Subsidence & Structural Movement in Blocks of Flats
A comprehensive guide to subsidence blocks of flats, covering the causes of structural movement flats, how to make a subsidence insurance claim, the process of underpinning block of flats foundations, and the question of subsidence responsibility leasehold properties. Understand clay soil shrinkage, tree root damage, drainage failures, and the role of your managing agent in protecting your building.
What Is Subsidence and How Does It Affect Blocks of Flats?
Subsidence is the downward movement of the ground beneath a building's foundations, causing the structure above to move unevenly and potentially sustain significant damage. Unlike normal settlement, which occurs shortly after construction and is generally uniform, subsidence is a progressive condition driven by external factors such as changes in soil moisture content, vegetation, or underground voids. In a block of flats, the consequences of subsidence are particularly serious because the building houses multiple households, the structure is often larger and heavier than a single dwelling, and the cost of investigation and repair is shared among all leaseholders through the service charge.
Structural movement in blocks of flats can manifest as cracking in walls, distortion of door and window frames, and in severe cases, a visible lean or displacement of part of the building. The impact extends beyond the physical damage: subsidence can affect property values, complicate insurance renewals, delay sales, and create anxiety among residents. Early detection and professional management are essential for minimising both the structural damage and the financial impact on leaseholders.
This guide explains the common causes of subsidence blocks of flats, the warning signs to watch for, the insurance claims process, the remediation options available, and the responsibilities of managing agents in monitoring and protecting the building. For related guidance on maintaining the fabric of your building, see our building maintenance guide.
Common Causes of Subsidence in Residential Buildings
Understanding the causes of subsidence is the first step in prevention and effective management. In England and Wales, the most common causes of structural movement flats are related to soil conditions, vegetation, drainage, and in some regions, historic mining activity. Each of these factors can affect the ground beneath a building's foundations in different ways.
Clay Soil Shrinkage
Clay soils are the single most common factor in subsidence across the United Kingdom. Clay is a highly reactive soil type that expands when wet and shrinks when dry. During prolonged dry periods, particularly in summer, the moisture content of clay soil can drop significantly, causing the ground to shrink and pull away from the foundations. This creates voids beneath the footings, allowing the building to settle unevenly. Properties built on London Clay, Weald Clay, Gault Clay, and other shrinkable clay formations are at the highest risk. The depth and type of foundation, the age of the building, and the severity of seasonal moisture variation all influence the likelihood and severity of clay-related subsidence in a block of flats.
Tree Root Damage
Tree roots are the most frequent trigger for subsidence claims in the United Kingdom. Certain tree species, including oak, willow, poplar, and ash, have extensive root systems that can extract large volumes of moisture from the surrounding soil. When these trees are growing in close proximity to a building on clay soil, the moisture extraction causes localised soil shrinkage beneath the foundations, leading to differential movement. The root zone of a mature tree can extend well beyond the canopy spread, and even trees on neighbouring land can cause subsidence to an adjacent building. Managing agents responsible for blocks of flats should ensure that trees on the building's land are regularly surveyed and that any vegetation management recommendations from structural engineers or arboriculturists are acted upon promptly.
Drainage Failures
Leaking drains can cause subsidence in two ways. First, water escaping from a damaged drain can wash away or soften the soil beneath the foundations, a process known as washout. Second, if a drain leak goes undetected for a prolonged period, the continuous flow of water can erode underground channels in the soil, creating voids that eventually collapse under the weight of the building. In blocks of flats, the drainage network is often extensive and runs beneath communal areas, car parks, and landscaped grounds. Regular communal repairs and CCTV drain surveys should form part of the building's planned maintenance programme to identify and address defects before they cause structural damage.
Mining and Underground Voids
In parts of England and Wales with a history of coal mining, tin mining, or other extractive industries, buildings can be affected by the collapse or gradual deterioration of former mine workings beneath the surface. This type of subsidence can be sudden and severe, and it is not always predictable from surface conditions alone. The Coal Authority maintains records of known mine workings and provides coal mining reports that should be obtained when purchasing a leasehold flat in an affected area. Managing agents should be aware of whether the building is in a mining-affected area and ensure that any signs of movement are investigated promptly with reference to the geological and mining history of the site.
Signs of Subsidence: What to Look For
Recognising the signs of subsidence early is critical for minimising damage and controlling repair costs. In a block of flats, both the managing agent and individual leaseholders have a role to play in monitoring the building for signs of structural movement. The following are the most common indicators that a building may be experiencing subsidence.
