Where this starts
Two quite different structures get called garden walls. A FREE-STANDING wall has air on both sides and has to resist only wind and its own slenderness. A RETAINING wall has ground on one side and has to resist that ground's push for as long as it stands, and the push is not a fixed number.
THE FIRST OBSERVATION IS WHICH. Look at the levels each side. Any difference in ground level means the wall is retaining, even if it is only half a metre and even if it was built as a garden wall. A great many domestic walls are retaining several hundred millimetres of soil that nobody has ever counted as retained.
WHY WATER MATTERS SO MUCH on a retaining wall: soil pushes, and saturated soil pushes considerably harder, because the water in the pores adds its own pressure on top of the soil's. A wall built to resist dry soil can be overwhelmed by the same soil after a wet week, which is why retaining walls need drainage behind them — weep holes, a drainage layer, a perforated pipe — and why a wall with none is relying on the weather.
MEASURE THE LEAN rather than describing it. Hold a spirit level or hang a plumb line against the face and measure the gap at the bottom, and write down the date and the height you measured over. A lean is only actionable as a rate, and a wall that has been half an inch out for thirty years is a different thing from one that has moved that far this winter.
THE THING THAT DECIDES URGENCY is not the angle, it is what is on the other side. A low wall leaning into a flower bed is a repair whenever convenient. A wall of any height leaning toward a path, a road, a play area or a neighbour's property is a public safety matter, and retaining walls fail suddenly rather than gradually once they start to rotate.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Soil pressure on a wall retaining ground
Most commonA difference in ground level either side, the lean toward the LOWER side, often with the top course moving furthest and the movement worsening after wet periods.
How to check: Whether the ground levels differ. Any retained height at all makes this a retaining wall whatever it was built as.
No drainage behind a retaining wall
Most commonA retaining wall with no weep holes, no visible drainage, and ground behind that stays wet — with movement clearly following rainfall.
How to check: Look for weep holes at the base and whether they run after rain. A dry wall face after a downpour means the water is staying behind the wall.
An inadequate or undermined foundation
CommonA whole wall tilting as a unit rather than bulging, sometimes with a visible gap opening at the base on one side, and often near a tree, a drain or a made-up ground area.
How to check: Whether the wall is tilting or bowing. A rigid tilt is the base; a belly in the middle is the wall itself.
Slenderness with no piers or returns
CommonA long, tall, single-skin free-standing wall with no piers, no returns and no copings, bowing in the middle of its longest unsupported length.
How to check: How long the wall runs without a pier or a corner. A half-brick wall is far more slender than it looks and relies on those.
Tree roots or vehicle loading close behind
CommonLocalised movement opposite a tree, a driveway, a parking area or a stored heap of material, rather than along the whole length.
How to check: What is on the high side within a couple of metres. A car parked behind a retaining wall is a load it was probably not designed for.
Frost or saturation in the wall itself
Less commonCrumbling mortar and spalled faces as well as movement, often on a wall with no coping so water enters the top, worsening after cold winters.
How to check: Whether the wall has a coping and whether the mortar is sound. A wall being destroyed from inside will also move.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Any retained height makes this a retaining wall, which is a different structure with a different failure mode, whatever it was built as or called.
- 2
A lean is actionable as a RATE. Without a dated measurement nobody can tell a wall that has been like that for decades from one that moved this winter.
- 3
A retaining wall with no drainage is relying on the weather, and saturated soil pushes considerably harder than dry soil.
- 4
A rigid tilt points at the foundation; a belly points at the wall's own slenderness or at pressure behind it, and those are different repairs.
- 5
The angle does not decide urgency; what the wall would fall onto does, and a retaining wall gives far less warning than a free-standing one.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Immediately, where a wall retaining any significant height is leaning toward a path, a road, a play area or a neighbour's property — retaining walls fail suddenly once they begin to rotate.
- Where a dated measurement shows the lean increasing, which turns a monitoring exercise into an investigation.
- Where the wall retains more than about knee height, since design for soil pressure and drainage is engineering rather than bricklaying.
- Where a vehicle is parked, or material is stored, on the high side — that is a surcharge the wall was almost certainly not designed for.
- Where the wall is shared with a neighbour, because the ownership and the obligation to repair are questions to settle before the work rather than after.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.