Why Is There a Crack Above Your Window? Lintels, and What They Are Telling You

Almost every house has one above every window and every door, and almost nobody could tell you what theirs are made of. Lintels do the least glamorous job in the building: they hold the wall up over a hole somebody deliberately cut in it.

They are also the reason for a very large share of the cracks people photograph and send to us. Sometimes correctly.

The short answer

A crack directly above a window or door usually means one of three things: the lintel is failing, something above the lintel has moved, or the opening is simply where a larger movement in the wall has chosen to show itself. Openings concentrate stress, so they are where cracks appear first regardless of the cause.

Which one it is can usually be read from the shape, the position and the direction of the crack, and that is what the rest of this page is about.

What a lintel actually does

Take a solid wall and cut a window into it. The masonry that used to sit in that space still has to go somewhere, and so does everything stacked above it. A lintel is the piece that catches that load and carries it sideways onto the wall either side.

What it does not do is carry the entire wall above it. In a wall with reasonable bond and mortar, the masonry arches naturally over the opening, and the lintel only ever picks up a rough triangle of brickwork directly above – plus anything bearing into that zone, like a floor joist or a roof truss landing in the wrong place.

That distinction matters, because it explains why two identical windows in the same house can behave completely differently. One has an empty triangle above it. The other has a purlin sitting in it.

Reading the crack

A horizontal crack along the top of the opening

A crack running horizontally along a mortar bed just above the window head, often with the brickwork above lifted very slightly, is the classic signature of a corroding steel lintel. Steel rusts, rust occupies several times the volume of the steel it came from, and that expansion jacks the masonry upwards. It is common in cavity-walled houses from the 1960s onwards, and it tends to run the full width of the opening rather than tailing off.

A stepped crack rising from the corner

A crack that starts at the top corner of the opening and steps diagonally up through the mortar joints is usually telling you about movement in the wall rather than failure of the lintel itself. Subsidence, heave, a loaded pier, or a bay that has rotated will all do this. The lintel is the messenger.

A crack straight down the middle of the stone

Stone and plain concrete are strong in compression and poor in tension, and the underside of a lintel is in tension. A single vertical split near mid-span, like the one in the photograph at the top of this page, is that material reaching its limit – usually because the span is a little too generous, or because something concentrated has landed on it since.

A crack that only exists in the plaster

Inside, a hairline that follows the edge of a timber lintel or beam is frequently nothing more than two materials with different habits parting company. Timber moves with moisture, plaster does not. Our guide to reading a crack in a wall goes into how to tell that apart from the real thing.

The four ways lintels fail

1. Steel lintels corrode

Galvanised pressed-steel lintels have a finite coating life, and the places that fail first are the ends, where water gets in and sits. Once the zinc has gone, corrosion starts, and the expansion does the damage long before the steel loses any meaningful strength. The crack arrives first and the structural problem arrives later.

2. Concrete lintels carbonate

Reinforced and pre-stressed concrete lintels fail from the inside out. Carbon dioxide gradually neutralises the alkalinity that protects the reinforcement, the steel begins to corrode, and the concrete spalls off in flakes from the underside – often showing rust staining and, eventually, exposed bar.

3. Timber lintels decay

Older solid-wall houses very often have a timber lintel carrying the inner part of the wall, hidden behind the plaster, with a brick arch doing the visible work outside. Where damp has reached it, or where somebody has later loaded it, the timber sags and the plaster above the opening cracks and bows. You cannot see any of this without opening up.

4. Brick arches lose their support

A flat or segmental brick arch is not really a lintel at all – it works by thrusting sideways into the masonry either side. If the wall beside it spreads, or the pier below moves, the arch drops slightly and the joints open out in a fan. Repointing it does nothing, because the arch has not failed; its abutment has.

When it is not the lintel at all

This is worth saying clearly, because a lot of money gets spent replacing lintels that were never the problem.

Openings are stress concentrations. Whatever is happening to a wall – foundation movement, thermal expansion along a long elevation, a tree drying the clay out one side of the house – it will surface at the weakest geometry available, and that is the corner of a window. Replacing the lintel in that situation treats the symptom and leaves the cause running.

The same applies at junctions between different parts of a building, which is a slightly different problem and one we have covered separately in cracks where an extension meets the house.

How to tell serious from cosmetic

Four things are worth recording before anybody is called out, and all of them are free:

• Width. Anything you can fit a 20p coin edge into is worth a professional opinion. Hairlines rarely are, on their own.
• Direction. Horizontal at the head suggests the lintel. Diagonal and stepped from the corner suggests the wall. Vertical through a stone suggests the stone.
• Whether it goes through. A crack visible on both faces of a wall in the same place is structural until proven otherwise. One that exists only in the decoration is usually not.
• Whether it is moving. Pencil both ends, write the date beside it, photograph it, and do it again in a month and in three months. Movement over a season tells you far more than width does on any single day.

If it is moving, or going through, or wide, that is the point to get it looked at rather than watched.

What it costs to get this wrong

A corroding steel lintel caught at the cracking stage is a contained repair. Left for several more years, the expansion lifts and distorts the masonry above it, and what began as a lintel replacement becomes a rebuild of the brickwork over the opening as well.

A decayed timber lintel behind plaster is the same story with less warning, because nothing is visible outside until the opening starts to deform. And a crack that was never about the lintel at all simply carries on, while the money has gone on the wrong thing entirely. If the picture is not settling it, a small opening-up exercise is usually the cheapest thing in the whole process – our note on what a structural investigation actually involves sets out what that looks like.

Lintels in Peterborough and London

Ora Labora Structural Engineers inspect and specify lintels across Peterborough, Stamford and the surrounding area, and across London – from a single cracked window head to a full elevation, including the opening-up and the design of the replacement where one is needed.

What you get is a straight answer about whether the lintel is the problem. If the crack is telling you about something else in the wall, we would rather say so at the survey than specify a lintel that will not fix it.

Get in touch with a photograph of the opening and a note of roughly when the crack appeared, and we will tell you whether it needs a visit.

Frequently asked questions

Why is there a crack above my window?

Most often because the opening is where stress concentrates, so it is the first place any movement in the wall shows. The specific cause may be a corroding steel lintel, a decayed timber one, a split stone head, or movement elsewhere in the building. The shape and direction of the crack is what separates them.

How do I know what my lintel is made of?

By age and construction, usually. Cavity-walled houses from about 1970 onward normally have galvanised steel. Mid-century houses often have concrete. Solid-wall Victorian and earlier properties frequently have timber inside with a brick arch or a stone head outside. Short of opening up, that is an informed guess rather than a fact.

Is a cracked lintel dangerous?

Rarely in the immediate sense. Lintels tend to fail slowly and give plenty of warning. What makes it urgent is movement that is still progressing, masonry that has visibly displaced, or a crack that has opened quickly over weeks rather than years.

Can a lintel be replaced without taking the wall down?

Usually, yes. The masonry above is needled and propped, the old lintel is cut out, the new one is installed and the brickwork is made good. It is a routine operation, but it is a temporary works job as much as a permanent one, and the propping arrangement needs designing rather than improvising.

Does a cracked lintel affect a house sale?

It can. A surveyor will flag visible cracking above openings, and buyers tend to assume the worst in the absence of information. A short report setting out what the crack is and what it needs will usually do more for the sale than a quick cosmetic repair, which surveyors recognise on sight.

Should I fill the crack?

Not until you know what it is. Filling destroys the only record you have of how wide it was, and a filled crack that reopens tells you less than an unfilled one you have been measuring. Record it first, repair it once the cause is settled.