Subsidence: Detection, Causes and Solutions for South Wales Properties

Subsidence is one of the most serious structural issues affecting properties in Cardiff and across South Wales. As building surveyors, we identify subsidence regularly during RICS surveys, and it's crucial that homebuyers understand what it means, how to detect it, and what can be done about it.

This comprehensive guide from Cardiff Surveyors explains everything you need to know about subsidence based on our extensive experience surveying residential properties throughout the region.

What is Subsidence?

Subsidence occurs when the ground beneath a property moves downward, causing the foundations to sink. This creates structural movement that manifests as cracks in walls, distorted door frames, and other visible damage.

Subsidence differs from settlement (which is normal in new builds as they adjust to ground conditions) and heave (where ground swells upward). Understanding these distinctions is crucial for proper diagnosis.

Why Cardiff and South Wales Are Susceptible

Several factors make South Wales properties particularly vulnerable to subsidence:

• Clay soils: Large areas of Cardiff sit on shrinkable clay that expands and contracts with moisture changes
• Historic mining: Coalfields beneath parts of South Wales can cause ground instability
• Mature trees: Many Cardiff suburbs have large trees with roots affecting foundations
• Victorian construction: Older properties often have shallow foundations vulnerable to movement
• Drainage failures: Leaking drains washing away support

Signs of Subsidence Our Chartered Surveyors Look For

During building surveys and Level 2 RICS home surveys, we systematically check for subsidence indicators:

1. Cracking Patterns

Not all cracks indicate subsidence, but certain patterns are concerning:

• Diagonal cracks: Running at 45-degree angles, especially near doors and windows
• Wider at top: Tapering cracks wider at the top than bottom
• Crack width: Cracks wider than 3mm (about the thickness of a £1 coin)
• Progressive cracking: Cracks that continue to widen over time
• External and internal: Cracks visible both inside and outside
• Pattern correspondence: Internal cracks aligning with external damage

During our surveys, we measure crack widths and document patterns carefully. Our survey reports include photographs and clear descriptions.

2. Distorted Doors and Windows

Subsidence causes door and window frames to distort:

• Doors sticking or not closing properly
• Windows becoming jammed or difficult to open
• Visible gaps appearing around frames
• Frames pulling away from walls

3. Gaps and Displacement

• Gaps between walls and ceilings
• Gaps between walls and floors
• Skirting boards pulling away from walls
• Stepped cracking in brickwork
• Visible displacement where walls have moved

4. External Evidence

Outside the property, we look for:

• Brickwork bulging or leaning
• Extension walls pulling away from main building
• Cracks in external render
• Distorted window and door lintels
• Chimney stacks leaning
• Cracks in paving or paths near the building

Common Causes of Subsidence

As RICS chartered surveyors with extensive local knowledge, we investigate potential causes:

1. Tree Root Activity

This is the most common cause of subsidence in Cardiff and South Wales:

• Large trees within influencing distance of the property
• Tree roots extracting moisture from clay soils
• Soil shrinkage as it dries out
• Ground sinking as volume decreases

High-risk trees include:

• Oak (can affect buildings up to 30 meters away)
• Poplar (25 meters)
• Willow (20 meters)
• Ash (20 meters)
• Large conifers

Interestingly, tree removal can cause heave as soil rehydrates and swells. Our professional guidance helps navigate these complex situations.

2. Clay Soil Shrinkage

Parts of Cardiff sit on London Clay and other shrinkable clays:

• During dry periods, clay shrinks significantly
• Foundations can drop as support is lost
• Particularly problematic during drought years
• Climate change may increase frequency

3. Drainage Failures

Leaking drains are a significant cause:

• Water from leaks washes away soil particles
• Voids form beneath foundations
• Foundation support is lost
• Building sinks into voids

During surveys, we check for evidence of drainage problems and recommend CCTV drain surveys when appropriate.

4. Mining Subsidence

Parts of South Wales are affected by historic coal mining:

• Old mine workings collapsing
• Ground surface sinking
• Often affects multiple properties in an area
• May be compensatable through Coal Authority schemes

5. Poor Original Construction

• Shallow or inadequate foundations
• Foundations on made ground (filled land)
• Foundations on variable ground conditions
• Poorly constructed extensions with inadequate foundations

6. Vibration

• Heavy traffic on nearby roads
• Construction work nearby
• Repeated vibration affecting foundation integrity

How We Diagnose Subsidence

Our experienced team of building surveyors uses systematic assessment:

Visual Inspection

We conduct thorough internal and external inspections, documenting:

• All cracking with measurements
• Distortion patterns
• Areas of displacement
• External evidence

Crack Monitoring

If subsidence is suspected but not certain, we may recommend crack monitoring:

• Tell-tales (movement monitors) installed across cracks
• Monitored over 6-12 months
• Seasonal changes considered
• Determines if movement is ongoing or historic

Investigating Causes

We look for:

• Nearby trees and their species
• Drainage condition
• Ground conditions (from trial pits if necessary)
• Historical issues (previous repairs, monitoring)
• Mining history (Coal Authority records)

Specialist Referrals

For serious cases, we recommend:

