Crack Where the Wall Meets the Ceiling: What It Means

It is one of the most photographed cracks in Britain. A thin line running along the very top of a wall, right where the wall stops and the ceiling starts. Sometimes it is a few inches in one corner. Sometimes it runs the whole length of the room, dead straight, as though somebody had ruled it in.

The short answer

A crack where the wall meets the ceiling is usually two surfaces moving at different rates, not a building in trouble. That junction is one of the most active joints in a house, and a hairline opening along it is normal and extremely common.

What matters is not that the crack exists. It is how wide it is, whether it stays on the junction or turns and travels, and whether anything else in the house has changed at the same time. Four of the five causes below are a decorating job. One of them is not.

Why that particular line cracks

A ceiling and a wall are rarely the same thing as each other.

In most UK houses the ceiling is plasterboard, or in older stock lath and plaster, fixed to timber joists. The wall is masonry: brick, block or stone with a plaster finish over it. Timber takes up and gives off moisture with the seasons and shrinks across its grain as it dries. Masonry, by comparison, barely moves at all.

So you have two materials with completely different habits, meeting along a line that is finished with a thin skim of plaster or a run of coving. That skim is brittle, and it is bridging a joint that moves. Every heating season the joint opens a fraction. Every summer it closes again. After a few cycles the plaster gives up, and you have a hairline.

It explains most of what people find puzzling about these cracks: why they sit on the junction and nowhere else, why they are so thin, why they are so straight, and why they come back a year after they are filled.

The five usual causes, commonest first

1. Seasonal movement between timber and masonry

The one described above, and by a distance the most likely.

What it looks like: a fine straight crack along the junction, usually well under a millimetre, often running most of the length of a wall. It does not continue down the wall or out across the ceiling. It is slightly more open in February than in August.

What to do: nothing structural. When you redecorate, use a flexible decorator’s caulk rather than a rigid filler, because filler will crack again in exactly the same line. Coving hides it, and then the coving cracks instead, which is the next item on this list.

2. The ceiling is deflecting

Ceiling joists bend under load. A small amount of deflection is designed in and harmless. More of it appears when something above has changed: a loft boarded out and filled with storage, a water tank relocated, a bathroom installed upstairs, or a conversion where joists sized for a ceiling are suddenly carrying a floor.

What it looks like: the junction crack is there, but the ceiling also reads slightly low across the middle of the room, or the cracking is worse mid-span than in the corners, or fresh cracks have started radiating across the ceiling itself. Doors on that floor may have begun to catch.

What to do: get the loft looked at before you load any more into it. Joists specified in 1890 to hold up lath and plaster were not specified for a quarter of a tonne of boxes, and whether a roof can take a conversion is a calculation, not a guess.

3. Coving or cornice bridging a moving joint

Coving is rigid and it spans the exact joint that moves. So it cracks, normally along one edge where it meets the wall or the ceiling, and at the mitres in the corners, which open into a little wedge.

What it looks like: the crack follows the top or bottom edge of the coving rather than the plaster line, the corner mitres have parted, and on older run-in-situ plaster cornices a section may have broken away or dropped.

What to do: cosmetic in almost every case. The exception is a heavy fibrous plaster cornice that has dropped or is hanging on a clean break. That is weight above head height and it should be taken down or re-fixed properly rather than painted over and hoped about.

4. Roof spread

Here we leave decorating behind. A pitched roof pushes outwards as well as downwards. If the rafters are not properly tied, by ceiling joists acting as ties, by collars, or by a ridge beam carrying the load, the roof slopes spread and take the tops of the external walls with them.

What it looks like: a horizontal crack at the top of an upstairs external wall, typically the wall under the eaves. The wall may lean outwards at the top. Sighting along the roofline from the garden shows a dip or a bow. Gaps may have opened where internal partitions meet the ceiling. It is usually worse on the elevation with the longest rafter span, and it is common in houses where the roof was recovered in heavier tiles than it originally carried, or where ceiling ties were cut out to make a vaulted room.

What to do: have it looked at. This is the reason we always ask which floor the crack is on and which wall. A long horizontal crack at ceiling level on an upstairs external wall is a different question from the same crack downstairs, and it is the one case on this list where leaving it alone makes it worse.

5. Foundation movement

Subsidence and heave do crack ceilings. They rarely start there, and they almost never stop there.

What it looks like: the junction crack is one part of a pattern rather than the whole story. It is wider at one end than the other. There is matching cracking outside, usually stepping through the mortar joints. Doors and windows have gone out of square in the same corner of the house. The crack is past about 3mm and still moving. And there is normally a cause close by: a mature tree, a leaking drain or a large shrub, a recent extension, or clay subsoil after a long dry summer.

What to do: treat the ceiling crack as a symptom and look at the building as a whole. The shape of a crack tells you far more than its location does.

Five questions that separate one from the other

1. Does it stay on the junction? A crack that lives only on the line between wall and ceiling is a very different thing from one that turns the corner and runs down the wall.
2. Is it the same width end to end? Parallel-sided cracks suggest two things quietly parting. Tapered cracks suggest rotation, which means one end has moved further than the other.
3. How wide is it? Anything you can get a fingernail into deserves measuring rather than estimating.
4. Which floor, and which wall? Downstairs, on an internal wall, is almost always finish movement. Upstairs, horizontal, on an external wall under the eaves, means somebody should look at the roof.
5. What else changed? Loft boarding, a new bathroom, a chimney breast taken out, an extension built, a tree felled, a drain repaired. Buildings respond to all of these, and usually a few months after the event rather than the week of it.

