Flooding is a consistent issue for many of the UK’s property owners. With the effects of global warming clearly being felt, we’re seeing a higher average rainfall year on year. This can be a big issue in certain areas of the country, one of them being London.
Flood risk varies significantly by property type, location, and the materials your property is constructed from. It also isn’t limited to riverside buildings: Those nearer the Thames might experience a higher risk, but groundwater and surface water flooding can be a significant in London’s more urban environments.
In this blog, we’ll discuss which London properties are prone to flooding, and help you understand why certain properties are more vulnerable to flooding than others.
Why Are Some London Properties More Vulnerable to Flooding?
There are several reasons why your London property might be more vulnerable to flooding.
Soil type and drainage limitations
London’s soil is predominantly composed of clay. This material is less porous than a typical dirt or sand-based soil, so it takes longer for water on the surface to drain away.
Groundwater behaviour
Clay has a lower saturation point than dirt or sand-based soil. Once it’s taken on enough water, it’s difficult to displace this. This water can remain in the soil for long periods, exhibiting increased hydrostatic pressure on underground structures like basements or cellars.
Urban density and impermeable surfaces
Through London’s periodic urbanisation, much of London’s green land has been transformed into less porous paving, tarmac, or concrete surfaces. This forces wastewater to remain on the surface for longer.
Age and construction methods of buildings
London has a rich tapestry of properties built over hundreds of years. Typically, much of London’s older building stock can’t or doesn’t take advantage of more modern materials or methods, mainly due to conservation of the original materials used in their construction. Our living habits have also changed over the years, and some methods for preventing damp now don’t function as well or actively work against keeping a building moisture-free.
Local drainage capacity
As London continues to urbanise and population density increases, with it comes an increased pressure on the drainage systems. Sewers and drains can become overwhelmed under the increased load, often leading to clogging or further damage.
What Are the Risks to Basement and Lower-Ground Properties?
Basement and lower-ground properties are some of the highest risk properties in London’s building stock where flooding is concerned. This is due to several factors:
- Exposure to groundwater flooding: Properties based partially or completely beneath the exterior floor are at an increased risk of groundwater flooding. This is where water cannot drain away effectively, and so the moisture in the soil looks for an easier path to travel through, which can often be through a defect in a wall or floor.
- Hydrostatic pressure on walls and floors: A prolonged increase in water levels within the ground increases the amount of hydrostatic pressure on walls and floors within a below-ground structure. This can cause defects that allow water to penetrate the room.
- Risks for commercial basements: Commercial properties, such as pubs or shops, are at a greater risk if they use these rooms for stock storage. Any leaks or moisture ingress could lead to damage or spoiling of stock, causing thousands of pounds of damage.
- Long-term moisture exposure vs one-off flood events: Moisture building over a longer period will gradually chip away at defences such as cellar tanking, eventually rendering them useless against water ingress. Incumbent defences may have a better chance of offering protection against one-off flood events, where the pressures are only felt for a short period.
What are the Risks to Riverside and Low-Lying Properties?
Riverside and low-lying properties also have their own associated risks. These include:
- Proximity to the Thames and tributaries: The most likely contributor of risk to these properties is how close they are to larger bodies of water. The Thames and its branching rivers provide a greater risk of flooding the closer your property is to them.
- Residual risk even with flood defences: Flood defences such as the Thames Barrier can alleviate some of the pressures of tidal flooding. However, there is also potential for these defences to become damaged during severe flooding, and so cannot be relied upon as the only form of protecting your property.
- Impact of high river levels combined with heavy rainfall: Low-lying properties or those near river systems have both the risk of the rivers themselves, as well as with groundwater or surface water flooding.
- Basement and foundation risks: Foundations, if not built correctly, can be at risk of collapse and further structural damage if they are not built to the correct specifications to prevent flooding or water ingress.
What are the Risks to Older Properties with Shallow Foundations?
Victorian and early 20th Century constructions were constructed to a different standard than modern buildings, and with different risks in mind at the time.
- Limited damp protection: Ventilation was the primary form of moisture removal in historic properties, especially as open fireplaces were the most common form of heating. Measures such as a Damp-Proof Course were not a regularity in many buildings of this age, and bridging plaster and render can also cause concern with greater levels of surface water on today’s London streets.
- Foundation movement in saturated soils: Shallow foundations rely on the surrounding soil to provide additional stability. With London’s more malleable clay soil, this can cause foundations to shift in wetter weather, leading to structural movement.
- Structural cracking and timber decay risks: Although timber fixtures are still readily used in modern construction, these are often treated or capped to prevent the transmission of moisture from adjoining masonry. Persistent moisture exposure can cause historic timbers to decay and cause structural cracking within the building.
What are the Risks in Densely Developed Urban Areas?
The greatest risk in these areas is the increase in surface water flooding, as well as increased demand on drainage and sewage systems.
- Hard landscaping prevents water from escaping through the ground, as it would with softer soil and sand. This moisture remains on the surface, or runs off into below-ground structures like basements or cellars.
