Subsidence usually happens when properties are built on clay soils, and either the water table drops due to a long, dry spell or water is sucked out of the soil by trees and bushes. As the clay contracts it pulls the foundations, triggering deflection which may cause structural damage to buildings. Different types of clay shrink and swell at different rates. Water leaks into the soil from, for example, a broken drainpipe and washes soil away from the foundations. This type of subsidence happens to soils with a high sand or gravel content.How can I tell if my house is subsiding?
There could be movement in the ground beneath your home if you find new or expanding cracks in plasterwork, new or expanding cracks in outside brickwork, doors or windows sticking for no particular reason and rippling wallpaper not caused by damp.
If you spot any of these problems and there is no other reason for them, get specialist help as soon as possible. If it is subsidence, the sooner it is diagnosed the better. Check that your buildings insurance covers subsidence. Most insurers will aim to be as helpful as possible in dealing with any claim and they will recommend that you get specialist advice. A chartered surveyor will be able to work out whether or not there is subsidence and what the likely cause is. If diagnosis is not straightforward, they may recommend you bring in a structural engineer to give a second opinion. You may also need specialist geological and drain surveys as moving soil can sometimes crack drains or water mains.How long will it take to rectify?
Establishing whether or not there is a problem often takes a long time. There is rarely any cause for real concern unless cracks appear suddenly and are more than 3mm wide. In most cases the first signs of a problem are visible cracks in a particular area of the house, and these will need to be measured and monitored, perhaps for as long as 12 months. Solving subsidence can be a lengthy process which can take up to two years.How to fix it
Underpinning (strengthening) the foundations usually prevents further movement, but it is a lengthy, costly and disruptive procedure. It is estimated that only 20-30% of properties suffering from subsidence need underpinning and the Institution of Structural Engineers recommend that it is only used as a last resort.
About 70% of all subsidence cases are due to tree roots sucking moisture out of the soil and where this is the case, trees may be removed. It may be a quick and easy way to solve the problem but can spoil the view from the house. More importantly, instead of solving the problem, removing a tree may add to it. Your chartered surveyor will be able to put you in touch with someone who can advise you on whether or not a tree should be removed or simply pruned to reduce the amount of moisture it takes out of the soil.
Where the soil beneath the property is being washed away because of leaking drains or water mains, a less intrusive remedy might be possible. In most cases, repairs to leaky pipework will be enough to stabilise the property without underpinning.Who pays?
Check whether your insurance policy covers the cost of investigation and repair. If the loss adjuster/insurance company considers this reasonable, you will get your costs back. A policy excess will probably be specified in your insurance documents, in which case, you will need to pay up to that amount before the costs are covered by insurance. Your chartered surveyor will deal with your insurance company and help with any claim you need to make. They can also help design and arrange for any work needed to fix the problem.