Subsidence vs Settlement: Identifying the Difference and What to Do

Cracks in a wall are alarming. They are also, more often than not, misdiagnosed. "Subsidence" gets attached to everything from drying plaster to a genuine foundation failure. The difference between subsidence, settlement, and heave matters — because each has a different cause, a different remedy, and a different impact on your insurance.

Definitions

Settlement

Downward movement of a building under its own weight, into ground that compresses slightly when loaded. Normal and expected in new buildings. Most settlement happens in the first 1–3 years after construction and stabilises.

Subsidence

Downward movement of the ground beneath the building, unrelated to load. Caused by changes in the ground itself — clay shrinking in drought, soil washout from a leaking drain, mining cavities, peat decay. Not normal. Requires investigation and usually remediation.

Heave

Upward movement of the ground beneath the building. Usually caused by clay re-hydrating after a previous drying episode — for example, after a large tree is removed from a clay-soil garden. Slow, sustained, and often misdiagnosed.

Landslip

Lateral movement of the ground beneath part of the building, usually on sloping sites. Often dramatic when it occurs.

How to tell them apart

Settlement signatures

• Cracks appear within the first 2–3 years of construction
• Cracks are usually hairline to 2 mm
• Cracks stabilise — they don't keep growing
• Frequently appear at junctions of different materials (e.g. plaster meeting brick)
• Often hairline diagonal cracks at corners of openings

Subsidence signatures

• Cracks appear in an established building, often after a hot summer
• Cracks are typically 3 mm or wider, sometimes much wider
• Cracks are diagonal — narrow at one end, wider at the other
• Cracks affect both internal and external faces of a wall
• Doors and windows start to stick or won't latch
• Cracks are localised to one corner or side of the building

Heave signatures

• Cracks appear after a tree was felled or after a wet winter following a drought
• Crack pattern is the opposite of subsidence — corners pushed up rather than pulled down
• Floor slabs lift or dome
• Often slower to develop than subsidence

The diagnostic process

1. Crack monitoring. Place a tell-tale or pair of marked points either side of the crack and measure regularly for 6–12 weeks. Movement that's progressing is live; static cracks are historic.
2. Site investigation. Trial pits or boreholes to identify foundation depth, soil type, and current moisture content.
3. Drain survey. CCTV survey of all drains within 3 m of the affected wall. Leaking drains are the most common cause of localised subsidence after clay shrinkage.
4. Tree and vegetation audit. Identify species, distance from the affected wall, and height. Some species (poplar, willow, oak) are notorious for clay-soil subsidence at significant distances.
5. Soil testing. If clay is suspected, plasticity index testing confirms shrink-swell potential.

Remediation options

If the cause is a leaking drain

Repair the drain. Monitor for 6–12 months. Most affected buildings recover without underpinning if remediation is timely.

If the cause is a tree

Options: tree removal (with heave risk), root pruning, or root barriers. Tree work in heritage protection overlay zones requires consent.

If the cause is generalised clay shrinkage

Usually requires underpinning. Resin injection is sometimes appropriate for modest movement.

If the cause is heave

More complicated — re-introducing moisture rarely works once the building has already cracked. Often requires partial demolition and reconstruction with heave-resistant foundations.

Insurance and disclosure

Most South African buildings insurance policies cover subsidence after a R20,000 excess. Coverage requires you to notify the insurer promptly. Once a property has had a subsidence claim:

• It must be disclosed to all future insurers
• Premiums typically increase 25–50%
• Some insurers decline cover entirely
• The condition must be disclosed on sale (TA6 form)

For cracks that turn out to be non-subsidence (settlement, thermal movement), there is generally no insurance involvement and no disclosure obligation.

Get an independent assessment from MCFAR's chartered engineers before assuming the worst — or the best.

Frequently Asked Questions