If every answer points the ordinary way, it is a decorating job. If two or more point the other way, it is worth a professional opinion.

How wide is too wide?

UK engineers and surveyors classify visible damage using BRE Digest 251, which sorts cracking into six categories, 0 to 5, based on how difficult the damage is to repair rather than on width alone.

Categories 0 to 2 cover cracking up to around 5mm and are treated as aesthetic: redecoration territory. Categories 3 and 4 begin to affect how the building performs, with doors and windows sticking and weather-tightness suffering, and repair means more than a tube of filler. Category 5 is a stability problem and needs structural work.

Two things are worth holding onto. First, width alone does not settle it. BRE is explicit that whether a crack is still widening matters as much as how wide it is today, and a stable 4mm crack and a 2mm crack that has doubled since the spring are not the same animal. Second, for a wall-to-ceiling junction specifically, the ordinary seasonal version sits firmly in category 0 or 1, which is hairline to about a millimetre. Once you are comfortably past that at a ceiling junction, something more than drying timber is involved.

How to monitor it properly, for nothing

Before anybody spends money, collect evidence. This is the single most useful thing a homeowner can do, and it is the first thing we ask for.

• Mark both ends of the crack with a fine pencil line drawn across it, and write the date beside each mark.
• Measure the width at the widest point. A crack gauge costs a few pounds; a steel rule and a close photograph will do.
• Photograph it with something in frame for scale, a coin or a ruler.
• Repeat after four weeks, then at three months, then after a full winter.
• Note what the weather has been doing and anything you have changed in the house.

A crack that is identical a year later, through a winter and a summer, has already told you what it is. A crack that has grown a millimetre by March has told you something different, and you now have a record to hand an engineer instead of a hunch.

When to call a structural engineer

• The crack is wider than about 3mm, or it is visibly tapered.
• It runs down the wall as well as along the ceiling, or it steps through the brickwork outside.
• It is horizontal, at the top of an upstairs external wall, and the roofline is anything other than straight.
• Doors and windows have started sticking in the same part of the house, or a floor upstairs has gone noticeably out of level.
• The ceiling sags, feels springy from above, or sounds hollow when tapped.
• It has measurably grown since you started monitoring.
• You are buying the house and the survey has flagged it, in which case this is what happens next.

And call someone if you simply cannot settle it in your own mind. A structural survey on a single issue like this is not an expensive exercise, and the usual outcome is an engineer confirming in writing that it is cosmetic, which is a useful document to own when you come to sell.

What we actually do when we come out

Mostly we read the building rather than the crack. The crack is one sentence in a longer story and it is very hard to interpret on its own.

So we look at the elevation outside for matching cracking, check the roofline and the eaves for spread, get into the roof void to see what the rafters are tied with and what has been added since, take levels on floors where they matter, look at the drains and at what is growing near the wall, and check for the junctions between old and new work that so often turn out to be the real answer. Where the pattern is ambiguous, we take a small area of finish off, because a crack that stops at the plaster and a crack that carries on into the masonry behind it mean entirely different things and look identical from a metre away.

You get a written report either way, with the cause, what it needs, and what it does not need.

Common questions

Is a crack between the wall and the ceiling serious?

Usually not. A fine, straight, parallel-sided crack that stays on the junction is almost always seasonal movement between a timber ceiling and a masonry wall. It becomes serious when it is horizontal at the top of an upstairs external wall, when it turns and runs down the wall, when it is tapered, or when it is wider than about 3mm and still growing.

Why does the crack come back every time I fill it?

Because the joint underneath it is still moving. Rigid filler sets hard and then splits along the weakest line, which is the same line it split along last time. A flexible caulk stretches with the seasonal movement instead of resisting it.

Should I use filler or caulk?

Flexible decorator’s caulk for a hairline at a wall-to-ceiling junction. Filler is the right product for a hole in a wall and the wrong product for a moving joint. If the crack is wide enough that caulk looks wrong, that is a reason to find out what is causing it before you decorate.

Can a loft conversion cause cracks where the wall meets the ceiling?

Yes, in two ways. The new floor loads joists and beams that may deflect more than the old ceiling did, and cutting ceiling ties to open up a vaulted space removes the restraint that was stopping the roof from spreading. Both show up at the top of walls, and the second one matters.

How long should I monitor a ceiling crack before calling someone?

Three months gives you a useful reading, and a full year through both a winter and a summer gives you a conclusive one. Do not wait that long if the crack is tapered, wider than about 3mm, horizontal on an upstairs external wall, or accompanied by doors that have started sticking.

Ora Labora Structural Engineers

We are chartered structural engineers working across Peterborough, Stamford and the surrounding area, and in London. Most of the cracks we are sent turn out to be cosmetic, and we would far rather tell you that in a short report than have you worry about it for another winter. If a crack is doing something more than drying timber, we will tell you that plainly too, along with what it needs.

Book a structural survey and we will read the whole building, not just the line at the top of the wall.