Underpinning in Melbourne — Stabilising Brick Homes & Footings

What underpinning actually does

Every house transfers its weight into the ground through its foundations. In a timber-floored weatherboard, that load goes down through dozens of individual stumps. In a brick house, it usually goes through a strip footing, a continuous concrete beam running under each load-bearing wall. In a slab home, the slab itself and its thickened edge beams carry the load.

When the soil beneath a footing loses strength or moves, part of the footing drops. Because brick is rigid, it cannot flex with that movement, so it cracks. Underpinning reaches below the existing footing to firmer, more stable ground and transfers the load down to it. The aim is to stop further movement, not to make the house look new.

Signs a house may need underpinning

Underpinning problems usually show in the walls before anywhere else. Look for:

• Stepped cracks in brickwork, running diagonally along the mortar joints like a staircase.
• Cracks that widen at the top or bottom, which suggest a section of footing has rotated or dropped.
• Gaps between walls and window or door frames, or frames pulling out of square.
• Cracks in internal plaster that line up with external brick cracks.
• Sloping floors in slab homes, or cracks in tiled floors running across the slab.
• Doors that jam in a brick house whose floors are on a slab or a brick perimeter.

Hairline cracks are common and not always structural. Cracks that are wide, keep growing, or reappear after patching deserve a proper look.

Why Melbourne footings move

Much of Melbourne sits on reactive clay, which changes volume with moisture. Suburbs across the north, the west and the south-east are known for it, and Oakleigh, Moorabbin, Bentleigh, Cheltenham, Preston and Reservoir all include areas of highly reactive soil. During a wet winter the clay swells and pushes footings up. Through a long dry summer it shrinks and footings drop. Over years, this movement can be uneven around a house, and that is when cracking appears.

Other common causes include leaking stormwater or sewer pipes softening the ground under one corner, large trees drawing moisture out of clay, footings that were too shallow for the soil, poor drainage, and excavation on a neighbouring property.

Underpinning or restumping?

This is one of the most common questions we hear, and the answer depends on how the house is built.

• Weatherboard on stumps: the stumps are the foundations. If they fail, the house needs restumping, not underpinning.
• Brick veneer with a timber floor: the timber floor usually sits on internal stumps, while the outer brick skin sits on a strip footing. Internal problems are fixed by restumping, and cracking in the brick skin may need the perimeter footing underpinned. Some houses need both.
• Double-brick or solid brick: the walls sit on strip footings. Movement is fixed by underpinning.
• Slab-on-ground: there are no stumps at all. Settlement of the slab edge is fixed by underpinning the edge beam.

Older double-brick homes are found right across the inner suburbs, while brick veneers on timber floors are everywhere through the east and south-east, from Mitcham and Ringwood to Mentone and Frankston. Working out exactly which system holds your house up is the first job at every inspection.

How underpinning is done

Every underpinning job is designed by a structural engineer, who considers the soil, the extent of movement and the structure. Common approaches include:

• Mass concrete underpinning. The traditional method. Short sections are dug out under the existing footing in a set sequence, never side by side, and filled with concrete down to stable ground. The house stays supported by the undisturbed sections while each pit cures.
• Piered underpinning. Concrete or steel piers are installed to reach deeper bearing ground and connected to the footing or slab. This suits sites where stable soil is a long way down.
• Localised underpinning. Only the section of footing that has moved is treated, rather than the whole house, when movement is confined to one area.

Underpinning in Victoria requires a building permit from a registered building surveyor. The excavations are inspected before concrete is poured, so the work matches the engineer's design.

Living with the work

Most owners stay in the house throughout. The digging happens outside along the footings, so disruption inside is usually limited to noise and restricted access around the affected walls.

After underpinning

Underpinning stops the footings moving, but it does not close existing cracks. Once the house has had time to settle on its new support, cracks can be repaired by a bricklayer or plasterer. Fixing the original cause is just as important: leaking pipes repaired, drainage improved and trees managed where they are drying out the clay. We will point out anything we see that could undermine the work.

Getting a real figure

Underpinning varies more than almost any other foundation work. The amount of footing involved, the depth to stable ground, the method the engineer specifies and site access all change the job. An on-site inspection gives a real figure and a clear view of whether your house needs underpinning, restumping or both.