Subsidence is a term that strikes fear in peoples' minds. However, it is worth remembering that damage caused by subsidence rarely means that your property is immediately unstable. As a general guide for more modern properties, the crack width/movement would need to be at least 15mm to warrant a severe damage classification.
Subsidence is mostly due to shrinkage of clay soils beneath the foundations. This is usually caused by roots from nearby vegetation which dry the soil out. Other causes include leaking drains causing the soil to soften as well as soil that has been excavated for building works, old mine workings and etc.
The first sign of possible subsidence is a series of cracks appearing in the building. The other tell tale sign may be that previous cracks reappear or if doors and window begin to show signs movement by sticking or not closing correctly.
Of all the properties that due suffer from subsidence, only about 20-30% of properties will require some form of underpinning. In many cases it may just mean that your builder needs to remove trees to repair leakages from the pipes underground.
It may be necessary to dig trial pits to determine the characteristics of the subsoil and foundations. Deeper soil samples may be needed and bore holes will be made with soil and root samples being sent off for tests. Cracks will need to be measured and monitored for changes.
Your buildings insurance policy will have details of your property's cover for subsidence. This is one of the main reasons why when you buy a property your lender will require you take buildings insurance which will include cover to the property in the event of it suffering subsidence.
What to do if you think you have subsidence
If you are concerned about subsidence you should immediately seek advice from a Chartered Engineer. They will initiate any investigations and monitor the situation. The engineer will be able to suggest any short term measure you should take and co ordinate any repairs required.
You should inform your buildings insurance provider incase they need to appoint a loss adjuster who will assist with any insurance claims. Depending on the type of subsidence it could take less than 18 months unless a third party is involved.
You can avoid the likelihood of subsidence occuring by ensuring trees and shrubs are regularly pruned to prevent excess growth and keeping your water disposal system in good condition. Your buildings insurance quote will give you details on the cost of covering subsidence.