In short
Many cracks are cosmetic: fine shrinkage cracks in plaster or at joints between different materials. Others need an engineer: cracks that are wide, growing, diagonal or stepped, cracks in beams, columns or slabs, and cracks with rust stains, damp or doors that start to stick. Record a crack, watch it, and have it inspected if it shows warning signs.
Key points
- Fine, stable cracks in plaster and at joints between materials are usually cosmetic.
- Wide, growing, diagonal or stepped cracks can indicate movement and need inspection.
- Cracks in structural elements, beams, columns, slabs, always deserve an engineer's view.
- Rust stains, damp or sticking doors and windows alongside cracks are warning signs.
- Photograph and date a crack, and mark its ends, so you can tell if it is growing.
Almost every building has cracks. Most are harmless; some are the first visible sign of a problem. The difference matters, and it can often be read from where the crack is, what it looks like, and whether it changes.
Usually cosmetic
- Fine hairline cracks in plaster, in random or map-like patterns, caused by shrinkage as the plaster dried.
- Straight cracks at the joint between two materials, for example where a block wall meets a concrete column, because the materials move differently with heat and moisture.
- Cracks that do not change over months.
These can usually be repaired as part of normal maintenance, ideally with a flexible filler at material joints.
Worth an engineer’s view
- Cracks wider than a hairline, especially if you can insert a thin card.
- Diagonal cracks from the corners of doors and windows, or stepped cracks following the joints of blockwork: these can indicate movement of the structure or the foundations.
- Cracks in beams, columns or slabs, the structural frame of the building.
- Cracks with rust stains or with concrete breaking away: signs that the reinforcement inside may be corroding.
- Cracks accompanied by damp, or by doors and windows that start to stick.
- Cracks that grow over weeks or months.
Record before you repair
Take a dated photograph with a ruler beside the crack, and mark its ends with a pencil and the date. If the crack extends past the marks, it is active, and that is valuable information for the engineer.
What an inspection does
An engineer maps the cracks, relates them to the structure, and finds the cause. Where needed, non-destructive tests confirm the condition of concrete and reinforcement. The result is a building safety report that tells you whether to monitor, repair or act urgently. Repairs that follow are covered in our guide to crack injection.
Questions
Can I just fill and paint a crack?
For a cosmetic crack, yes. For a crack caused by movement or corrosion, filling hides the symptom while the cause continues; it often reopens.
Are cracks normal in a new building?
Some fine shrinkage cracks in plaster are common as materials dry and settle. Cracks in structural elements, or cracks that keep growing, are not normal.