Plenty of older homes across Werribee, Hoppers Crossing and Altona Meadows have been extended at some point. A family room off the back, a rumpus room, a sunroom or a second bathroom, added in the 1980s, 1990s or later. And on a lot of those homes, the first place movement shows up is the line where the new part meets the old.
You might notice a crack running up the wall at the join, a gap opening between the ceilings, a door at the transition that will not close, or a small step in the floor as you walk from the original house into the extension. Here is why it happens and how it is usually fixed.
Why the join is the weak point
The original house and the extension were built at different times, often by different builders, and very often on different foundations. The common combinations we see around Wyndham are:
- Original house on stumps, extension on a concrete slab. Very common with 1980s and 1990s family room additions. The slab and the stumps respond to the clay in completely different ways.
- Original house on old timber or concrete stumps, extension on newer stumps. The extension's stumps may be a different depth, size or material, and the soil under the original house has had decades longer to settle.
- Extension built on shallower footings than the original, sometimes because it was built as a cheaper add-on or by an owner-builder.
Werribee sits on reactive clay that swells when wet and shrinks when dry. Two structures sitting on that clay with different foundations will move by different amounts, at different times of year. The join between them is where that difference has to go, so that is where the cracks appear.
Drainage makes it worse
The junction is also where water tends to collect. Box gutters and valleys where the extension roof meets the original roof are notorious for overflowing. Downpipes often discharge right at the corner where the two buildings meet. And extensions frequently block the subfloor vents on that side of the original house, so the ground under it stays damp.
Wet clay at the junction swells, dry clay elsewhere shrinks, and the movement at the join gets bigger every season. Our article on why Wyndham's reactive clay soils cause stump failure covers this cycle in more detail.
Is it the house or the extension that has moved?
This is the question that decides the fix, and it is not always obvious. A crack at the junction only tells you the two parts have moved relative to each other. It does not tell you which one moved.
To answer it, we measure floor levels across the whole house, not just the room where the crack is. If the original house has dropped along the back wall because its stumps have failed, the fix is under the original house. If the extension has sunk at its outer edge or dropped where it meets the house, the fix is under the extension. Sometimes both have moved.
How it is usually fixed
The repair depends on what is underneath each part:
- Original house on failed stumps. Restumping or partial restumping along the affected walls, then releveling so the original floor matches the extension again. See our guide to partial restumping in Werribee for when replacing only some stumps makes sense.
- Extension on stumps that have moved. The same approach applied to the extension, often with the stumps at the junction replaced and the bearers properly connected.
- Extension on a slab that has dropped at the edge. Stumps cannot fix a slab. This is where underpinning comes in, supporting the slab edge on new footings taken down to more stable ground. Our Underpinning service and our article on restumping vs underpinning explain the difference.
- Floor framing damaged at the junction. Where bearers or joists have been poorly connected or have softened from damp, our Subfloor & Foundation Repair service deals with the framing before the floor is releveled.
In almost every case, the drainage at the junction needs fixing as part of the job. Overflowing box gutters, misdirected downpipes and blocked vents need to be sorted, or the clay will keep moving and the repair will not last.
What to expect afterwards
Once the floors are back to level, the cracked plaster and cornice at the join can be repaired. Some homeowners also choose to put a proper movement joint at the junction, a deliberate, flexible line in the plaster, so that any small future movement shows as a neat line rather than a ragged crack. Doors and windows near the junction often start working properly again once the levels are corrected.
It is worth being realistic about two things. First, a house with two different foundation types will always move slightly differently, and the aim is to reduce and manage that movement rather than eliminate it. Second, some releveling work can open or close existing cracks elsewhere as the house moves back towards level, which is normal and is repaired at the end.
Permits and paperwork
Whether a building permit is needed depends on the scope of the work, and restumping and underpinning are generally permit work in Victoria. If you are not sure whether your extension was built with a permit, it is worth checking with Wyndham City Council, because that can affect what a building surveyor asks for. Our article on restumping building permits in Werribee covers this.
Book an inspection
If you have a crack, a gap or a step where your extension meets the original house, the first job is working out which part has moved and why. We inspect the subfloor and the slab edge, check the drainage at the junction, measure floor levels across the whole home, and give you a fixed written quote for the fix that suits your house. Book your free inspection online anywhere across Werribee and Wyndham.