Frequently Asked Questions - Clay-related Subsidence

Desiccation cracking in shrinkable clay soils

Desiccation cracking in shrinkable clay soils

What is clay-related subsidence?

Clay-related subsidence is a significant risk for low-rise properties in many areas of the UK where ‘shrinkable’ clays are present, where soils shrink and swell with changes in moisture content - in dry summers shrinkage of these clays can cause downward movement of building foundations, which can result in cracking of the walls above.

Growth of tree roots increases the shrinkage effects by drawing water out of soils, so building damage can be particularly severe where trees are near buildings. Cracking of buildings may only be cosmetic, but in severe cases repair work is required, normally involving underpinning – extending foundations below the upper layers of soil which may moving. This kind of repair work can be very expensive. Clay shrinkage can also damage buried pipes and cause damage to hard surfaces when movement is uneven.

Is clay-related subsidence getting worse?

Dry summers in 2003, 2006 and 2018 have caused significant increases in insurance claims for clay-related subsidence. The summer of 2018 showed the largest quarter-on-quarter increase since records began in 2001 (The Insurance Times, December 2018). These effects are likely to increase if more extreme weather occurs due to future climate change.

What can I do about it before I buy?

The Terrafirma Ground Report assesses the potential for clay-related shrinkage, and how this might change in the future.

The Ground Report:

  • Uses the latest Cranfield University data on the types of soils and their water content (their ‘shrinkability’).

  • Uses satellite tree mapping to work out the likely influence of nearby recorded trees that could affect ground movements.

  • Highlights if the property is in an area of higher than average recorded insurance claims investigations for subsidence.

With this information a building surveyor will be able to look for evidence of building or drainage damage that might require repair. The details of any previous repairs should be checked to make sure they have been properly completed. If there has been a previous claim, a check on the insurance status of the property will be needed.

What can I do about it after I buy?

If there is a risk from clay-related subsidence, good subsidence insurance is essential – not all buildings insurance policies cover subsidence and some have a high excess or low level of cover. If in doubt, advice from an insurance broker will be needed. New houses should also be covered by NHBC guarantee but this should be checked.

Where there is a risk of shrink-swell movements, good maintenance of buried pipework is important – leakage from damaged pipes can increase ground movements. Locating soakaways or overflows near to buildings are also therefore not advisable.

Pruning or removal of trees may reduce the risk of seasonal ground movements affecting foundations, and may even reverse some of the effects, but advice from an arboriculturalist (tree specialist) should be sought first. A check must be made to with the Local Authority to make sure there is no legal protection on any trees - known as a Tree Preservation Order (TPO). You must apply for a licence from the Local Authority before any work on trees with a TPO - always check with your Local Authority even if you don’t think your trees are affected by TPOs as this can change and you may not be notified..

Care should be taken to ensure any new trees are planted at least their full mature height away from foundations and buried pipes to reduce the risk of seasonal ground movements. Advice from an aboriculturalist will be helpful since some types of trees (like willows or oaks) may affect ground movements more than others. Specialist advice on the depth of new foundations will be required, particularly where these are within a distance equivalent to the mature height of any trees.

Where you believe your property is affected by clay-related subsidence, advice from a building surveyor will be required. They should be able to confirm the cause of any movement and make recommendations for further investigations, repairs or tree removal.

Insurers should be advised of property damage and they may be able to arrange for suitable repairs, which may be cosmetic if the movement have stopped (filling in cracks, re-plastering etc), or may require more extensive structural repairs such as underpinning. Insurers should also be informed of any significant repair work if not arranged through them. 

Underpinning works should only be undertaken by specialist contractor and Building Control approval from your local council will be required. The underpinning should provide a warranty or guarantee for the work completed – for example members of the ASUC trade organisation offer an insurance-backed guarantee.

Further Information

Terrafirma’s professionals are always available for advice on your Ground Report and the Terrafirma website has a range of helpful guidance, including more information on other kinds of subsidence.

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