Cracks in walls: When should you worry about them?

What causes cracks in walls?

If you’re wondering what causes cracks in walls, there are several common explanations – and most aren’t a cause for alarm.

1. Normal settlement and movement

Are cracks in walls normal? In many cases, yes, because all homes move. Whether it’s a new build or you've got cracks in walls in an old house, buildings naturally settle over time.

• Settlement cracks in walls often appear within the first few years of construction.
• Minor movement cracks in walls can happen as foundations adjust to the ground.
• Small cracks in internal walls are especially common and usually cosmetic.

2. Temperature changes (thermal expansion)

Brick, plaster and timber expand and contract as temperatures rise and fall. These shifts can cause thermal expansion cracks in brick walls, particularly:

• After hot summers or freezing winters
• Around window and door frames
• In painted plaster surfaces

These cracks are often thin and seasonal. You might notice these if you leave a property unoccupied for some time, for example.

3. Damp and water damage

Water weakens building materials. If you have persistent damp, leaks or poor drainage, cracks appearing in walls could follow. Look for:

• Peeling paint
• Crumbling plaster
• Staining around the area

4. Plaster and finish shrinkage

Many cracks in plaster walls are simply surface-level. As plaster dries, it can shrink slightly and create fine lines – particularly in new builds.

These are classed as non-structural types of cracks in walls.

5. Structural movement (subsidence or heave)

More serious causes of cracks in walls include:

• Subsidence, which is the ground beneath the property sinking
• Heave, where the ground beneath swells
• Lintel or wall-tie failure
• Foundation movement

These tend to produce larger or patterned cracks – which we’ll cover below.

Cracks in a new-build house: Are they normal?

Yes, to an extent.

In the first year or two after construction, it’s common to see small settlement cracks as the building materials dry and the structure beds in.

Developers often return after this period to repair minor cosmetic cracks once movement has stabilised. In particular, hairline vertical cracks on internal walls are typical.

That said, wide horizontal or diagonal cracks should still be checked.

What are the different types of cracks in walls?

Understanding the types of cracks in walls helps you judge whether they’re cosmetic or structural.

Hairline cracks (usually minor)

These are typically non-structural and easy to repair. They are:

• Very thin (less than 1mm)
• Often random or vertical
• Common in internal walls
• Usually caused by plaster drying or minor movement

Vertical cracks

Vertical cracks in walls can be either minor or more serious.

• Thin, straight vertical cracks are often due to shrinkage or settlement.
• Wider vertical cracks in outside walls – especially above windows or doors – might suggest structural movement.

If vertical cracks are wider than 3-5mm (about the thickness of a £1 coin), you'll need to investigate them further.

Horizontal cracks

Horizontal cracks in walls are more concerning, particularly in external brickwork.

They can point to:

• Wall-tie failure
• Pressure build-up
• Foundation movement

Horizontal cracks in plaster walls internally can sometimes be cosmetic – but in brickwork, they warrant caution.

Diagonal or staircase cracks

These follow mortar lines in a stepped pattern and are commonly associated with subsidence.

Staircase cracks in walls or diagonal cracks from window corners are among the most recognisable signs of foundation movement.

When should I worry about cracks in walls?

If you’re unsure whether you should be concerned about cracks in walls, use this quick ‘worry’ checklist.

You should seek professional advice if:

• Width: The crack is wider than 3-5mm
• Direction: It’s horizontal or diagonal, especially in external walls
• Location: It runs from window or door corners
• Pattern: It forms a staircase shape in brickwork
• Progression: It’s getting longer or wider
• Other signs: Doors or windows stick, floors slope, or you get bulges in walls

Cracks appearing in walls and ceilings together can also indicate structural movement.

If you suspect structural cracks in walls:

• Consult a structural engineer or a RICS surveyor
• Avoid repeatedly filling cracks that keep reopening
• Check whether your buildings insurance includes subsidence cover

Does home insurance cover cracks in walls?

Whether cracks in walls are covered by home insurance generally depends on the cause.

What might be covered

Buildings insurance can sometimes cover damage caused by:

• Subsidence
• Heave
• Landslip
• Sudden accidental damage (in some policies)

However, subsidence home insurance claims usually carry a high excess – often £1,000 or more.

What’s usually excluded

Most insurers won’t cover:

• Normal settlement cracks
• Poor maintenance
• Cosmetic plaster cracking

Before contacting your insurer, it’s a good idea to confirm whether the crack is structural. Multiple enquiries could be flagged on your record.

Are you unsure whether your current policy offers adequate protection? If so, compare quotes to find a level of home insurance that better suits your needs. Just remember that insurance is unlikely to cover pre-existing issues.

How to fix and fill small cracks in walls

If you’re dealing with minor hairline cracks in walls, you might be able to repair them yourself.

For hairline cracks in plaster or paint

• Use decorator’s caulk or a light, flexible filler
• Smooth with a filling knife
• Sand lightly once dry
• Repaint

Note that this is only suitable for fixing small cosmetic cracks.

For larger cracks (up to 5mm)

If you’re filling cracks in walls that are slightly wider:

1. Widen the crack slightly into a shallow ‘V’ shape
2. Brush out dust and debris
3. Apply a hard-setting powder filler
4. Allow to dry fully
5. Sand smooth
6. Prime and repaint

Use the best flexible filler (something fibre-rich) for cracks in walls if you’re expecting minor movement.

For spider or alligator cracking

Older plaster can develop fine surface patterns, which resemble reptile skin (hence ‘alligator’ cracking). In this case:

• Scrape back loose material
• Apply plaster repair filler
• Sand and redecorate

Important warning

If the crack is still moving, filling it won’t solve the problem – the filler will simply crack again. Structural cracks require professional assessment before any cosmetic repair.

If you’re unsure about cracks in your wall, get expert advice early, as this can prevent bigger repair bills later.