House subsidence threat from concrete drives


Over 100,000 London homeowners - and many more in the South East of England - could face an increased risk of subsidence because they, or their neighbours, have succumbed to the urban trend for turning front gardens into off-road parking spaces, according to new research by esure home insurance.

The research includes a report commissioned by esure from the British Geological Survey which describes how new driveways - and other impermeable surfaces such as patios - can cause rainfall to 'run off' into drains rather than penetrating lawns or flowerbeds where it will seep down to reach thirsty tree roots.

This can then create the conditions for subsidence as roots searching for water upset the moisture balance in the soil that underpins nearby foundations.

The number of properties in Greater London at risk from this phenomenon is estimated by esure home insurance to top 100,000. This is equivalent to the 7% of homeowners in the South East who have already converted their front gardens into parking spaces (source ICM) - plus their direct neighbours - as a percentage of the estimated half million homes in the Greater London area built on London Clay with a large tree nearby (source: Addressology 2006).

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."

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

Lee Jones of the British Geological Survey said, "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."

The risk of subsidence is further compounded if homeowners have laid impermeable surfaces such as concrete driveways or patios rather than gravel or brick paving which still allow some water to filter through.

Without adequate moisture entering the soil, esure's research concludes that tree roots may extend under properties in search of moisture. In clay soil areas - particularly during dry periods - this can contribute to soil 'desiccating' under properties. This can significantly increase the chances of subsidence, particularly if a number of adjacent neighbours all do the same thing, effectively cutting off one of the main water sources for a tree.

Tips for converting a driveway or adding a patio safely:

Be very conscious of nearby trees when making any change to lawns or front gardens that will reduce or increase the amount of water entering the ground. Tree roots can extend up to 30m for some trees and much of the moisture absorption comes from the last few metres.

Avoid laying concrete in a place where soil or lawn was previously close to a large tree (ie one with a potential full grown height of over 15ft). This can make an area of ground practically impermeable to moisture. Instead consider block or brick paving with permeable gaps.

Consider leaving areas of open soil or flower beds adjacent to newly paved areas and to ensure the paving 'cambers' to allow moisture to run off into them - particularly if it slopes right down to meet pavement.

Watch out for new cracks - particularly diagonal cracks bigger than a 10p piece - in the front bay, porch way or brickwork of your home. These can be the first signs of subsidence. Notify your insurer early if concerned.