Why Your Sydney Home Is Cracking After This Year’s Drought

A photo I keep on my phone

I have a photo on my phone of a kitchen wall in a Penrith home I looked at three months ago. There’s a crack running from the corner of the doorframe up to the ceiling cornice. It’s about 4 millimetres wide at the top, fades to nothing at the bottom, and runs at a slight angle.

The owner showed me the same wall in a photo she took in 2023. Same wall, same corner, no crack at all.
That’s not the house ageing. That’s the ground underneath her house moving — and this year’s dry summer is the reason.

I’ve been underpinning Sydney homes for 28 years. Every time we hit a long dry spell, the phone starts ringing more. The last bad drought, 2017 to 2020, pushed our callout volume up by about 40 percent. This year is looking similar.

If you’ve started noticing cracks, sticking doors, or floors that feel a bit off — and you live anywhere from Penrith out to Campbelltown, or across the Hills District — I want to explain what’s actually happening under your feet, so you can decide whether it needs urgent attention or can wait.

The science behind it.

Most of Sydney sits on clay. Not the soft potter’s clay you might picture — much harder stuff, mixed with sand and rock. But it still behaves like clay in one important way: when it gets wet, it expands. When it dries out, it shrinks.

This is called “reactive” soil. Engineers classify it from S (stable) up to E (extremely reactive). Most of Western Sydney is H (highly reactive). Parts of the Eastern Suburbs and Lower North Shore sit on sandstone and don’t have this problem nearly as much.

When clay shrinks, it doesn’t shrink evenly. The clay around the edge of your house dries out faster than the clay deep underneath. So one side of the house can drop relative to the other. Or one corner can sink while the rest stays put.

When that happens, your foundation tries to follow the soil. But your house is rigid — bricks, mortar, plaster, tiles. Rigid things don’t bend. They crack. That’s what you’re seeing.

Why this year specifically

A normal Sydney summer has wet and dry patches. The clay moves up and down a bit, the house moves with it, and nothing major breaks. The cycle balances out.

A drought is different. The clay just keeps shrinking. There’s no rain to put the moisture back. And the longer it goes on, the deeper the drying goes.

By the third or fourth dry month, the clay that’s been holding up your footings is starting to pull away. Footings that were sitting nicely on solid ground are now sitting on ground that’s shrunk away below them. Sometimes the footing follows the soil down. Sometimes the footing stays but cracks form where the load is no longer supported evenly.

This is the part that catches people out: the damage isn’t from the heat. It’s from the lack of water.

The five things I look for in your home

When I come to a free assessment, here’s what I’m actually checking:

1. Crack direction and width

Diagonal cracks running corner to corner are a much bigger deal than vertical hairline cracks. Width over 5 millimetres is a structural concern. Width over 10 millimetres needs urgent attention.

2. Where the cracks are.

Cracks around door and window openings are normal stress points and don’t always mean foundation movement. Cracks running through the middle of a wall, or stair-step cracks through external brickwork, almost always do.

3. Door and window operation.

If a door has started sticking in the last six months — and nothing else has changed (no humidity, no new carpet) — that’s the house moving. The doorframe is no longer square.

4. Tile cracks and grout failures.

Especially in bathrooms and kitchens with tile floors. Long cracks running across multiple tiles, particularly in a line, are a sign the slab has moved.

5. Outside the house.

I walk the perimeter. I look for gaps between the path and the wall. I check for cracks in concrete paths or driveways. I look at the soil itself — has it pulled away from the brickwork? Are there visible gaps?

If three or more of these show up, we have a real problem. One or two on their own — could be cosmetic, but worth keeping an eye on.

The mistake I see homeowners make

The mistake is waiting.

I get it. Cracks are scary. Calling an underpinner sounds expensive. The natural reaction is to hope it stops.

Here’s the problem: if foundation movement is causing cracks, the cracks don’t just stay the same size. They grow. The next dry summer makes them worse. The next wet patch causes other parts of the house to move too. The damage compounds.

A job that costs $12,000 if you catch it early can easily turn into $25,000 in two years. Not because we’re charging more — because more piers are needed, more areas have moved, and there’s now collateral damage like cracked tiles, broken windows, and damaged plumbing where pipes have been pulled apart.

The free assessment exists for a reason. If your cracks are cosmetic, I’ll tell you. I’d rather give you 30 minutes of free time and have you sleep better than scare you into work you don’t need.

What we actually do about it

If the diagnosis is foundation movement, the fix is underpinning.

Underpinning sounds dramatic but the basic idea is simple: we go below your existing footing, down to where the soil is stable, and we build new support down to that depth. The house then sits on the new, deep support — and the reactive clay near the surface can do whatever it wants without affecting you.

For most Sydney homes, we use mass concrete underpinning. We dig down underneath the existing footing in small sections — typically 1.5 to 4 metres deep — and pour concrete to fill the gap. Each section is engineer-designed and certified.

When to call us, and when not to

Call us if:

• You’ve noticed cracks appearing or growing in the last 6-12 months
• Doors have started sticking that used to close fine
• You see diagonal cracks wider than 3 millimetres
• Your floor feels uneven or has visible slope
• You’re in Western Sydney, the Hills, or South-West Sydney and your house is over 20 years old

Don’t worry yet if:

• Cracks are hairline (under 1 mm) and haven’t changed in a year
• Cracks are only around door frames and the doors still work fine
• You’re on sandstone (Lower North Shore, parts of Eastern Suburbs) and haven’t seen new cracks appear

Either way, if you’re not sure, the assessment is free. Give me a call on 0411 487 203 and I’ll come out, walk around the house, and give you an honest opinion. If it’s cosmetic, I’ll tell you. If it’s serious, you’ll have a clear plan.

The bottom line

Sydney’s clay soils plus a dry summer equals foundation movement. It’s not your house’s fault, it’s not your fault, and it’s not getting better on its own.

The good news: caught early, it’s a defined job with a defined cost and a permanent fix. Caught late, it gets expensive.

If you’ve been looking at a crack and wondering whether it’s serious — stop wondering and book an assessment.

Worried about cracks appearing this summer? Get an honest assessment from Mark and the A1 Underpinning team. Call 0411 487 203 or book online — we cover all of Sydney and surrounding areas.