Millions of UK homes could be at growing risk of subsidence due to deteriorating clay drainage systems, according to new insights from ground engineering specialist Mainmark UK.
With the Association of British Insurers (ABI) estimating that more than 5.76 million properties are exposed to a medium to high risk of subsidence, Mainmark warns that ageing underground drainage networks could be a significant but often overlooked contributor.
The company suggests that over one million homes may face increased risk due to defective or leaking clay drains and water supply pipes.
Leaking Drains a Major Driver of Subsidence Claims
The last full year of ABI subsidence data highlights the leading causes of repair-related claims:
- Root-induced clay shrinkage (60 per cent)
- Leaking drains or mains water pipes (18 per cent)
- Poor ground, infill, or consolidation (18 per cent)
Mainmark’s Residential Lead, Freya Chapman, said leaking drains remain a “hidden but critical” driver of ground movement beneath older housing stock. “Many people associate subsidence with extreme weather or tree roots, but in reality, leaking drains are one of the most common unseen causes,” Freya explained. “A damaged clay pipe beneath a home can alter the soil’s moisture content, triggering gradual ground movement that leads to structural damage, and, ultimately, costly insurance claims.”
Legacy Infrastructure Under Pressure
Clay drainage pipes, widely used in homes built before the 1970s, are particularly prone to deterioration due to their porous and brittle nature. Cracks in these pipes allow water to seep into the surrounding soil, eroding or softening the ground that supports foundations.
This issue is especially acute in areas with largely clay soil, including London and the South East, the North East, parts of the North West, and the Trent Valley. In these regions, repeated wet–dry cycles cause the ground to expand and contract, amplifying structural movement.
It has been estimated that approximately 20 per cent of UK housing stock was built between 1945 and 1964 when clay pipes were standard. “Many of these homes still have their original pipework. Because drainage systems are typically only replaced when problems arise, latent defects often remain undetected for decades,” she said.
Early Detection Can Reduce Claim Severity
Mainmark is urging homeowners and insurers alike to prioritise preventive assessment of older drainage systems, recommending that policyholders arrange drain and ground surveys to identify vulnerabilities before they escalate into full subsidence events.
“Replacing damaged clay pipes with modern PVC or lining systems can dramatically reduce the risk of ground movement. Modern materials are more watertight and resilient, and early intervention can prevent far more costly foundation repairs later.” Commented Freya.
Climate Pressures Heightening the Risk
She added that climate change is compounding the issue, as more intense rainfall and prolonged dry spells drive wider fluctuations in soil moisture. “Managing ground moisture around foundations is becoming a key aspect of protecting structural stability.”
Homeowners are advised to watch for classic warning signs of subsidence, such as stepped wall cracks, uneven floors, and doors or windows that stick. Prompt investigation can save significant cost and disruption compared to delayed remediation.
Mainmark Ground Engineering (UK) Ltd is a subsidiary of the Mainmark group of companies, established in 2016. The company, located in Milton Keynes, has over three decades of experience and has established itself as a pioneering force in the industry, leading the way in developing and delivering cutting-edge solutions. Mainmark has earned international recognition as an award-winning ground engineering organisation.