Diagnosing Cracking

How can I tell if cracking is serious?





Cracking in residential buildings


Cracking is one the more complex subjects that surveyors address in RICS home surveys.

There are a wide range of potential causes so it’s important to identify specific factors such as the age and type of construction (eg modern cavity walls or older solid walls), the location and ground conditions (type of subsoil, nearby trees etc), and the type of cracking – pattern, dimensions, age etc.


Potential causes of significant cracking

  • Ground instability –  e.g. where large trees on clay subsoils are in close proximity, or homes built on made up / backfilled ground. Or even nearby mines and sinkholes.
  • Botched structural alterations  – such as where internal walls and chimney breasts have been removed without supporting the remaining loadings above, and without Building Regs consent (therefore illegal !).
  • Cavity wall tie failure – where rusted wall ties have expanded and cracked mortar joints, found in some older cavity walls.
  • Rot – where timber decay or beetle has weakened load-bearing beams or lintels.
  • Defective design & construction –  aka ‘inherent defects’ where the original builders or designer made mistakes that left the structure vulnerable to movement.
  • Roof thrust – where the roof rafters succeed in pushing out the upper walls that they’re resting on (often because the walls and roof are not sufficiently tied in).
  • Thermal movement –  Different materials expand and contract at different rates.
  • Differential movement – e.g. where bay windows or extensions have different foundation depths to the main house



The Building Research Establishment (BRE) published the following guide –  known as ‘BRE Digest 251’.  This defines cracking in terms of size and related damage.

Cracks visible on both sides of the wall and wider than about 5mm (BRE categories 3 to 5) and cracking extending below DPC level are more likely to be structurally significant. But due to its complexity cracking is sometimes misdiagnosed leading to unnecessary works and expense. So for a definitive professional opinion the best advice is to instruct a structural engineer to assess specific cracks.

Buildings insurers are also likely to request a structural engineer’s report, as well as possible monitoring of cracks over time, before agreeing to fund remedial work such as underpinning.


                                         BRE Digest 251

 Assessment of Damage in Low Rise Buildings


Category of Damage 

Description of Typical Damage



Hairline cracks of less than about 0.1mm which are classed as negligible.
No action required.



Fine Cracks which can be treated easily using normal decoration.
Damage generally restricted to internal wall finishes, cracks rarely visible in external brickwork.
Typical crack widths up to 1mm.



Cracks easily filled. Recurrent cracks can be masked by suitable linings.
Cracks not necessarily visible externally; some external repointing may be require to ensure weather- tightness.
Doors and windows may stick slightly and require easing and adjusting. Typical crack widths up to 5mm.



Cracks which require some opening up and can be patched by a mason.
Repointing of external brickwork and possibly a small amount of brickwork to be replaces. Doors and windows sticking.
Services pipes may fracture. Weather tightness often impaired.
Typical crack widths are 5 – 15 mm or several of say, 3mm.



Extensive damage which requires breaking -out and replacing sections of walls, especially over doors and windows.
Windows and door frames distorted, floor sloping noticeably.
Walls leaning or bulging noticeably, some loss of bearing in beams. Service pipes disrupted.
Typical crack widths are 15-25mm but also depends on number of cracks.



Structural damage which requires a major repair job involving partial or complete rebuilding.
Beams lose bearing, walls lean badly and require shoring. Windows broken with distortion. Danger of instability.
Typical crack widths are greater than 25mm but depends on number of cracks.











Different homes require different types of survey.

This video on the RICS Facebook page explains which survey is recommended for each property type:-