Subsidence has a fingerprint. Once you know what to look for, the difference between cosmetic cracking that nobody needs to worry about and movement that genuinely warrants an independent surveyor is straightforward. The nine signs below are the ones that consistently come up in our diagnostic casework. Spotting any one in isolation is rarely cause for alarm. Spotting two or three together is the point at which you should send a photo for a professional opinion before you let anyone with a quotebook over the threshold.
1. Diagonal cracks from the corners of doors or windows
Door and window openings are stress concentrators. When a wall has to span across a settling foundation, the corners of openings are where the tension shows first. Diagonal cracks running away from the upper corners of a door or window, wider at the top than at the bottom, are the most characteristic subsidence pattern in UK homes.
2. Stepped cracks in external brickwork
On a brick wall, subsidence cracks tend to follow the path of least resistance, the mortar joints. The result is a stepped or zig-zag crack climbing the wall in a diagonal direction. Stepped cracks that mirror themselves between inside and outside, and that widen as they go upward, are the textbook external sign.
3. Cracks visible on both inside and outside of the same wall
Internal-only cracks are almost always cosmetic, plaster shrinkage, thermal movement, or lath-and-plaster keying failure. Cracks that appear on the same wall both inside and outside, in roughly the same position and direction, indicate that the wall itself has cracked through the full thickness rather than just the plaster surface.
4. Cracks wider than the edge of a 10p coin (≈3mm)
Hairline cracks (under 1mm) and fine cracks (1–3mm) are usually cosmetic. Cracks wider than the edge of a 10p coin, particularly when they meet the other criteria here, push into the territory where a structural opinion is genuinely warranted.
5. Cracks that are growing over months
A historical crack of any width is far less significant than an active one. Mark each end of a crack with a pencil line and the date. Photograph it next to a tape measure. Check again in four to six weeks. If the marks have not moved, you almost certainly have a historical record of a single past event, not an active structural problem.
6. Sticking doors and windows that used to operate cleanly
Where a foundation has moved enough to crack a wall, it usually moves enough to distort the frames within it. Doors that have started catching, windows that no longer open and close as they used to, or visible gaps opening between frames and the surrounding masonry, particularly when they appear at the same time as new cracking, point to live distortion.
7. Sloping floors
Use a digital level on the floors of the rooms above any flagged crack. A gradient of more than 1:100 across a room (a 4mm fall across a 4m room) is meaningful in a building that was originally level. Older properties will always show some slope; what matters is whether it is concentrated in the area where other signs are showing.
8. Cracks that follow a single line, not multiple random plaster cracks
Plaster shrinkage produces multiple, fine, random cracks across a ceiling or wall. Subsidence cracks tend to be solitary, decisive, and follow a single geometric line, diagonally across a wall, or running between two stress concentrators.
9. Cracks adjacent to extensions, additions or alterations
Where an extension or alteration has been added to an older property, the new structure and the original structure often have different foundations and respond differently to seasonal soil movement. Cracking at the junction between original and extension is one of the most common subsidence-like patterns we are called to inspect, and not all of it is genuine subsidence; differential movement between two structures is a separate and usually less severe phenomenon.
What is not a sign of subsidence
- Multiple fine, random plaster cracks (almost always cosmetic).
- A single hairline crack on a freshly-plastered wall (drying shrinkage).
- Crack-lines along ceiling-to-wall junctions (movement between materials, not subsidence).
- Vertical cracks on internal walls that do not appear externally (typically plaster, lath or block-bedding issues).
- Cracks above lintels that follow the lintel line (lintel sag, not ground movement).
- Hairline cracks around radiator pipes and electrical boxes (thermal and service movement).
When any of these signs warrants a professional survey
The threshold is not a single dramatic sign but the combination of two or more, particularly when the pattern is mirrored inside and outside, or when the cracks are visibly progressing. At that point an independent subsidence survey is the cheapest and fastest way to put the situation on an evidential footing.
Send your photographs through the form on this page first. A qualified surveyor will review them and give you a no-obligation opinion on whether a paid independent survey or a period of crack monitoring is warranted before you spend anything.