What Causes Subsidence and heave?

Much of the UK’s housing is founded on what are known as shrinkable clays. Such clays are strong enough to support a building of up to four storeys on a simple foundation. But they shrink when their moisture content decreases and swell when it increases. Slight movement of houses founded on these soils is therefore inevitable as a result of seasonal changes in moisture content: downward movement or subsidence occurring during the summer and upward movement or heave during the winter. However, these movements rarely cause damage because the whole house is affected more or less equally. Damage is therefore usually associated with trees, which enhance the extraction of moisture locally, especially during prolonged periods of dry weather. Conversely, removing a large tree can cause heave as moisture gradually returns to the soil. Shrinkage and swelling of the surface soil are not the only causes of subsidence and heave, but they are by far the commonest.

How can I tell whether the cracks have been caused by foundation movement?

It can be very difficult to distinguish damage caused by foundation movement from other causes, especially where damage is relatively slight – typically, where the cracks are no wider than 2 mm. Nevertheless, there are a number of characteristics to subsidence and heave damage. Wherever possible measurements should be made to confirm that there is a significant slope to floors and brick courses before jumping to the conclusion that the damage has been caused by foundation movement.

When should I start to worry about the damage?

The incidence of damage caused by clay shrinkage increases dramatically during dry summers – so called “event years”. Recent event years include 1989, 1990, 1995, 1996, 1997, 2003, 2018 and 2019. Following an event year, there is a tendency for insurance claim figures to remain at a relatively high level suggesting that many homeowners are prompted to report damage because of media coverage or because they are aware that their neighbours have had problems. In many cases, the damage reported may be unrelated to foundation movement, but if the affected property is founded on shrinkable clay, it is likely to be moving seasonally making it difficult for investigators to distinguish between genuine subsidence and general wear & tear.