Cracks in Walls: When to Worry (London Homes, 2026)

Key Takeaways

Most fine cracks are harmless settlement or shrinkage, not structural.
Cracks wider than about 5mm, diagonal, or growing are more concerning.
Warning signs: doors and windows sticking, cracks inside and out in the same place.
Sudden or widening cracks should be assessed promptly.
A structural engineer can confirm the cause and whether action is needed.

Almost every home gets cracks, and most are nothing to worry about. But a few signal something more serious. This guide helps you tell the difference and know when to call an expert.

Most cracks are normal

Fine hairline cracks are very common and usually harmless, caused by settlement, seasonal movement, or plaster drying out. They are stable, only a few millimetres wide, and easily filled and redecorated.

When to worry

Be more cautious when a crack:

Is wider than about 5mm, or growing.
Runs diagonally, often near windows and doors, and is wider at one end.
Appears suddenly.
Shows on both the inside and outside in the same place.
Comes with sticking doors and windows.

These can indicate structural movement such as subsidence.

Who to call

For a worrying crack, a chartered structural engineer can confirm the cause, and a reputable builder can advise on repair. Do not simply fill a significant crack, as that hides the problem. GS Renovation carries out structural repairs and making good across London. For advice, contact us or call 07472 424 226.

The GS Renovation Team

GS Renovation & Home Improvements has delivered kitchen, bathroom, structural and extension projects across London for over 30 years. This guide reflects current UK industry pricing and our hands-on site experience.

Frequently Asked Questions

When should I worry about a crack in my wall?

Worry when a crack is wider than about 5mm, runs diagonally, is wider at one end, appears suddenly, keeps growing, or shows on both the inside and outside of the wall in the same place, especially alongside sticking doors and windows. These can indicate structural movement such as subsidence and should be assessed by a professional.

Are hairline cracks in walls normal?

Yes, fine hairline cracks are very common and usually harmless, caused by normal settlement, seasonal movement, drying out of plaster, or shrinkage of new plaster and paint. They are typically less than a few millimetres wide, stable, and easily filled and redecorated. It is sudden, wide, diagonal or growing cracks that warrant closer attention.

What size crack is structural?

As a rough guide, cracks up to around 5mm are usually cosmetic, while cracks wider than 5mm, and especially those over 15mm, are more likely to indicate structural movement and should be investigated. Width alone is not the whole story, though: the direction, location, pattern and whether the crack is active also matter, so a professional view helps.

What causes cracks in walls?

Common causes include normal settlement of a building, seasonal expansion and contraction, drying out of new plaster, thermal movement, and minor foundation movement. More serious causes include subsidence (often from clay soils, trees or leaks), heave, and structural overloading. Identifying the cause from the crack's pattern and behaviour is the key to knowing whether it matters.

Who do I call about a worrying crack?

For a crack you are concerned about, a chartered structural engineer can assess it and confirm the cause and whether action is needed, and a reputable builder can advise on repairs. If you suspect subsidence, your buildings insurer will usually arrange investigation. Avoid simply filling a significant crack, as that hides a problem that may need addressing.

Cracks in Walls: When to Worry

Key Takeaways

Most cracks are normal

When to worry

Who to call

Frequently Asked Questions

Planning a Project in London?