Where this starts
A steel lintel sits in the wall carrying the masonry over an opening. Where it is exposed to moisture and the protective coating has gone, it corrodes — and rust is bulkier than the steel it came from, by a large factor. That expansion has nowhere to go inside a wall, so it lifts whatever is above it.
THE PATTERN IS ALMOST DIAGNOSTIC. Horizontal cracking in a bed joint at one consistent height directly above the opening; the courses above sitting slightly proud or lifted; sometimes a brown stain bleeding down from the joint; occasionally the lintel's own painted face visible and flaking. Very little else produces a horizontal crack at exactly window-head height across the width of the opening.
TELL IT APART FROM THE OTHER THINGS THAT CRACK THERE. A crack running DIAGONALLY from the corner of the opening is movement in the wall or the ground, not the lintel. A crack directly above with the courses SAGGING rather than lifted is the lintel deflecting or short of bearing — a different fault with the opposite geometry. And regularly spaced horizontal cracks at several heights up a wall are wall ties, not a lintel.
WHY IT IS FED, and this decides whether a repair lasts. A lintel corrodes because water reaches it: a failed or absent cavity tray above it, a defective cill above, blocked weep holes, a render crack, or a cavity bridged with mortar droppings. Replacing or treating the lintel without finding the water source restarts the same process.
THE VERSION THAT MATTERS MOST is not domestic steel lintels at all but older buildings with embedded iron or steel — cills, balcony supports, structural sections in stonework — where the same mechanism is called Regent Street disease and can split substantial stonework. The diagnosis is the same; the consequences and the specialism are not.
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.
A corroding steel lintel expanding
Most commonA horizontal crack in the bed joint directly above the opening, courses above lifted rather than sagging, sometimes with rust staining bleeding down.
How to check: Whether the masonry above is lifted or dropped. Lifted is expansion from below; dropped is the lintel failing to carry.
Water reaching the lintel from above
Most commonThe corrosion pattern together with a defect above — a cracked cill, failed pointing, blocked weep holes, render cracking — often on the weather elevation.
How to check: What is directly above the crack. A lintel corrodes because something is feeding it and that something is usually visible.
A missing or failed cavity tray over the opening
CommonDamp appearing internally at the same height as well as the cracking, with no weep holes visible above the opening outside.
How to check: Whether there are weep holes above the window and whether they run after rain. No tray means water sits on the lintel permanently.
The lintel deflecting or short of bearing
CommonCourses above the opening SAGGING in a shallow curve rather than lifted, sometimes with the crack widest at mid-span.
How to check: Sight along the courses. A dip is a load or a bearing problem and is the opposite geometry from corrosion.
Wall tie corrosion rather than the lintel
CommonHorizontal cracking at several regular heights up the wall rather than only at the opening, sometimes with the outer leaf bulging.
How to check: Whether the cracking repeats at intervals. Ties sit at a regular spacing and corrode along the whole wall.
Embedded ironwork in an older building
Less commonSplitting or spalling of stone rather than brickwork, around a cill, a balcony, a railing or a structural section, on a building of some age.
How to check: Whether the material is stone and the building old. Embedded iron expanding in stonework is a specialist conservation problem.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Lifted means something below it is expanding — corrosion. Sagging means it is not being carried — deflection or bearing. Opposite geometries, opposite causes.
- 2
A brown stain bleeding from a bed joint at window-head height is close to conclusive, and it costs a look from the ground.
- 3
One crack at the opening is the lintel; regularly spaced horizontal cracks up the wall are wall ties, which is a whole-wall problem.
- 4
A lintel corrodes because something feeds it, and treating the lintel without finding the cill, the pointing or the blocked weep restarts the clock.
- 5
No weep holes usually means no cavity tray, so water sits on the lintel permanently — which is both the cause and the thing a repair must add.
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 masonry above an opening has lifted or cracked, since the lintel is carrying the wall and its condition has to be established rather than guessed.
- Where the courses above are sagging rather than lifted, which indicates the lintel is not carrying its load and is a more urgent question.
- Where horizontal cracking repeats at regular heights up the wall, which indicates wall tie corrosion across the whole elevation.
- Where the building is old and the affected element is stone with embedded iron, which is a conservation specialism and the wrong repair splits more stone.
- Where any masonry is loose above an opening, because what is above a window is above a door or a pavement.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.