Foundations, Paths and Slabs

Who owns the foundations?

Foundations and the slab they support are structural elements of a building. They are common property (with very rare exceptions) owned and maintained by the corporation, regardless of which unit sits on top.

The same usually applies to paths and external concrete around the buildings, although this depends on whether the path crosses common property or sits inside an exclusive-use area.

Strata Title: Section 5 of the Strata Titles Act

Under Section 5 of the Strata Titles Act 1988 (SA), the boundary of a unit is the inner surface of the floor slab. Everything below that surface such as the slab, footings, piers and foundation soil is common property. Section 25 then makes the corporation responsible for maintaining it.

Paths are common property when they sit on common property. Where a unit has an exclusive-use area (for eg. a courtyard or private garden) and the path runs through that area, the path is generally still common property unless a by-law says otherwise. At the end of the day, the By-law is what matters, so check it.

Community Title: Sections 28 and 75

Under the Community Titles Act 1996 (SA), lot boundaries are defined by the deposited plan. In strata divisions (one lot above another), the same logic as strata title applies - the slab and footings are common property under Sections 28 and 75. In lot-beside-lot divisions, the foundations of buildings within a lot are the lot owner's. Foundations of buildings on common property (a shared shed, a community hall) belong to the corporation.

Footing types in South Australian unit groups

Concrete strip footing

Concrete strip footing is most common in older buildings. It is a continuous concrete strip that runs under each load-bearing wall. Although cheap to build, it is unsuited to South Australia's reactive clay soils as strip footings often fail to handle the seasonal soil movement, which is why so many older units in Adelaide's clay belt show diagonal cracks above doorways.

Stiffened raft slab

The most common modern footing for unit groups is stiffened raft slab. It is a reinforced concrete slab with downward-projecting beams under load points; the whole thing acts as a single rigid platform that rides over soil movement. When properly designed and drained, a raft slab handles reactive clay well.

Bored piers

Bored concrete piers are drilled deep into stable soil, with bearers spanning between them. They're most commonly used on poor or unstable sites such as coastal sand, made ground and sloping blocks. They're expensive to install but extremely stable once in.

What causes foundation movement?

South Australia's reactive clays, particularly across the Adelaide Plains and the eastern suburbs toward the Hills, swell when wet and shrink when dry. The seasonal movement is enough to crack masonry, jam doors and pull tiles off walls.

The three biggest contributors to foundation trouble are:

Plumbing leaks under or near the slab: Most Australian soils are clay-heavy and clay swells when it gets wet and shrinks when it dries out. A slow leak creates a wet patch that behaves differently to the soil around it, and the slab ends up sitting on ground that's moving unevenly.
Trees too close to the building: Trees are thirsty, especially in summer. A big gum or fig within a few metres of the slab will pull moisture out of the soil on that side of the building, causing the ground to shrink and the slab to drop slightly where the tree is.
Poor surface drainage: Water pooling against the slab edge, blocked agricultural (aggie or ag) drains or downpipes emptying onto garden beds instead of into stormwater all push extra moisture into the soil right next to the building. Over time that soft, wet ground can't support the slab the way it should.

Early signs of a foundation problem

Diagonal cracks above door and window openings which usually start at the corner of the opening and run upward at 30–45 degrees.
Doors that suddenly start sticking after years of working fine, particularly if multiple doors on the same side of the building start binding around the same time.
Tiles cracking in a line across a kitchen or bathroom floor.
A horizontal crack in the brickwork running along a course of mortar is usually a sign that one section of the building is moving differently to another.
Gaps appearing between cornices and ceilings or skirting boards lifting away from walls.

Hairline cracks in plaster are often cosmetic. Cracks wider than a 5-cent coin, cracks that step across multiple courses of brickwork or cracks that are still growing month-on-month all warrant a structural engineer's opinion.

What to do if you suspect a problem

Check for plumbing leaks first. A leaking pipe under or near the slab is the most common cause of localised movement and the easiest to fix.
Photograph and date the cracks. A simple set of measurements (a ruler held against the crack in a monthly photo) tells you whether it's active or stable.
Get a structural engineer to inspect if the cracks are growing or wider than 3–4mm. A report costs in the low four figures and tells you whether the problem is cosmetic, can be monitored, or needs underpinning (extending or strengthening the foundations to stop further movement).
Notify the insurer if the trigger looks like a one-off event (a burst water main, a tree blown down in a storm). See our article on Insurance Claims.

Underpinning

Where settlement is severe, underpinning extends the foundations down to stable soil, usually with concrete piers driven beneath the existing footing. It's expensive (tens of thousands of dollars per affected wall) but it's the only durable fix when reactive clay has caused real movement.

Always get at least three quotes. Specifications vary widely between contractors, and the cheapest is rarely the right answer for a load-bearing structural repair. See our article on Working with Contractors for tendering guidance.

Paths and external concrete

Paths crack and lift for the same reasons foundations do, due to reactive clay, tree roots and water. Most cracking in paths is cosmetic and can be left alone. Lifted slabs that create a trip hazard need attention either by grinding down the lip or by lifting and re-laying. Trip hazards are a public liability issue; document any reported and don't let them sit to worsen.

Get in touch

Foundation issues are stressful and expensive, but the worst of them are usually preventable with good drainage, prompt attention to plumbing leaks and an eye on what's growing near the building. If you're seeing cracks and want a second opinion before commissioning an engineer, we're happy to take a look. Acacia Collective manages strata and community title groups across South Australia.

Call us on 1300 792 255 or email hello@acaciacollective.com.au.