Diagonal Cracking
The hallmark of subsidence is diagonal cracking, typically emanating from the corners of windows and doors and extending towards the corners of the building. These cracks are usually wider at one end than the other, reflecting the differential movement of the foundations. Cracks wider than three millimetres that appear suddenly or worsen over time should be treated as a potential sign of subsidence and reported to the managing agent immediately. It is important not to dismiss cracking as cosmetic without professional assessment, as what appears to be a minor crack on the surface may indicate more significant movement within the structure.
Sticking Doors and Windows
When the structure of a building moves, door and window frames can become distorted, causing doors and windows to stick, jam, or fail to close properly. This is particularly noticeable in ground-floor flats or in the parts of the building closest to the area of subsidence. While sticking doors and windows can have other causes, such as humidity or poor installation, they should be considered alongside other signs of movement as part of an overall assessment of the building's condition.
Gaps and Separations
Visible gaps appearing between walls and ceilings, walls and floors, or between the building and any attached extensions or outbuildings can indicate that one part of the structure is moving relative to another. In a block of flats, these gaps may be observed in communal areas such as stairwells, corridors, and entrance halls, as well as within individual flats. Any new or widening gaps should be documented with photographs and measurements and reported to the managing agent without delay.
External Ground Movement
Changes in the ground level around the building, such as paths pulling away from external walls, cracking in paved areas, or visible soil shrinkage around the foundations, can all indicate that the ground beneath the building is moving. These external signs are often easier to observe than internal cracking and can provide early warning of subsidence before it causes significant damage to the structure above.
Insurance and Subsidence Claims
Making a subsidence insurance claim on a block of flats policy is a complex process that requires careful management and patience. Most buildings insurance policies include cover for subsidence, heave, and landslip, but the claims process is more involved than a standard insurance claim due to the investigative work required to establish the cause and extent of the movement. For a broader overview of insurance in leasehold buildings, see our insurance claims guide.
Notifying the Insurer
The first step in any subsidence insurance claim is to notify the building's insurer as soon as subsidence is suspected. The managing agent should provide the insurer with a detailed report of the signs observed, photographs and measurements of any cracking, and information about the building's construction, age, and surrounding environment. Early notification is essential because most policies require prompt reporting of any event that may give rise to a claim. Delay in reporting can complicate the claims process and in some cases may give the insurer grounds to decline the claim.
Investigation and Monitoring
Once a claim is notified, the insurer will typically appoint a loss adjuster and instruct a structural engineer to carry out an investigation. This usually involves installing crack monitors on the affected parts of the building to track the pattern and rate of movement over time. Monitoring is typically carried out for a minimum of twelve months to capture a full seasonal cycle, as clay-related subsidence often follows a seasonal pattern of worsening in summer and stabilising or partially recovering in winter. The investigation may also include trial pits to examine the foundations and soil conditions, level surveys to map the profile of the building, borehole investigations to analyse the soil at depth, and root identification tests where tree root involvement is suspected.
Excess and Cost Sharing
Subsidence claims typically carry a higher policy excess than standard claims. In a block of flats, the excess is usually payable from the service charge account or the reserve fund, meaning the cost is shared among leaseholders. Managing agents should ensure that leaseholders are informed about the excess amount and how it will be funded before the claim proceeds. In some cases, particularly where the claim is complex or the remedial works are extensive, the insurer may fund the investigation and monitoring costs in full, with the excess applied only to the final remedial works. Understanding the policy terms and the insurer's approach to cost sharing is essential for managing leaseholder expectations throughout the claims process.
Remediation: Underpinning and Stabilisation Options
Once the cause and extent of subsidence have been established through investigation and monitoring, the next step is to determine the most appropriate remedial solution. The choice of remediation method depends on the cause of the subsidence, the type and depth of the existing foundations, the soil conditions, and the severity of the movement. For a block of flats, the scale and cost of remediation can be substantial, and the process of underpinning block of flats foundations requires specialist engineering expertise and careful project management.
Addressing the Cause
Before any structural remediation is undertaken, the underlying cause of the subsidence must be addressed. If tree roots are responsible, this may involve removing or heavily pruning the offending trees, subject to any tree preservation orders that may be in place. If drainage failure is the cause, the damaged drains must be repaired or replaced. If clay soil shrinkage is the primary factor and no specific trigger such as a tree or drain is identified, the remediation strategy will focus on stabilising the foundations to resist future movement. Addressing the cause is a prerequisite for any successful long-term solution, as underpinning alone will not prevent further movement if the underlying cause remains active.
