Subsidence risk behind off road parking

Friday, 23 February 2007 12:00 AM

With parking places at a premium hundreds of homeowners, especially in London, have converted their front gardens into off road parking space.

However, a leading insurer warns that paving or concreting over lawns and flower beds increases the risk of subsidence, as moisture levels become disrupted under the property's foundations.

As increasing numbers of urban dwellers have opted to transform their front garden, esure commissioned research into the extent of the potential problem facing homeowners in clay-rich areas such as London.

Conducted by the British Geological Survey, the report found 100,000 Londoners could be at risk of subsidence; based on the seven per cent of homeowners who have paved over their garden combined with their neighbours, as a percentage of homes in Greater London built on London clay with a large tree nearby.

Lee Jones of the British Geological Survey explained: "It is a simple fact that trees need moisture, so anything that deprives them of moisture over time in clay soil areas can create the conditions for subsidence. The dramatic rainfall we have seen recently will only reach trees if the ground above allows the water to penetrate the soil and reach their roots."

Although a practical solution to many car parking problems, concreting over a garden risks structural damage as it makes the ground virtually impermeable to moisture. This leaves large trees struggling to absorb sufficient water, encouraging roots to grow up to 30 metres. Often they will extend under nearby properties and, as most water is absorbed in the final few metres of root, this disrupts the water balance in the soil and can cause the ground to desiccate under foundations.

Nikki Sellers, head of home insurance at esure, said: "It is very appealing for homeowners in busy streets to create a bespoke parking space for their property. Unfortunately, for houses with a street tree or large front garden tree nearby this can significantly increase the risk of subsidence. A dedicated space may increase the value of your home but subsidence damage will do quite the reverse."

She added: "It is essential that driveway or patio conversions are sympathetic to the fact that nearby trees will always need moisture."

Esure recommends a number of ways homeowners can reduce their subsidence risk. Anyone tempted to convert their garden should be aware of any large trees and consider how their plans will disrupt its water supply.

People should avoid laying concrete near a tree, as this can make the ground completely impermeable. Instead opt for block or brick paving which will allow some moisture to reach the soil. Better still, consider leaving areas of lawn or flower beds unpaved and camber any hard surfaces to encourage water to drain into the soil. Homeowners should also keep any eye out for any new cracks, especially diagonal cracks larger than a ten pence piece.

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