Where this starts
Render is a coat of mortar or a proprietary system applied to a wall, and it depends on two things: sticking to the background, and being able to cope with the small movements the background makes. Most defects are a failure of one of those, and tapping tells you which.
TAP THE WHOLE ELEVATION with a knuckle or a light hammer, not just the cracked part. Solid render gives a dull, dead sound. Debonded render gives a noticeably hollow, drummy one, and the hollow area is almost always larger than the cracked area — often much larger. Mapping that boundary before deciding anything is the most useful half-hour available, because it turns a question about a crack into a measurement of how much render is actually still doing its job.
THE SHAPE OF THE CRACK is the second reading. Fine, closely spaced, map-like cracking over an area is usually the render's own shrinkage. A single straight crack running through the render in a continuous line — especially one that continues past a change of material or lines up with an opening — is the wall behind moving and the render simply showing it. Cracks radiating diagonally from the corner of a window or door belong in the second family.
WHERE WATER GOES matters more on render than on most finishes. A crack that lets water in but does not let it out keeps the wall behind it wet, and the freeze-thaw that follows widens the crack and pushes the render off. That is why a hairline crack with staining below it is more advanced than a wider dry one, and why the base of a wall, the area under a sill and the line under a coping are where failure usually starts.
One thing worth settling early, because it decides the whole repair: what kind of render it is. A cement render, a lime render and a modern polymer or silicone system respond to water and movement quite differently, and a repair in the wrong material on an old building can do more damage than the crack.
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.
The render has debonded from the background
Most commonA hollow, drummy sound over an area — usually well beyond the visible cracking — sometimes with the render moving slightly under hand pressure.
How to check: Tap across the whole elevation and mark where the sound changes. The hollow area is the real extent of the problem.
Shrinkage of the render itself
Most commonFine cracks in a random, map-like pattern over an area rather than a single line, appearing within months of application, with the render still solid when tapped.
How to check: Whether the cracking is a network or a line. A network is the coat shrinking; a line is the wall moving.
Movement in the wall behind
CommonA single continuous crack, often diagonal from the corner of an opening or following a line in the structure, and frequently traceable on the inside as well.
How to check: Whether the crack continues past a change in the render — through a bell cast, across a bay, onto an adjoining surface. A crack that ignores the render's own boundaries belongs to the wall.
Water getting in behind and freezing
CommonCracking with staining, moss or salts below it, worst at the base of a wall, under a sill or beneath a coping, and clearly worse after cold winters.
How to check: Whether there is a water path above the damage. Damage that sits under a defective sill, coping or downpipe is being fed.
An incompatible coat over the background or over an older render
CommonFailure over one material and not another — commonly a hard cement render on soft brick or on an old lime-rendered building — often debonding in sheets.
How to check: What the wall is made of and what age the building is. A hard render on a soft, breathable wall is a mismatch rather than a defect.
Applied in the wrong conditions or onto an unprepared background
Less commonWidespread failure of a recent application, sometimes with a dusty or smooth surface visible behind removed render, and often on a job done in very hot, cold or wet weather.
How to check: The dates and the weather when it was applied, and whether the background was keyed or primed.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It converts the question from how big the crack is into how much render has let go, and the second number is the one that decides the repair and its cost.
- 2
A random map-like network is the render shrinking; a single continuous line is the wall behind moving, and only one of those is a decorating question.
- 3
A crack that carries on past a bell cast, across a bay or onto the next material is structural in origin, and repairing the render will not stop it.
- 4
Defective sills, copings, gutters and downpipes feed render failure from above, and repairing the render without fixing the feed simply restarts the clock.
- 5
Cement, lime and polymer systems are not interchangeable, and a hard repair on a soft old wall relocates the damage rather than ending it.
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
- Where tapping finds large hollow areas, since render that has let go can fall, and the extent is a safety question above a pavement or an entrance.
- Where a straight or diagonal crack continues onto other materials or shows on the inside of the same wall, which is a movement question rather than a finish one.
- Where the building is old, solid-walled or listed, because the choice between lime and cement is not a preference and the wrong one traps moisture in the wall.
- Where damage is concentrated below a sill, coping or gutter that is itself defective and at height, since the render is the symptom and the repair starts above it.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.