Traditional Mass Concrete Underpinning
Traditional underpinning involves excavating beneath the existing foundations in a carefully planned sequence and filling the excavations with mass concrete to extend the foundations to a greater depth where the soil is more stable. This method has been used for decades and is well understood, but it is labour-intensive, disruptive, and can be particularly challenging in a block of flats where access is restricted and the works must be carried out while residents remain in occupation. Traditional underpinning is most commonly used where the foundations need to be taken to a significantly greater depth to reach stable ground and where other methods are not suitable.
Mini-Piling and Micro-Piling
Mini-piling and micro-piling involve installing small-diameter piles through or adjacent to the existing foundations, which are then connected to the foundations with a reinforced concrete beam or bracket. This method is less disruptive than traditional underpinning, can be carried out with smaller equipment, and is often suitable for buildings with restricted access. For blocks of flats, mini-piling offers the advantage of reduced noise, vibration, and disruption to residents compared with traditional underpinning, making it an increasingly popular choice for residential buildings.
Resin Injection Stabilisation
Resin injection is a modern ground stabilisation technique that involves injecting expanding resin into the ground beneath the foundations. The resin expands to fill voids, compact the soil, and raise the foundation back towards its original level. This method is fast, minimally invasive, and can often be completed in a single day, making it an attractive option for blocks of flats where minimising disruption to residents is a priority. However, resin injection is not suitable for all ground conditions or all types of subsidence, and a thorough site investigation is required to determine whether it is an appropriate solution for the specific building and soil conditions involved.
Managing Agent Responsibilities and Ongoing Monitoring
The question of subsidence responsibility leasehold properties is one that managing agents must address proactively. The managing agent acts as the first line of defence against subsidence, responsible for monitoring the building, coordinating professional investigations, managing insurance claims, and overseeing remedial works. A competent managing agent will have systems in place to detect and respond to subsidence early, minimising the risk of serious structural damage and protecting the interests of all leaseholders.
- Regular visual inspections of the building exterior and communal areas for signs of cracking or movement
- Maintaining a photographic record of any existing cracks and monitoring their progression over time
- Ensuring the buildings insurance policy includes adequate subsidence cover with appropriate excess levels
- Promptly notifying the insurer when subsidence is suspected and managing the claims process on behalf of leaseholders
- Instructing qualified structural engineers and surveyors to investigate and report on suspected subsidence
- Implementing tree management programmes to control vegetation within the influence zone of the building
- Scheduling regular CCTV drain surveys as part of the planned maintenance programme to detect drainage defects
- Keeping leaseholders informed at every stage of the investigation, claim, and remediation process
- Maintaining comprehensive records of all subsidence-related investigations, reports, and works for future reference
- Coordinating with neighbouring landowners where trees or drainage on adjacent land may be contributing to the subsidence
Ongoing monitoring is particularly important for buildings that have a history of subsidence or that are located in areas with known risk factors such as shrinkable clay soils or historic mining activity. A structured planned maintenance programme that includes foundation and structural monitoring will help to identify any recurrence of movement early and ensure that appropriate action is taken before damage becomes severe.
Frequently Asked Questions About Subsidence in Blocks of Flats
Who is responsible for subsidence in a block of flats?
Responsibility for subsidence in a block of flats depends on the terms of the lease and the nature of the building's ownership structure. In most cases, the freeholder or management company is responsible for the structural integrity of the building, including the foundations, external walls, and roof. This means that the cost of investigating and remedying subsidence will usually fall on the service charge, shared between all leaseholders in the building. However, if the subsidence is caused by something attributable to an individual leaseholder, such as unauthorised alterations that have affected the foundations, that leaseholder may bear direct responsibility. The managing agent plays a critical role in coordinating the response, instructing surveyors, liaising with insurers, and ensuring that the correct parties are held accountable. Leaseholders should check their lease carefully and seek professional advice if there is any uncertainty about who is liable for the costs of subsidence investigation and repair.
Does buildings insurance cover subsidence in a block of flats?
