Both slab heave and slab settlement cause cracking, sloping floors, and sticking doors, the visible symptoms overlap significantly. But the underlying mechanisms are opposite, and treating one as if it were the other will either waste money or make things worse.
Understanding which is which is the first step to an effective repair in Wagga Wagga’s reactive clay environment.
What Is Slab Settlement?
Settlement occurs when the clay beneath part of the slab shrinks and loses volume, causing that zone to move downward. The overlying slab, deprived of support, bends or drops into the void created by the clay contraction.
Causes of settlement in Wagga:
- Summer clay desiccation (seasonal or extended drought)
- Tree root moisture extraction causing localised clay shrinkage
- Leaking drains removing fine soil particles from the foundation zone
- Consolidation of poorly compacted fill beneath the slab
- Seasonal moisture cycle edge-fall (the perimeter dries faster than the covered interior)
Characteristic symptoms:
- Floor slopes downward in the affected zone
- External brickwork shows diagonal stepped cracks with the wider end at the bottom of the crack (tension on the lower/settled side)
- The slab edge may visibly drop, daylight visible between the bottom of the wall and the top of the slab
- Internal doors on the settled side of the house bind at the bottom of the frame
Seasonal pattern: Worsens during summer and drought. May partially recover in wet seasons but rarely fully reverses if movement has been occurring for years.
What Is Slab Heave?
Heave occurs when clay beneath the slab absorbs moisture and expands, pushing the slab upward. Heave is the opposite of settlement but can cause the same structural symptoms from a different direction.
Causes of heave in Wagga:
- Excessive garden irrigation adjacent to the foundation introducing moisture to the clay perimeter
- Leaking water or sewer pipes beneath or adjacent to the slab saturating the clay
- Tree removal (clay previously desiccated by the tree rehydrates rapidly after removal)
- Flood events saturating previously dry clay
- Poor drainage directing surface water toward the foundation
Characteristic symptoms:
- Floor slopes upward in the affected zone (affected zone is higher than surroundings)
- A hump or ridge in the floor in the heaved zone
- Doors bind at the top of the frame rather than the bottom (the frame has been pushed upward or skewed)
- Stepped cracks in external brickwork with the wider end at the top of the crack (tension on the upper/heaved side)
- In severe cases, brickwork courses are visibly pushed upward, you can see a horizontal “push” in the brickwork pattern
Seasonal pattern: May worsen after rainfall or irrigation events. May partially recover in dry seasons.
The Diagnostic Distinction: Which Way Is the Crack Opening?
The most reliable diagnostic tool for distinguishing heave from settlement is the crack taper direction:
Settlement crack: Wider at the top, tapering toward the bottom. The top of the crack has been pulled open as the foundation below it has dropped.
Heave crack: Wider at the bottom, tapering toward the top. The bottom has been pushed upward by expanding clay, opening the lower part of the crack.
This applies to:
- Diagonal stepped cracks in brickwork
- Cracks at corners of window and door openings
- Vertical cracks in walls
In a typical Wagga brick veneer home experiencing edge-fall settlement, the diagonal cracks at the window corners will be wider at the top and taper downward, the classic presentation. If you see the opposite (wider at the bottom), the foundation is heaving upward, not settling.
Combined Movement: When You Have Both
In the Riverina’s reactive clay environment, a single home can experience settlement in some zones and heave in others simultaneously. The most common combined scenario:
Edge-fall + interior heave: The slab perimeter settles (edge-fall) during dry conditions while the slab interior, protected from moisture loss by the slab above, maintains its moisture and height, or even heaves slightly if irrigation is directed toward the interior of the floor. The result is a slab that tips: the edges are lower, the interior is relatively higher. This is the most common slab movement mode in Wagga’s brick veneer homes.
Post-tree-removal pattern: A large tree has caused drought-like desiccation on one side of the house. That side has settled. After the tree is removed, clay on that side rehydrates faster than the rest of the site. The previously-settled side now heaves. The crack pattern reverses over one to three seasons, the top of cracks that were once wide at the top become wider at the bottom.
