You have noticed a crack running straight down an internal corner of a room. It is easy to jump straight to “subsidence”. Often the explanation is simpler, and sometimes it is not. This post explains the usual causes, which differ between older stone houses and newer cavity-walled ones, the signs that change the picture, and what a sensible next step looks like.
Key Takeaways
- A clean vertical crack in an internal corner, with nothing matching outside, often comes from the walls drying out or moving with temperature rather than from the ground. It takes a look on site, and sometimes monitoring, to tell which applies to your house.
- In houses with cavity walls and a concrete block inner leaf, the blocks shrink as they dry and move with temperature. Where the walls have no movement joints, that movement tends to show where the wall changes, such as at a corner.
- In older solid stone houses, a corner crack can mean a wall is moving away from the walls or floors that hold it in place. That is a different matter, and it needs looking at.
- Whether a crack is cosmetic depends on its cause, and the cause is something to confirm on site, not to assume from a photograph.
- The repair depends on the cause. Filling the crack without knowing what it is doing can bring it back. Underpinning does nothing for movement in the walls themselves.
Where These Cracks Come From
This post is written from general engineering principles, not from a single case. The patterns below are well known. Which one applies to your house is the part that needs someone to look.
Movement in a block inner leaf. Many newer houses have cavity walls: a brick or block outer leaf, a cavity, and an inner leaf of concrete blocks, often lightweight aircrete blocks. Concrete blocks shrink as they dry out after building, and they expand and contract a little with temperature and moisture through their life. Design guidance for blockwork recommends movement joints at intervals along the wall, so that this movement has somewhere to go. Where they were left out, the movement tends to show where the wall changes: at a corner, a junction, or an opening.
A junction between two walls. A crack in a corner is sometimes the line where an internal wall meets an external wall. If the two were built separately, or tied rather than bonded together, they can move slightly apart as the materials dry and shrink, leaving a neat vertical line.
Plaster and plasterboard. In dry-lined houses, and in timber-framed ones, a vertical crack in a corner can be no more than a taped joint in the plasterboard opening up as the building dries out, especially in the first few years or when the heating first goes on. Once that is confirmed, it is a decorating matter. A crack in the lining can also reflect movement in the wall behind it, so it is worth checking which one you have.
Older solid stone walls. Much of the older housing around Huddersfield has thick stone walls, and the internal walls and floors help hold the outside walls in place. If a front or gable wall is moving outwards, it can pull away from an internal wall and leave a vertical crack in the corner, often worse upstairs and wider towards the ceiling. Where a roof is pushing its walls outwards, the crack may show at the top of the upstairs rooms. None of these is shrinkage, and none of them should be filled and forgotten.
Movement in the ground. Foundation movement can crack internal corners too, but it usually shows in other places as well: outside, around windows and doors, and as cracks that run diagonally or step through the joints. In this area, the ground-related causes include leaking drains, made or filled ground, sloping sites, and old shallow coal workings, as well as clay soils. Where an extension joins the original house, the two parts sit on foundations of different ages and depths, and can move differently.
Signs It Needs Looking at Properly
Any of the signs below means the crack needs looking at properly, and sooner. Where none of them is present, the simpler causes above become more likely, but that is a reason to have it checked calmly, not proof that it is harmless.
- Cracking in the outside walls at or near the same corner.
- Diagonal or stepped cracks, especially from the corners of windows and doors.
- A crack that is getting wider over weeks or months.
- Doors or windows that have got steadily harder to open or close.
- Floors that have gone out of level, or gaps opening between floors and skirtings.
- A crack where an extension, bay, or porch meets the main house.
- In a stone house, an outside wall that looks out of line or bulging, or gaps opening where floors or ceilings meet an outside wall.
Some signs should not wait. If a crack is opening quickly, brickwork, stonework, blockwork, or plaster is loose, or has started to bulge, or a door or window has jammed at the same time as new or widening cracks appear around it, get advice straight away. If you think part of the building could fall, keep people clear of it and get a structural engineer out urgently. If it could fall onto a road, a path, or a neighbour’s property, also tell your council’s building control team, which deals with dangerous structures.
Whether a crack is wider at the top or the bottom tells an engineer something, but it does not settle the cause on its own. Several different causes, including some that are not cosmetic, produce a crack wider at the top. It has to be read with everything else.
Is It Structural?
Where the cause really is shrinkage or temperature movement in the walls, the cracking is usually a matter of appearance rather than safety. The difficulty is knowing that is the cause. That cannot be settled from a description or a photograph. It needs someone to look at the cracks, the walls outside, the openings, the floors, and how the cracking has behaved over time.
That is also why a single visit sometimes cannot settle it, and a period of monitoring with dated measurements is the right next step.
What the Repair Involves
Where the cause is movement in the walls themselves, underpinning is not the answer. It deals with foundations, not with the walls above them.
The repair depends on what the crack is doing. Where the crack has stopped growing, which dated measurements can show, it can often be repaired and redecorated with a finish that allows a little movement. In a newer house, where the movement is still going on, a crack at a junction is sometimes best turned into a neat, flexible joint rather than fought. Where the wall itself needs tying back together, stainless steel bars or ties can be set into the masonry across the crack, but tying a wall that is still moving can simply push the crack somewhere else. Cutting a new movement joint into a wall that carries the floors or roof is a bigger job than it sounds.
In older stone houses where a wall is moving outwards, the repair is about holding the wall back in place, not about the crack.
None of this should be done from a description. The right repair, and whether and where anything can safely be cut or fixed into a wall that carries load, should be decided by an engineer for your house. Do not cut a joint into a wall without that. The work itself is then for a competent builder.
What to Do Now
- If any of the signs that should not wait are present, do not wait to record it. Get advice straight away, and if you think part of the building could fall, keep people clear of it.
- Record it. Photograph the crack with a ruler held against it, note the date, and mark the ends lightly in pencil and write the date beside each mark, so you can see whether it grows.
- Look outside. Check the outside of the same corner, and the walls around nearby windows and doors, for cracks.
- Do not fill it yet if you want to know what it is doing. Filling removes the evidence.
- Get it looked at if any of the signs above are present, or if you want to know the cause before you repair and redecorate.
Need expert eyes on your project?
I am a Chartered Structural Engineer (CEng, MIStructE) based in Huddersfield. The Beam Doctor offers homeowners and builders:
- Defect investigation — I attend your property, inspect the cracking or movement, and give you a written opinion on the likely cause, or, where one visit cannot settle it, what monitoring or further investigation is needed. The site visit is included in the price. Monitoring, if it is needed, is priced separately. I take on a limited number of these, and I will confirm whether I can take yours once you have answered a few scoping questions. The report is for your own decisions. I do not take on reports that a lender, valuer, or buyer will rely on. At present I also do not take on reports that an insurer will rely on, or that are to be put to another party, such as a builder or neighbour, to establish what caused the damage. See the fee bands →
- On-site attendance — a Chartered Engineer at your property, tiered by duration. A brief verbal summary on site, then site visit notes within 2 working days: photographs, annotated sketches, and anything still needed for the design. The notes are a record of the visit, not a report. If you want to know what is causing a crack, Defect Investigation is the right service. See site visit fees →
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