Subsidence is the technical term used to describe the movement of foundations due to the removal of support from the adjoining ground. The resulting movement can cause either full or partial structural failure of the wall above (often evidenced by tapered cracks above ground level). There are various causes of subsidence, including landslip, mining, tunnelling and leaking drains. However the most common cause in London and the South East, is desiccation (drying out) of clay sub-soils. Clay will shrink in volume if its moisture content is reduced. This typically occurs when a relatively dry winter is followed by hot, dry summer weather, causing a lowering of the water table in the ground. Desiccation of clay sub-soils is also caused by the roots of trees and hedges sucking up large volumes of water from the clay below. Most clay related subsidence occurs when hot, dry summer weather is combined with the growth of nearby trees/hedges. The problem is exacerbated by the inadequate, shallow foundations used in older buildings. In most cases, clay related subsidence can be remedied by the full or partial removal of nearby trees/hedges. It may also be necessary to repair/replace any nearby defective drains. Only in exceptional cases, will it be necessary to replace the foundations, by full or partial underpinning.