This complexity is why a professional foundation inspection is valuable, the inspector looks at the full pattern of movement across the house, not just one crack in isolation.
Treatment Differences
Treating Settlement
The primary treatment for foundation settlement is underpinning, adding deep-bearing elements (screw piles or concrete mass piers) that bypass the reactive clay and bear in stable material below. This arrests further settlement.
Drainage improvement to reduce clay desiccation in the affected zone is complementary, but if the foundation has already settled, drainage alone will not return it to level.
After underpinning, the structure can sometimes be relevelled, though the degree to which existing cracks can be closed (rather than stabilised) depends on how long the settlement has been occurring and how much distortion has accumulated in the structure.
Treating Heave
Heave treatment focuses on eliminating the moisture source. The typical intervention sequence:
- Identify and fix the moisture source, leaking pipe, irrigation, poor drainage
- Allow the clay to drain, do not attempt relevelling while the clay is still swelling
- Improve drainage around the affected zone to prevent recurrence
- Re-inspect after one dry season to determine residual condition
Heave rarely needs underpinning, the problem is too much moisture, not inadequate bearing. Adding deep piles to a heaving foundation zone can actually complicate matters by tying the slab down while the clay is still pushing up.
Quick Diagnosis Table
| Observation | Settlement | Heave |
|---|---|---|
| Floor slopes downward in zone | Yes | No |
| Floor humped upward in zone | No | Yes |
| Door binds at bottom of frame | Yes | No |
| Door binds at top of frame | No | Yes |
| Crack wider at top | Yes | No |
| Crack wider at bottom | No | Yes |
| Worsens in dry/drought | Yes | No |
| Worsens after rain/irrigation | No | Yes |
| Associated with trees | Yes (if roots drying clay) | Possible (if tree removed) |
| Associated with leaking pipe | Possible (if drain removes fines) | Yes (pipe saturating clay) |
When Both Look the Same
Frustratingly, in some cases the visual presentation of advanced heave and settlement can be similar, especially when both have been occurring for years and the crack pattern has accumulated from multiple cycles. In these situations, quantitative floor level surveying is the most reliable diagnostic tool: a laser level survey across the full floor plan, compared against any previous surveys or the known design floor level, shows exactly where the floor is high and where it is low.
Frequently Asked Questions
Q: My slab is cracking and my neighbour said it’s “just settlement”, but could it be heave? A: Yes, in Wagga, heave from garden irrigation or leaking pipes is common in newer slab homes. Check the crack taper direction and ask whether there is a significant water source (irrigation, recently repaired leak) on the heaved side.
Q: My floor has a hump in the middle, is that heave? A: Possibly. A hump in the centre of a slab (with the perimeter being lower) could be interior heave if there is a moisture source under the interior (leaking pipe, for example), or it could be differential edge-fall where the perimeter has settled more than the centre. A floor level survey will distinguish them, the survey shows the shape of the floor, not just whether it slopes.
Q: Can heave and settlement happen in the same house? A: Yes, and it is common. One side of a house may be settling (next to a large tree) while another side is heaving (from garden irrigation). The two sides of the house crack in mirror-image patterns.
Q: After the 2021 Wagga floods, my slab has a hump, was that heave? A: Most likely yes, flooding saturated the clay beneath the slab, causing heave. This is a well-documented post-flood phenomenon in Wagga. The hump typically reduces as the clay dries over the following 12-24 months. Monitor with regular floor level surveys.
Q: Underpinning was recommended for my heaving slab, is that correct? A: Underpinning is the treatment for settlement, not heave. If your slab is heaving, you need to eliminate the moisture source, not add piles. Be cautious if underpinning is recommended without a clear explanation of why the heave would benefit from piles.
Not sure whether your Wagga slab is heaving or settling? Book a free assessment, we’ll carry out a floor level survey and crack analysis to give you a definitive diagnosis.