Slab heave
Slab heave is the upward movement of a slab-on-ground when reactive clay swells with moisture, lifting it unevenly and cracking walls and floors.
Ask Chalkline about this →Slab heave is the upward movement of a slab-on-ground when reactive clay beneath it swells with moisture, lifting the slab unevenly and cracking the structure above. It is the swelling half of ground movement on reactive sites, and the opposite of settlement (downward).
Why it happens
Reactive clay soils expand when they take up water and shrink when they dry. When the soil under part of a slab gains moisture (a leaking pipe, poor surface drainage, garden beds watered hard against the edge, or a large tree removed so soil it had been drying out rehydrates), that zone swells and pushes the slab up. The moisture change is rarely even across the footprint, so the lift is differential, and that uneven movement is what cracks the building. See reactive soil for how the soil itself behaves.
What it looks like
- Cracking in brittle finishes (cornices, plaster, brickwork), often diagonal from openings.
- Doors and windows that bind or stop closing.
- Floors that hump, tiles that drum or lift.
Heave damage often shows up months or years after completion as moisture conditions change, which is why it is a common warranty and dispute trigger.
How it is managed
AS 2870 is the standard that fights heave (and its mirror, differential movement). It classifies the site by reactivity (Class M, H1, H2, E) and sets the slab and footing design to suit, a stiffened raft such as a waffle-pod or conventional reinforced slab being the usual answer. The build and the owner then manage moisture: drainage falling away from the slab, sound plumbing, and consistent conditions around the edge.
For a builder
- It is a moisture problem first. Most heave traces back to water reaching the soil under the slab unevenly; control drainage and plumbing and you control most of the risk.
- Match the slab to the site class. The AS 2870 classification drives the design; do not put a standard slab on a highly reactive site.
- Warn the owner. Watering gardens against the edge, or planting or removing big trees, changes soil moisture and can trigger heave after handover.
Also known as: reactive soil heave, footing heave.
Related
- Reactive soil
- AS 2870 (residential slabs and footings)
- Differential movement
- Waffle-pod slab
- Conventional reinforced slab
- Ground movement
See also
Last updated: 2026-05-26. Verified: 2026-05-26. Quarterly review for currency.