Are cracks in my house serious?
Nearly every house has cracks. The skill is telling the harmless ones from the few that need attention, and knowing what to do about those.
The short answer
Most cracks in houses are not serious. Hairline cracks in plaster, cracks along ceiling joints and short cracks running from the corners of doors and windows are usually caused by drying shrinkage, temperature changes or minor settlement, and are cosmetic. A crack is more likely to be structural if it is wider than about 5 mm, wider at one end than the other, runs diagonally through brickwork, appears inside and outside in the same place, or keeps growing. Those cracks need a structural engineer to look at them.
Width alone does not tell the whole story. The pattern, the location, whether it is still moving and what else is happening in the building all matter.
How engineers grade crack width
In the UK, engineers commonly use a damage classification based on crack width and how easily the damage can be repaired. It runs from category 0 to category 5. It is a guide to the severity of the damage, not a diagnosis of the cause.
- Category 0: hairline cracks under about 0.1 mm, negligible
- Category 1: up to about 1 mm, very slight, dealt with by normal decoration
- Category 2: up to about 5 mm, slight, easily filled, possibly some repointing outside
- Category 3: about 5 to 15 mm, moderate, may need opening up and some brickwork replaced
- Category 4: about 15 to 25 mm, severe, extensive repair, doors and windows distorted
- Category 5: over about 25 mm, very severe, major repair or partial rebuilding
Cracks that are usually harmless
Fresh plaster and new plasterboard shrink as they dry, so fine cracks in the first year or two after building work are expected. Cracks along the line where a ceiling meets a wall, or along plasterboard joints, are usually caused by the timber floor above moving slightly as people walk on it.
A vertical crack where a newer extension meets the original house is also common. The two parts sit on different foundations and move slightly differently. Unless it is widening or the extension is visibly tilting, this is usually a movement joint that was never formed rather than a structural problem.
Short cracks running up from the corner of a door frame or window, especially in older lath and plaster, often come from doors being slammed for decades or from timber lintels shrinking slightly. If the crack is fine, stable and stops after a short distance, it is rarely anything more. Small cracks that open in summer and close in winter, by a millimetre or less, are typical of houses on clay and are often not a cause for concern on their own.
Warning signs that need an engineer
Any one of these is worth a professional look. Two or three together, especially if they have appeared recently, should be inspected promptly.
- Diagonal or stepped cracks in brickwork, especially near corners, doors or windows
- Cracks that are tapered, noticeably wider at the top or bottom
- The same crack visible inside and outside the wall
- Doors or windows that have started to stick or no longer latch
- Cracks that are growing over weeks or months
- Horizontal cracking in mortar joints at regular heights, which can indicate wall tie corrosion
- A bay window pulling away from the main wall
- Bulging or leaning walls, or floors that have started to slope
Common causes in London houses
Much of London sits on London Clay, which shrinks as it dries and swells when it wets. Trees and large shrubs draw water from the clay, so houses with shallow Victorian foundations near trees can move with the seasons and in long dry spells. Our guide to London Clay, trees and foundations explains this in more detail.
Leaking drains can soften clay or wash fine material out of sandy and gravelly soils, causing localised settlement. Other common causes are failed timber lintels over openings, roofs spreading and pushing walls outwards, and earlier alterations such as chimney breasts or walls removed without proper support. If you suspect the ground is moving, read the signs of subsidence and what to do.
What to do next
Start by recording the crack. Photograph it with a ruler or coin for scale, measure its width at the widest point, and mark the ends lightly in pencil with the date. Check it again every few weeks. Do not fill it straight away, because that removes the evidence an engineer needs.
If you think the house may be subsiding, tell your buildings insurer early, as most policies require prompt notification. For a single crack you want explained in writing, a Specific Structural Inspection report is usually the right level. All our structural inspection reports are signed off by a chartered engineer who is a Member of the Institution of Civil Engineers (MICE).
Where the cause is unclear or movement may still be active, the engineer may recommend monitoring. A crack monitoring baseline fixes gauges across the cracks and records their starting position, so later readings show whether anything is changing.
How cracks are repaired
Cosmetic cracks are simply filled and redecorated, ideally after the house has been through a full year so you can see they are stable. Structural cracks are repaired once the cause has been dealt with, otherwise they come back. Typical repairs include stitching cracked brickwork with stainless steel helical bars, replacing failed lintels, installing new wall ties and repairing leaking drains.
Underpinning is needed far less often than people fear. Many cases of clay-related movement settle down once the cause, such as a tree or a broken drain, has been dealt with.
Where repairs are needed, the engineer can specify them so your builder prices the same scope as everyone else, and Building Control can be involved where the work requires it. Getting the order right, cause first and cosmetic repair last, is what stops the same crack reappearing a year later.
Frequently asked questions
Are horizontal cracks worse than vertical cracks?
Not automatically. Vertical cracks are often shrinkage or thermal movement. Horizontal cracks in render or brickwork at regular intervals can point to corroded wall ties expanding in the mortar joints, while a horizontal crack low in a wall can suggest bowing. Diagonal and tapered cracks are usually the ones most associated with foundation movement. The pattern and location matter more than the direction alone.
Should I tell my insurer about cracks?
If you suspect subsidence or heave, yes. Most buildings policies require you to notify the insurer promptly, and delay can affect a claim. Cosmetic cracking from shrinkage is not usually an insurance matter. If you are unsure which you have, an engineer's inspection will tell you, and the report can be shared with the insurer or loss adjuster if a claim follows.
Will cracks affect selling my house?
They can, if a buyer's surveyor or the lender's valuer flags them. The best protection is evidence. A structural report explaining the cause, confirming the cracking is historic or cosmetic, or showing it has been repaired, usually answers the buyer's questions. Unexplained cracks tend to lead to lender conditions, renegotiation or delays.
Are cracks in a new extension normal?
Fine cracks in new plaster are very common as materials dry out and settle in the first year or two. A crack where the extension joins the original house is also common because the two parts move slightly differently. Cracks wider than a few millimetres, cracks in the extension's brickwork or doors that stick are worth reporting to your builder and engineer.
How long should cracks be monitored?
Long enough to see a pattern. On clay soils, where movement follows the seasons, engineers often recommend monitoring through a full seasonal cycle so both the dry summer and wet winter readings are captured. Shorter monitoring can work where the cause is clear. The engineer will recommend a programme based on what they find at the first visit.