If you own a slab-on-ground home in Wagga Wagga, there are two distinct ways that reactive clay can distort your slab, edge lift and edge fall. These terms come from structural engineering and describe the two failure modes that AS 2870, the Australian Standard for residential slabs, specifically designs against. Understanding which mode your slab is experiencing is essential for diagnosing problems correctly and choosing the right repair. For the full picture of how reactive clay affects Wagga foundations, see our guide on reactive clay soils and foundation implications.
Quick answer (BLUF)
Edge fall is when the perimeter of the slab settles lower than the centre, caused by the exposed slab edge drying out faster in summer. Edge lift is when the perimeter rises higher than the centre, caused by the perimeter soil absorbing more moisture than the protected interior. Both produce characteristic cracking patterns. Edge fall is more common in Wagga’s dry summers; edge lift can follow wet winters or irrigation-related moisture changes.
What is a concrete slab on reactive clay doing?
A concrete slab on reactive clay is not sitting on a stable substrate. The clay beneath it is constantly changing volume with moisture. The slab, a rigid reinforced concrete structure, cannot change shape, so when the ground beneath it changes shape, the slab either bends (developing internal stresses) or cracks.
AS 2870 designs slabs to resist a defined amount of bending, set by the site class. The reinforcement and edge beam dimensions specified for an H2 site are specifically calculated to prevent cracking under the expected moisture-driven movement for that soil class.
When the actual movement exceeds what the slab was designed for, because drainage has worsened, because the original design was inadequate, or because the reactive zone is deeper than assumed, the slab cracks.
Edge fall
What causes it
Edge fall is the more common failure mode in Wagga Wagga’s dry climate. The mechanism is straightforward: the exposed perimeter of the slab, including the edge beam, is in contact with soil at the building line, which is exposed to sun, wind, and direct moisture loss. In summer, the soil at the edge dries faster and more completely than the soil beneath the interior of the slab, which is protected by the slab itself.
As the perimeter soil dries and contracts, it pulls away from the edge beam. The perimeter of the slab is no longer fully supported, it drops relative to the centre. The slab is now domed upward at the centre and sagging at the edges.
What edge fall looks like
- Floors that appear to slope toward the outer walls
- Tiles debonding or cracking in a pattern that follows the room perimeters
- Diagonal cracks in brickwork at wall corners
- Skirting boards that gap at the bottom near external walls
- External wall cracks, stepped or diagonal, that are worst at the building perimeter
How it is repaired
Underpinning the edge beam is the structural solution. By installing additional support beneath the edge, either mass concrete pits or screw piers, the edge beam is stabilised at or near its original position. Where the edge has already dropped significantly, relevelling the edge as part of the underpinning process may also be undertaken.
Critically, drainage must also be corrected. If the cause of the edge drying is excessive surface moisture loss (downpipes discharging against the edge, poor surface drainage), these issues must be addressed or the underpinning will be undermined by continuing differential drying.
Edge lift
What causes it
Edge lift is the reverse condition, the perimeter of the slab rises relative to the centre. This is less commonly associated with Wagga’s hot dry summers, but can occur:
- Following wet winters or La Niña years when the soil surface is repeatedly re-wet
- Where the perimeter soil is exposed to more moisture than the interior, for example, where garden beds are built against the wall and regularly watered
- Where irrigation or leaking stormwater introduces moisture at the perimeter that the protected interior does not receive
- In covered areas (under verandahs, in garages) where rain does not reach the interior but moisture enters from adjacent wetter soil
What edge lift looks like
- Floors that slope toward the centre of the building
- Cracks in internal walls that run horizontally or in an arch pattern
- Doors in the centre of the house that jam while perimeter rooms are unaffected
- A “tented” or domed ceiling pattern visible in some lighting conditions
- Tiles that crack in the middle of the room
How it is repaired
Edge lift is more difficult to correct than edge fall because the problem is excess moisture at the perimeter, which is harder to remove than it is to add. The primary intervention is drainage correction: removing the source of excess moisture at the perimeter (correcting garden irrigation, improving surface drainage, clearing blocked subfloor vents). In some cases, passive sub-slab drainage can be installed to intercept moisture before it reaches the edge beam subsoil.
Structural correction of edge lift, lifting the centre of the slab to match the risen perimeter, is rarely practical. In most cases, the goal is to stabilise the perimeter at its current level and prevent further lift.
FAQs
How do I know if my slab is experiencing edge fall or edge lift?
A floor level survey, measuring the height of the floor surface at a grid of points across the home, will distinguish between the two clearly. If the perimeter is lower than the centre, edge fall has occurred. If the perimeter is higher, edge lift. Without a floor level survey, it can be hard to tell from observations alone, though the location of cracking provides clues. A foundation inspection includes a floor level survey.
My slab was built to H2 standard but it is still cracking, how?
AS 2870 H2 design provides a defined level of performance against expected movement. If the actual movement exceeds the design assumption, because the reactive zone is deeper than assumed, because drainage has worsened, or because tree root activity has created additional moisture variation, the slab can crack despite being built to the correct standard. The standard is a minimum, not a guarantee for all conditions.
Is edge fall or edge lift more expensive to fix?
Edge fall, because it typically requires underpinning the edge beam, is generally more expensive to repair than edge lift, which may be addressable primarily through drainage correction. However, if drainage correction alone is insufficient for edge lift (for example, where the edge has risen so much that tiles and internal walls are damaged), structural correction adds cost. Every project is different, a foundation inspection will give you an accurate picture.