What is Subsidence and Why Should You Worry?

Subsidence represents one of the most serious structural issues affecting UK properties. It occurs when the ground beneath a building settles or moves, causing the structure above to shift and crack. Whilst minor subsidence may present only cosmetic concerns, severe cases can render properties unmortgageable and require expensive underpinning works. Understanding subsidence, recognising its visual indicators, and seeking professional assessment early can save substantial time and money.

Understanding Ground Movement and Subsidence

The UK’s geology varies significantly. Some areas, particularly in southern England, suffer from clay soils that shrink dramatically during dry periods and expand when wet. Other regions experience subsidence due to old coal mining activity, chalk dissolution, or deteriorating historical foundations. Identifying your area’s subsidence risk involves understanding local geology and historical land use.

Why Does Subsidence Occur?

Subsidence develops when the soil supporting a building’s foundations loses strength or volume. Common causes include clay soil shrinkage during droughts, water table fluctuations, nearby tree roots extracting moisture from clay, inadequate or failing foundations, mining subsidence, and dissolution of chalk or limestone substrata. In some instances, load-bearing defects or poor construction accelerate subsidence.

Recognising Subsidence Cracks: Visual Indicators

Identifying subsidence requires distinguishing subsidence cracks from normal settlement cracks and other fractures. This distinction is crucial because treatment differs fundamentally.

Crack Characteristics

• Location and Pattern:
  Subsidence cracks typically run diagonally from corners of windows and doors, progressing towards roof level. They often appear in patterns suggesting the building is moving as a unified mass—for instance, cracks radiating from several points suggest differential movement across the structure. Multiple cracks affecting the same side of the property are particularly concerning.
• Width and Severity:
  Minor cracks under 3mm wide may indicate normal settlement. Cracks between 3mm and 15mm suggest more significant movement and warrant investigation. Cracks exceeding 15mm indicate serious structural movement and require immediate professional assessment. Measure cracks using a ruler or crack monitor to track whether they’re expanding, stable, or healing.
• Directionality:
  Cracks running at approximately 45 degrees from horizontal are most consistent with subsidence. Horizontal cracks, by contrast, often result from water penetration or freezing damage rather than ground movement.
• Multiple Cracking:
  A single crack may reflect localised settlement. Multiple cracks affecting different parts of the property suggest systematic structural movement—a key subsidence indicator.
• Stair-Step Patterns:
  In brick properties, subsidence sometimes produces characteristic stair-step cracking patterns following mortar joints. This distinctive pattern reflects the masonry’s geometry as differential settlement occurs.

Where Subsidence Cracks Typically Appear

• Exterior Brickwork:
  Cracks on external walls represent the most obvious subsidence indicator. Examine all elevations, particularly around window and door openings where stress concentrates. Look for cracks extending through multiple brick courses rather than following mortar joints exclusively.
• Internal Plasterwork:
  Interior cracks, especially where walls meet ceilings or around door openings, often mirror external cracking. However, internal cracks can also result from thermal movement or timber shrinkage, necessitating careful assessment.
• Around Openings:
  Subsidence frequently produces cracks radiating from window and door frames. The lintel above the opening experiences stress, and the surrounding masonry cracks as the building settles unevenly.
• Roof Damage:
  Severe subsidence causes roof coverings to slip or break, valleys to separate from guttering, and roof structure to crack. If you notice tiles missing or roof line distortion, subsidence may be responsible.

Distinguishing Subsidence from Other Cracks

• Normal Settlement Cracks:
  New buildings often experience minor cracking during the first two years as the structure settles. These cracks are typically thin (under 2mm), follow mortar joints, appear mostly internally, and stabilise over time. Professional assessment distinguishes these harmless settlement cracks from subsidence.
• Thermal Movement Cracks:
  UK properties experience seasonal thermal expansion and contraction. These cracks typically appear at predictable seasons, follow consistent patterns, and don’t worsen progressively. They differ from subsidence cracks, which generally don’t reverse seasonally.
• Water Damage Cracks:
  Freeze-thaw damage, water ingress, and efflorescence produce characteristic horizontal cracks, often accompanied by visible water staining. These differ from diagonal subsidence cracks.

Professional Subsidence Assessment

• Surveyor Inspection:
  A surveyor conducts a thorough internal and external examination, measuring and photographing cracks, assessing their progression, and examining the building’s overall structure. We assess foundation adequacy, identify ground conditions, and evaluate whether movement patterns align with subsidence-related behaviour. This includes reviewing drainage, checking for nearby trees, analysing soil type, and determining whether cracks correspond to ground shrinkage, washout, or structural failure.
• Monitoring Movement:
  In some cases, ongoing monitoring is required. Crack monitors may be installed to measure movement over weeks or months, helping determine whether subsidence is active, historic, or seasonal.
• Soil and Drainage Investigation:
  Where necessary, further investigations such as soil testing or CCTV drainage surveys provide crucial insights into the cause of foundation movement.
• Reporting and Recommendations:
  Following assessment, we provide a detailed subsidence report outlining the cause of movement, the severity of structural impact, and recommended next steps. This may include remedial works, monitoring, or insurance involvement.

Conclusion

If you suspect subsidence, early action is essential. Professional assessment helps distinguish harmless cosmetic cracks from genuine structural movement and prevents unnecessary cost, stress, and risk. At Structura Surveying, we provide clear, evidence-based subsidence diagnosis and guidance to protect your home and investment.