Two Opposite Problems That Look Similar
A homeowner notices cracks, sticking doors, and a floor that is not level. Those symptoms are produced both by a foundation sinking and by a foundation rising, and from inside the house they present almost identically.
The distinction matters because the repairs are opposite in kind. Settlement is addressed by supporting the structure from below. Heave is addressed by finding and removing whatever is pushing it up. Applying the first response to the second problem is one of the more expensive mistakes in this field, and it happens.
Telling them apart requires an elevation profile, which is what a foundation evaluation produces.
Definitions
Settlement is downward movement of a foundation as the soil supporting it loses volume or capacity. Uniform settlement across a whole structure rarely produces damage.
Differential settlement is uneven downward movement, where one area drops relative to another. This is the mechanism that actually damages structures, because the difference is what the building has to accommodate.
Heave, also called upheaval, is upward movement as soil beneath a foundation gains volume. In expansive clay, that means the clay is taking up moisture and swelling.
Center heave describes a rise concentrated in the interior of a slab, producing a floor that is high in the middle and low around the edges.
Edge lift describes a rise around the perimeter, producing a floor that is high at the edges relative to the centre. It is commonly associated with perimeter clay swelling after significant rainfall.
Why Heave Happens
Seasonal moisture return. After a drought, clay that shrank substantially takes up water and expands. Where that happens unevenly around a perimeter, edge lift is the result.
Under-slab plumbing leaks. A leaking supply or drain line beneath a slab introduces water continuously to a localised area. Clay there swells and stays swollen. This is the most common cause of pronounced center heave in slab homes and it is a plumbing problem presenting as a foundation problem.
Drainage directed under the structure. Grading or drainage that channels water toward and beneath a foundation produces the same effect more slowly.
Tree removal. Removing a large tree that had been drawing substantial moisture from clay near a foundation allows that soil to rehydrate and swell. This is a genuine and underappreciated cause.
Chemical or fill-related expansion. Less common, but certain fill materials expand over time.
How the Elevation Profile Distinguishes Them
This is the whole answer, and it is why measurement rather than inspection is what resolves the question.
| Profile shape | Most likely mechanism | Typical next step |
|---|---|---|
| Steady fall toward one corner | Differential settlement | Underpinning that area |
| Fall around the full perimeter | Perimeter settlement | Perimeter underpinning |
| Rise in the centre, low edges | Center heave | Hydrostatic plumbing test first |
| Rise at edges, low centre | Edge lift | Drainage and moisture assessment |
| Flat within tolerance | No significant differential | Monitoring, document and re-check |
The symptoms visible inside the house barely distinguish among the first four rows. The profile distinguishes all of them.
Why this matters commercially
A contractor who proposes piers without producing a profile has not established which of these situations applies. Underpinning a slab that is heaving because of a leaking sewer line does not fix the leak, does not stop the heave, and adds the cost of piers to the cost of the plumbing repair that was always going to be necessary.
The Plumbing Question
Where the profile shows a rising centre, the plumbing question moves to the front of the queue.
A hydrostatic plumbing test pressurises the drain system to establish whether it holds, which identifies leaks beneath the slab. On older homes with original cast-iron lines, this is worth doing before any structural work is committed to. Our guide on hydrostatic plumbing testing covers how it works and when it is recommended.
Other indicators that make the plumbing question more pressing:
- Unexplained increases in water usage.
- Damp or warm spots in the floor.
- A musty smell without an obvious source.
- Sewer odours.
- Foundation movement that developed relatively quickly rather than over years.
What Happens If You Get It Wrong
Installing piers under a heaving slab has several unwanted consequences. It does not address the source of the moisture. It fixes the elevation of the piered points while the unpiered area continues to rise, which increases rather than decreases the differential. And it adds significant cost to a project that was, at root, a plumbing repair.
The reverse error, treating settlement as a moisture issue and waiting for it to normalise, allows continued movement while nothing is done about the loss of support.
Both errors come from the same origin: a conclusion reached without a measured profile.
What to Do If You Are Unsure
Document what you observe: where the floor feels high or low, which doors bind and at which corner, where cracks appear, and whether anything changed after a particular event such as a wet season, a drought, or a tree removal.
Then measure. The elevation survey resolves the direction of movement, and the direction of movement determines what comes next. Until that is established, any proposed repair is a guess about which of two opposite problems you have.