Underpinning in Melbourne's West — Steadying Brick Walls & Footings

Footings, not stumps

Brick houses do not stand on stumps the way weatherboards do. A double-brick home rests on a continuous strip footing: a ribbon of concrete running under every structural wall. A slab home rests on the slab itself, which is thickened into edge beams around the outside. A brick veneer usually has a strip footing under the outer brick skin and stumps inside carrying the timber floor.

Brickwork is rigid. If one length of footing sinks or tilts, the wall above cannot flex with it, so it cracks along the weakest line, which is usually the mortar. Underpinning goes down beneath the footing that has moved and gives it a deeper, firmer base. It is about stopping the movement. It will not make an old wall perfectly straight.

What footing movement looks like

The walls usually give it away:

• Staircase cracking zig-zagging through the mortar joints of a brick wall.
• Cracks that taper, wide at the top and fine at the bottom or the reverse, hinting that a footing section has tipped.
• Window and door frames pulling away from the brickwork, or no longer square.
• Inside cracks that mirror outside ones, appearing in the plaster on the opposite face of the same wall.
• Tiles cracking in a line across a slab floor, or a slab floor that has developed a fall.
• Doors that jam in a house with brick walls and a slab or brick perimeter.

Plenty of fine cracks are nothing to worry about, particularly in old lime mortar. The ones to have checked are wide, growing, or back again after being filled.

How basalt country affects footings

The west is basalt country. The clay that has formed over the rock is dark, heavy and strongly reactive, and in a dry summer you can see it split open in wide cracks across nature strips and backyards. Footings resting in that clay rise and fall with it. Where a house has a mix of conditions under it, such as clay on one side and shallow rock on the other, the two sides can move differently and the brickwork between them pays the price.

Shallow rock can make underpinning simpler in one sense, because sound basalt gives an excellent base to found on. It also makes the digging harder, since the pits may need rock broken out by hand or with machinery to reach the level the engineer wants. Every site is different, which is why the design comes from an engineer rather than a rule of thumb.

The steps on an underpinning job

• Assessment. We record the crack pattern, check floor levels and look at drainage, nearby trees and plumbing.
• Design. An engineer specifies the depth, size and sequence of the underpinning for that particular house.
• Approval. The building surveyor signs off on the design and issues the permit.
• Staged digging. Pits go in under the footing a few at a time, in a set order, so the wall never loses support.
• Pit check. An inspector looks at the excavations before they are filled.
• Concrete. Each pit is poured and packed hard up under the old footing.
• Repairs. After the concrete has cured and the house has had time to settle, the cracks can be dealt with.

Brick, timber or both

A lot of houses in the west mix materials. A brick veneer in Sunshine or Altona North often has a brick outer skin on a strip footing, with a timber floor on stumps inside. A period cottage in Williamstown may have a brick plinth across the front and timber stumps everywhere else. In those cases the internal stumps may need reblocking, the perimeter footing may need underpinning, or both, depending on where the movement is coming from. We look at the whole house before recommending anything.

Fix the water too

New concrete under a footing will not help much if the clay around it keeps getting soaked and dried. Broken stormwater pipes, downpipes emptying against the wall, paving that falls toward the house and thirsty trees close by can all keep the ground moving. We flag these at the inspection so they can be fixed at the same time as the structural work.