Coburg, Reservoir, Glenroy or Thomastown, the first half-metre is usually dark topsoil. Below that, the spade starts to stick. That heavy, greasy grey-black clay is the reason so many floors in the north are out of level.

Where the clay comes from

Much of northern and western Melbourne is built over basalt laid down by ancient volcanic eruptions. Weathering broke the upper layers of that rock down into fine clay. Geologists call it highly reactive, which means it changes volume a lot as its moisture content changes.

The basalt plains run roughly from the Merri and Darebin creek corridors westward. That takes in most of Merri-bek, northern Darebin, Whittlesea west of the Plenty River, the southern parts of Hume, and Moonee Valley. East of the Darebin Creek, through Banyule, the geology changes to older sedimentary rock and valley soils.

What the clay does through the year

Winter. Rain soaks into the clay. It swells, and anything sitting in it is pushed upward. Stumps rise slightly, and the floor goes up with them.

Summer. The clay dries from the top down. It shrinks and cracks, sometimes with fissures wide enough to put your fingers into. Stumps lose support around their sides and can settle.

Over the years. The cycle repeats. If every stump moved by the same amount, the floor would simply go up and down and you would barely notice. The problem is that they do not.

Why movement is uneven

Several things make one stump move more than another:

• Sun and shade. The north-facing side of a house dries out faster in summer; the south side stays damper.
• Trees. Large trees draw water out of the clay within their root zone, causing extra shrinkage near them.
• Downpipes and leaks. A downpipe that discharges at the base of a wall keeps that patch wet; a leaking pipe under a bathroom does the same.
• Paths and paving. Concrete laid against the house can stop the clay under it drying, or channel water toward the subfloor.
• Load. Internal stumps under walls and the roof structure carry more weight and press down further.

Over time, these differences add up. A floor that was level when it was built ends up with low spots and high spots.

Why timber stumps suffer most

Timber stumps in reactive clay face two problems at once. The ground pushes and pulls them, and the moisture cycling at the ground line rots them. Once the timber softens, the stump can no longer resist the movement, and settlement speeds up.

That is a big reason why post-war houses in Reservoir, Fawkner, Glenroy, Lalor and Broadmeadows, many stood on cheaper timber, are now needing restumps in large numbers.

How footings deal with it

When a house is restumped, each new stump sits on a concrete footing dug as deep as the permit drawings require. The aim is to get below the zone where the clay moves most, or to reach firmer material underneath. On the plains, the crew sometimes hits basalt before the specified depth, which gives a solid base.

New footings do not stop the clay moving, but they reduce how much of that movement reaches the house.

The Banyule side is different

East of the Darebin Creek, through Ivanhoe, Heidelberg, Rosanna, Macleod and Greensborough, the ground is more varied: silty clays, weathered siltstone and river sediments near the Yarra and Plenty. Reactivity varies from one block to the next.

There, the bigger issues tend to be slope and drainage. Water running downhill collects under the low side of a house, and that side moves more. Fill brought in to level a block can also settle over time.

What helps

• Get water away from the house. Downpipes into stormwater, not onto the ground beside the walls.
• Fix leaks promptly. A slow leak under a bathroom can soften clay and rot stumps quickly.
• Keep subfloor vents clear. Good airflow helps the subfloor dry evenly.
• Think about tree placement. Large trees close to the house can cause localised shrinkage.
• Consider adjustable stumps. Steel stumps with a threaded head can be tweaked later without new packers.

When to get it checked

Seasonal movement is normal in small amounts. A door that binds through July and swings freely by February is common on the plains. What is not normal is movement that keeps growing year after year, or a floor that has clearly dropped in one area. If that sounds like your house, a subfloor inspection will show whether the stumps, the ground or the plumbing is to blame.