How to spot the signs of subsidence
— 11 November 2021 by Claudia Miedzianowska Posted in: Surveying
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Update your browser!— 11 November 2021 by Claudia Miedzianowska Posted in: Surveying
Are those cracks settlement, subsidence or something completely different? Noticing cracks in your property can be a worry. It’s important to identify the cause and, in certain cases, carry out remedial work to prevent your property from becoming seriously defective.
Settlement is generally defined as “a downward movement as a result of the soil being compressed by the weight of a building”. Whereas subsidence is a ground issue, being “the downward movement of the ground beneath the building other than by settlement”.
As surveyors, we tend to notice signs of subsidence following a prolonged period of dry weather. However, this may not be the only cause. Ground conditions and the type of soil in the local area are also important. If your property sits on highly shrinkable clays, this combined with the close proximity of certain trees and a long hot summer, could create the perfect environment for subsidence.
Subsidence can occur if ground soils contract when moisture is lost. Cohesive soils, like clay are particularly susceptible. Subsidence is most likely to be discovered after dry weather and high temperatures. Mature trees, shrubs and other vegetation can accelerate the movement as they also absorb water from the ground.
But it’s not only highly shrinkable clay soils that lead to subsidence. Wash-out of fine particles of granular or sandy soils due to leaking rainwater goods or underground drainage can also trigger it. Other causes include nearby mining activities, a change in ground water levels or decaying peat conditions.
The mechanics of subsidence mean that part of the foundations of the property move downwards, as they lose their support from the soil, which affects the structure above ground. This superstructure then starts to move and cracks appear, normally diagonal in nature and around windows and door openings. The pattern of cracking is key to identifying and diagnosing subsidence. Cracks are often widest towards the top of the property, becoming narrower towards ground level, occurring in areas of weakness. We’ve seen entire rear extensions come away from the main building before!
When considering subsidence, it’s vital for surveyors to understand the preceding weather conditions and to consider the level of moisture in the soil. The British Geological Society Soil Map gives us an indication of soil conditions across the UK, whilst local bore hole data allows us to take a closer look at the soil make-up. Both sources are reviewed as part of a subsidence inspection, along with looking into defective drainage systems.
If a property is found to be suffering from subsidence, the fix usually involves eliminating the cause, for example by removing trees or large shrubs, or by repairing pipework. If the damage is severe, the property may require unpinning to repair the foundations. This will usually prevent any further movement but it’s a costly and disruptive procedure.
If subsidence is caused by a neighbouring tree or drainage issues, it’s crucial that you speak with your insurance company and seek professional guidance.
Ensure that your buildings insurance policy covers issues relating to ground movement, including subsidence. A Chartered Surveyor can help you deal with any insurance claims, managing them on your behalf, and can arrange for any works required to fix the problem.
If you suspect subsidence at your property, get in touch.