Crack Classification
The Building Research Establishment (BRE) classifies cracks by width:
- Category 0 (negligible): Hairline cracks up to 0.1mm — normal and harmless
- Category 1 (very slight): Up to 1mm — cosmetic, easily filled
- Category 2 (slight): Up to 5mm — may indicate some movement, easily repaired
- Category 3 (moderate): 5–15mm — requires professional assessment, may need structural repair
- Category 4 (severe): 15–25mm — significant structural damage, needs investigation
- Category 5 (very severe): Over 25mm — major structural damage, possible danger
Which Crack Patterns Are Concerning?
Diagonal Cracks
Cracks running diagonally from window or door corners — especially if wider at one end — often indicate differential movement or subsidence. These always warrant investigation.
Stepped Cracks in Brickwork
Cracks following the mortar joints in a stepped pattern suggest foundation movement. The direction and width indicate which part of the building is moving and how severely.
Horizontal Cracks
Horizontal cracks in walls can indicate lateral pressure (from ground movement or roof thrust) and are always worth investigating.
Cracks Around Openings
Cracks around windows and doors are common stress points. Minor cracking may be cosmetic, but widening or recurring cracks need assessment.
When to Commission a Survey
Get a professional survey if you see:
- Cracks wider than 2–3mm
- Diagonal or stepped crack patterns
- Cracks that have been filled but reopened
- Multiple cracks in the same area
- Cracks combined with doors/windows sticking
- Any crack that concerns you — peace of mind is worth the investment
A Level 3 Building Survey includes detailed crack assessment with mapping, classification, cause analysis, and remediation recommendations. For specific structural concerns, a structural survey provides focused investigation.
Crack Repair Costs
- Cosmetic filling: £100–£500
- Repointing: £500–£3,000
- Structural stitching: £1,000–£5,000
- Underpinning (if subsidence): £10,000–£50,000+