Why would I be talking about subsidence when all we are seeing on TV is rain and localised flooding? Well they are very closely related and similar in terms of scale — the opposite sides of a coin. The average number of properties affected by flood in a year is the same as the average number of properties affected by subsidence, and equally even in a year like 2007 with the flooding of York, Sheffield, Hull and Tewkesbury, the number of properties affected were the same as the number of properties affected by subsidence in 2003 through dry weather.
The main reason why we are all aware of flooding but not subsidence is simple — newsworthiness! The images of flood are far more powerful, attracting film crews in droves — whereas with subsidence it is not quite the same as standing in front of Mrs Miggin’s house in the sunshine showing a diagonal and tapered crack in the wall going down from the window frame into the ground!
So why opposite sides of the coin?
Flood is driven by excessive and extensive rainfall, whereas in simplistic terms subsidence is driven by the opposite (although water washing away fine soils and soluble rocks does cause subsidence).
To explain a little further I’ll provide a definition of subsidence — ‘Subsidence is the vertical downward movement of a building foundation caused by the loss of support of the ground beneath the foundations i.e. the soil has shrunk or been removed’. By far the largest cause of subsidence in the UK is soil shrinkage (predominantly clay and leaking drainage) and this accounts for over 75% of all subsidence claims.
Soils such as clay and silt vary in their ability to change their volume when wetted or dried (known as volumetric change), therefore when wet they will expand (swell) and when dry they will shrink. Clay is particularly prone to ‘shrink-swell’ and is found extensively across England and Wales. The British Geological Society estimate that as many as one in five homes in England and Wales are likely to be damaged by ground that swells when it gets wet and shrinks as it dries out, and it is has cost the economy £3bn in the last 10 years.
Whilst we continue to shelter from the rain it is also worth considering European research by Swiss RE (a leading global reinsurance company) which predicts a 50% increase in subsidence by 2040. This follows an actual 50% increase in subsidence we are seeing today compared with the period 1951-1970. So subsidence will only grow as an issue.
Before dry winters, hose pipe bans and drought concerns become the hot topic again our advice is to “think ahead”. The first step is to understand your subsidence risk because the good news is that some relatively easy and cost effective steps can reduce the likelihood of subsidence.