Most buildings insurance policies for blocks of flats do include cover for subsidence, heave, and landslip, although the specific terms and excess amounts vary between insurers. Subsidence claims typically carry a higher excess than standard claims, often in the region of one thousand pounds or more per claim. It is important to understand that insurers will usually require a thorough investigation before accepting a claim, including monitoring of crack movement over a period of months and sometimes a full site investigation involving trial pits and soil analysis. The insurer may also require that the cause of the subsidence is addressed before they will fund underpinning or other stabilisation works. Managing agents should ensure that the building's insurance policy is reviewed annually to confirm that subsidence cover is in place and that the sums insured and excess levels are appropriate for the risk profile of the building. Leaseholders who are concerned about subsidence cover should request sight of the insurance policy summary.
How long does it take to resolve a subsidence claim?
Subsidence claims are among the most complex and time-consuming insurance claims to resolve. The process typically takes between twelve and twenty-four months from the initial report to the completion of remedial works, although more severe cases can take considerably longer. The first phase involves investigation and monitoring, which alone can take six to twelve months as crack monitors are installed and readings are taken over a full seasonal cycle to understand the pattern and cause of movement. Once the cause is established, the insurer will consider the most appropriate remedial option, which may range from tree management and drainage repair to full underpinning. The remedial works themselves can take several months depending on their scope and complexity. Throughout this process, the managing agent must keep leaseholders informed, manage the relationship with the insurer and loss adjuster, and ensure that any temporary safety measures are in place. Patience and thorough documentation are essential for a successful outcome.
What are the signs of subsidence in a block of flats?
The most common signs of subsidence in a block of flats include diagonal cracking in the external or internal walls, particularly around windows and door openings. These cracks tend to be wider at the top than the bottom and are often tapered in shape. Other warning signs include doors and windows that stick or no longer open and close properly, visible gaps between walls and the ceiling or floor, rippling or distortion of wallpaper not caused by damp, and in more advanced cases, a noticeable lean or tilt to part of the building. It is important to distinguish between cracks caused by subsidence and those caused by normal settlement, thermal expansion, or superficial plaster shrinkage. Subsidence cracks are typically wider than three millimetres, appear suddenly or worsen over a short period, and are often diagonal rather than vertical or horizontal. If any of these signs are observed, the managing agent should be notified immediately so that a structural engineer or surveyor can assess the situation and recommend further investigation.
Can subsidence affect the value of a leasehold flat?
Subsidence can have a significant impact on the value of a leasehold flat, both during the period of investigation and remediation and potentially in the longer term. While a subsidence claim is ongoing, prospective buyers and their mortgage lenders may be reluctant to proceed with a purchase, as the extent of the damage and the cost of repair remain uncertain. Even after successful remediation, a history of subsidence must be disclosed when selling the property, and some buyers and lenders may view a previously affected building as a higher risk. However, it is important to note that a properly remedied subsidence issue, supported by a certificate of structural adequacy and a full claims history from the insurer, should not significantly reduce the long-term value of a well-maintained building. The key factors are the quality of the remedial work, the ongoing monitoring arrangements, and the transparency of the information provided to prospective buyers. A proactive managing agent who maintains comprehensive records and manages the building effectively can help to reassure buyers and lenders alike.
What is the difference between subsidence and settlement?
Subsidence and settlement are often confused but are fundamentally different issues with different implications for a block of flats. Settlement is the natural downward movement of a building that occurs shortly after construction as the weight of the structure compresses the underlying soil. Settlement is expected, usually uniform across the building, and typically ceases within a few years of construction. It rarely causes significant structural damage, and any cracks that result are usually hairline and cosmetic. Subsidence, by contrast, is the downward movement of the ground beneath the foundations caused by factors unrelated to the weight of the building, such as clay soil shrinkage, tree root moisture extraction, or the collapse of underground features. Subsidence is not part of the normal life of a building and can cause progressive, significant structural damage if left unaddressed. The distinction matters because buildings insurance typically covers subsidence but not normal settlement. A structural engineer can determine whether movement is caused by subsidence or settlement through investigation, monitoring, and soil analysis, and this determination is essential for establishing the correct course of action.
Concerned About Subsidence in Your Block?
Whether you have noticed signs of structural movement in your building, need help managing a subsidence insurance claim, or want professional advice on underpinning and stabilisation options, Block is here to help. Our experienced team manages subsidence investigations and remediation projects across buildings of all sizes, working with structural engineers, insurers, and loss adjusters to protect your property and keep leaseholders informed throughout the process. See our insurance claims guide and communal repairs guide for further information on related topics.