FAQ’s: What is subsidence?

As a property owner, the word ‘subsidence’ can give you a sinking feeling (pardon the pun). This month, at Blount and Maslin, we identify the top four causes of subsidence in the UK.

What is Subsidence?
Subsidence occurs when the ground under a property is unstable and sinks, or collapses, causing a building to sink on one side. This can put pressure on the structure causing cracks to appear.  About 37,000 people were badly affected by subsidence last year, which cost the insurance industry almost £200 million.
Causes of Subsidence
The biggest cause of subsidence in the UK is soil shrinkage. In the UK, around 75% of insurance claims for subsidence are the result of shrinking soils.

Soils such as clay and silt have a capacity to swell and shrink. Foundations built on clay or silt can witness changes from day to day, month to month and season to season. Clay soil can shrink, crack and shift during hot, dry weather, making the ground unstable and potentially causing foundations to sink.

For more information on UK soil types, including detailed maps, visit the UKSO website.
Trees and shrubs can also encourage soils to shrink, particularly if they’re close to your foundations. Plants need to water in order to survive and send out roots in search of moisture. In clay areas, plants may take away so much water from foundation support that subsidence can occur. Species of trees such as Oak, Willows and Lleyandii absorb a lot of water, making the soil very dry and unstable. Interestingly, subsidence risk has nothing to do with the height that a tree can reach, only to do with its thirst. A mature deciduous tree drinks an average of 50,000 litres a year.
Around 15-20% of subsidence claims are caused by leaking pipes and drains. drains and water mains can wash away or soften soil, causing it to compact under the weight of your home. Water can also makes the ground softer meaning that supporting soils lose some structural support, this can also lead to sinking foundations.
Age of homes:
Old homes may have shallower foundations – although if they’re built of bricks and soft lime mortar, they might also be more flexible, and less likely to be badly damaged by any movement.
Coming up in our next blog post: How to identify and prevent subsidence!

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