• Structural engineers: For detailed structural assessment
• Arboriculturists: For tree reports
• Drainage specialists: For CCTV drain surveys
• Geotechnical engineers: For ground investigation

Distinguishing Subsidence from Other Issues

Not all cracks are subsidence. Our chartered surveyors specialising in structural issues differentiate between:

Subsidence vs. Settlement

• Settlement: Normal in new builds, stops within 1-2 years
• Subsidence: Ongoing movement in established properties

Subsidence vs. Thermal Movement

• Thermal: Minor cracks from temperature changes, cyclical
• Subsidence: Progressive, doesn't close up seasonally

Subsidence vs. Lintel Failure

• Lintel failure: Cracking above windows/doors from inadequate support
• Subsidence: Broader pattern affecting multiple areas

Subsidence Solutions and Remedial Work

Treatment depends on cause and severity:

1. Underpinning

The most common solution for serious subsidence:

• Excavating beneath existing foundations
• Installing deeper, more substantial foundations
• Typically concrete mass or mini-piled underpinning
• Cost: £15,000-30,000+ for a typical semi-detached house
• Duration: 4-8 weeks typically

Underpinning effectively stops further movement but doesn't reverse existing damage. Cosmetic repairs (re-plastering, repointing) are needed afterward.

2. Tree Management

If trees are causing the problem:

• Pollarding/pruning: Reducing canopy to reduce water uptake
• Root barriers: Installing barriers to prevent root growth toward property
• Tree removal: Last resort, can cause heave as soil rehydrates

Important: Work may need approval if trees are protected or in conservation areas.

3. Drainage Repairs

If drains are causing subsidence:

• CCTV survey to identify leaks
• Repair or replacement of damaged sections
• Much cheaper than underpinning
• Cost: £500-5,000 depending on extent

4. Monitoring

For minor cases:

• Install tell-tales to monitor movement
• Check over 6-12 months
• If movement stops, no further action needed
• If continuing, proceed with repairs

5. Crack Repairs

Once movement has stopped:

• Repointing external brickwork
• Re-plastering internal walls
• Repairing or replacing damaged lintels
• Cost: £2,000-10,000 depending on extent

Subsidence and Insurance

Understanding insurance implications is crucial:

Making a Claim

• Most home insurance policies cover subsidence
• Typical excess: £1,000 for subsidence claims
• Insurers appoint loss adjusters to assess
• May require structural engineer's reports
• Claims can take 6-12 months to resolve

Impact on Future Insurance

• Subsidence claims must be declared when selling
• Future insurance may be more expensive
• Some insurers won't cover properties with subsidence history
• Specialist insurers may be needed

Buying a Property with Subsidence

Our impartial advice for buyers encountering subsidence:

If Subsidence is Active

• Most mortgage lenders will refuse finance
• Property may be unmortgageable until repaired
• Significant price reduction needed to reflect risk and repair costs
• Consider walking away unless you're a cash buyer willing to repair

If Previously Repaired

• Check repairs were done properly with structural engineer certification
• Verify insurance is available (get quotes before exchange)
• Understand ongoing monitoring requirements
• Negotiate price to reflect subsidence history
• Most lenders will lend if repairs properly completed and guaranteed

If Being Monitored

• Understand monitoring duration and results so far
• Consider delaying purchase until monitoring complete
• If proceeding, negotiate retention (holding back funds until resolved)
• Ensure insurance is available

Real Example: Semi-Detached House in Llanishen

Last year, I surveyed a 1930s semi in Llanishen. The sellers hadn't mentioned any issues, but our survey revealed:

• Diagonal cracking in the front elevation
• Cracks up to 5mm wide
• Distorted front door frame
• A large oak tree 8 meters from the house
• Evidence the cracks had been recently filled and painted over

Further investigation showed:

• Active subsidence caused by tree roots
• Clay soil shrinkage during recent dry summers
• The sellers had concealed the problem

Our survey report estimated £20,000 for tree management and underpinning. The buyers initially withdrew, but eventually renegotiated a £35,000 reduction to cover repairs and future insurance difficulties.

This demonstrates why thorough professional surveys from independent firms are essential.

Preventing Subsidence

While not all subsidence is preventable, you can reduce risk:

• Maintain adequate distance between trees and buildings
• Consider tree species before planting
• Maintain drainage systems properly
• Address small leaks promptly
• Ensure adequate foundations for extensions
• Monitor for early warning signs
• Address minor cracks before they worsen

Frequently Asked Questions

Book Your Professional Survey

Subsidence is too serious to risk. Our experienced team of RICS chartered surveyors has extensive experience identifying and assessing subsidence in properties across Cardiff and South Wales.

We provide comprehensive building surveys with expert analysis of any structural movement, clear professional guidance on implications, and realistic cost estimates for remedial work.

Contact Cardiff Surveyors today for your online quote. We survey residential properties across Cardiff, Swansea, Newport, Vale of Glamorgan, Bristol and all surrounding areas of South Wales.

Regulated by the Royal Institution of Chartered Surveyors. Providing expert structural assessments and impartial advice since 1998.