- Limited drainage and Overloaded sewer systems reduces the amount of wastewater that can be drained away. Overloaded sewer systems can also be prone to blockage or damage, reducing their functionality during periods of heavy rainfall
- Flash flooding during heavy rainfall creates a short-term risk to properties, but they are no less damaging than long-term flooding if measures aren’t taken to prevent this.
- There is an increased risk to ground floors and structural elements such as foundations or sub-floor voids. If moisture is allowed to ingress into structural elements, this can cause greater damage in other areas of the property if not correctly remedied.
What are the Risks to Converted and Redeveloped Buildings?
Converted and redeveloped buildings have perhaps the greatest crossover of risk, as they combine concerns with both historic and modern constructions.
- Added basements and extensions carry the same risks as historic buildings, and proper measures should be taken to prevent moisture ingress in below-ground structures.
- Changes to drainage patterns such as removing soil surfaces or increased occupancy can prevent surface moisture from draining away effectively.
- Increased hydrostatic pressure on below-ground surfaces means that flooding is always a risk unless a structure is sufficiently waterproofed during construction.
- Redevelopment has structural implications on existing buildings, especially where modern materials are used alongside historic ones. Bearing loads, foundation strength and subsidence concerns are all valid considerations when choosing to redevelop an existing property, as even small changes in a structure can create significant risks.
What are the Common Structural Issues Linked to Flood-Prone Properties?
Persistent flooding or moisture ingress can create several structural issues if left untreated.
Timber decay and rot
Untreated timbers are subject to moisture ingress. Continuous moisture ingress can lead to the development of Wet Rot or Dry Rot. These distinct types of fungal decay attack moisture-rich timbers, weakening their structural integrity.
Concrete cracking and spalling
Persistent exposure to moisture can cause concrete to crack or spall, especially during cyclical weather changes.
Wall tie corrosion
Steel wall ties can corrode if not weather treated, reducing their effectiveness and leading to structural movement in previously treated walls.
Masonry movement
Foundational shifts or subsidence from malleable soil can cause masonry to crack further up a wall.
How do I Assess Flood Risk in London Properties?
You should have your property checked periodically by a professional surveyor, especially if your London property is in a high-risk area for flooding. A flood risk assessment will highlight any potential defects that may cause ingress into a property, and will highlight measures you can take to reduce the risk of flooding affecting your property.
If your property has suffered from a recent flood event, a structural survey should be conducted to highlight any recent damage caused by the event. Swift remedial action will prevent issues from getting worse, and are often less expensive to treat the sooner you act.
You can also conduct property checks yourself during periods of regular maintenance or when between surveys. Looking out for early warning signs such as internal damp patches, or cracking plaster could help to identify potential concerns before they become major renovations.
You may require each method described above over the period of owning your property. Regular and thorough investigations will prevent the likelihood of expensive remedial work and protect your asset over the long term.
Conclusion
Flood risks in London properties will vary depending on the type of property you own, its age, and its proximity to flood plains or river systems. If you’re in a high-risk area for flooding of any kind, early and regular assessments of your property will reduce the likelihood and costs of long-term repairs.
In any situation, it is important to seek professional advice prior to beginning any work on your property. Request a survey to have one of our expert surveyors assess your property’s condition, and provide recommendations for remedial structural or basement waterproofing treatments.
FAQs
What types of London properties flood most often?
The London properties most prone to flooding are basements, riverside buildings, low-lying developments, and densely built urban properties with limited drainage. Older buildings with shallow foundations and converted properties with added basements are also at higher risk. Flood risk in London buildings depends on soil type, proximity to rivers, drainage capacity, and construction methods, rather than location alone.
Are basements in London prone to flooding?
Yes, basements in London are highly prone to flooding due to groundwater pressure and clay-heavy soil. When the ground becomes saturated, hydrostatic pressure builds against basement walls and floors, forcing water through defects. Basement flooding in London can result from groundwater, surface water runoff, or overloaded drainage systems. Without effective waterproofing and maintenance, long-term moisture exposure can lead to structural deterioration.
Do older London buildings have a higher flood risk?
Older London buildings can have a higher flood risk because many were built with shallow foundations and limited damp-proofing measures. Increased surface water and saturated clay soil can cause foundation movement, rising damp, and structural cracking. Historic timber elements may also be vulnerable to decay after prolonged moisture exposure. Regular structural inspections help identify early signs of flood-related damage.
Can surface water flooding cause structural damage?
Yes, surface water flooding can cause structural damage if moisture penetrates foundations, masonry, or sub-floor voids. In urban London, impermeable surfaces increase runoff during heavy rainfall, allowing water to collect around buildings. Repeated exposure can lead to timber decay, concrete cracking, wall tie corrosion, and foundation movement. Even short-term flooding can weaken structural materials if not addressed promptly.
Should commercial properties be inspected after flooding?
Yes, commercial properties should be inspected after flooding to assess hidden structural damage. Floodwater can weaken timbers, increase pressure on foundations, and accelerate corrosion of structural components such as wall ties. Moisture trapped within walls or floors may not be immediately visible but can cause progressive deterioration. A professional structural survey helps identify defects early and reduce long-term repair costs.
