Damp, mould and structural
Cracks in walls: which are cosmetic and which are serious?
Most cracks in walls are cosmetic, caused by drying plaster, normal settlement or seasonal movement. The ones to watch are wide (above about 5mm), diagonal, visible inside and out in the same place, or still growing. These can signal subsidence or structural movement and should be assessed by a professional.
- Also known as
- wall cracking, plaster cracks, fracture lines
- Easily confused with
- subsidence (a cause, not the crack itself), plaster shrinkage, thermal movement
- How serious
- Varies: most are cosmetic, but wide growing diagonal cracks are serious
- Typical cost
- Usually no cost for DIY control
How to identify cracks in walls
Surveyors often think in terms of crack width as a rough guide. Hairline and fine cracks up to a couple of millimetres are usually cosmetic. Cracks around 5mm and above, especially if diagonal and near openings, are the ones to take seriously.
Look at pattern as well as width: vertical cracks following plasterboard joints or where an extension meets the original house are often movement between materials. Diagonal cracks radiating from window and door corners are more associated with ground movement.
Whether a crack passes through to the outside in the same position, and whether it is still moving, tells you more than its current size alone.
How serious is it?
Varies: most are cosmetic, but wide growing diagonal cracks are serious
The seriousness sits on a spectrum. A fine crack in plaster after a hot summer or a new radiator is almost always harmless and a redecoration job.
A wide, diagonal, growing crack that appears inside and out, or that comes with sticking doors and uneven floors, can indicate subsidence or structural movement and is the point at which you stop filling and start assessing. When unsure, treat it as the more serious case until a professional says otherwise.
How to fix it yourself
- Measure and monitor first. Note each crack width with a ruler, photograph it with the date, and check again after a few weeks to see if it is moving.
- Repair genuinely cosmetic cracks (fine, stable, plaster only) with flexible filler and redecorate once you are confident they are not growing.
- Address obvious triggers you can see: a clearly defined cause such as a leaking gutter softening the ground, or a heavy door slamming a frame.
- Do not fill or paper over a crack you are unsure about, because doing so hides movement a surveyor needs to see.
When to call a professional
- A crack is wider than about 5mm, diagonal, near a window or door, or visible on both the inside and outside of the same wall.
- Cracks are still widening week to week, or come with sticking doors and windows or sloping floors.
Who to call
- A structural engineer or a Royal Institution of Chartered Surveyors (RICS) member to assess whether movement is structural.
- A builder or plasterer only once a professional has confirmed a crack is cosmetic and stable.
Frequently asked questions
When should I worry about a crack in my wall?
Worry about cracks wider than about 5mm, diagonal cracks near windows and doors, cracks visible on both sides of the same wall, and any crack that is still widening. These can indicate structural movement. Fine, stable hairline cracks are almost always cosmetic.
What does the width of a crack tell me?
Width is a rough guide: hairline and fine cracks up to a couple of millimetres are usually cosmetic, while cracks around 5mm and above warrant closer attention. Pattern and movement matter too, so a thin crack that keeps growing can be more telling than a wider stable one.
Can I just fill cracks in my walls?
Fine, stable cracks in plaster can be filled and redecorated. Do not fill a crack you are unsure about, because filling hides whether it is still moving, which is the information a structural engineer needs. Monitor first, fill only once you are confident it is cosmetic.
Sources
Editor, HomesAndHedge
Oliver leads HomesAndHedge's editorial coverage of home and garden problems. He researches and writes the plain-English explainers on pests, invasive plants, damp and mould, drainage and wildlife, drawing on guidance from bodies such as the Property Care Association, the RHS and the NHS, and is clear about when a job needs a qualified professional.
Last reviewed: 8 June 2026