What is subsidence?
Subsidence is the downward motion of the ground causing buildings above to sink into the ground.
What causes subsidence?
Houses built on clay soil are particularly susceptible to subsidence. This is because the soil expands during wet weather but during a dry spell can dry out and start to crack, causing the ground beneath the house to sink. Older houses built with shallow foundations are most vulnerable.
Other possible causes include mining activity and filled in pits and quarries, leaking water mains that can soften the ground and wash away soil, and trees that are growing close to your home's foundations that can draw moisture from the ground, causing the ground to shift during dry weather.
How to spot the signs of subsidence
The key signs of subsidence are:
- Diagonal cracks that are wider at the top than at the bottom
- Cracks thicker than a 10p coin
- Cracks close to doors and windows
The first signs of subsidence
The first obvious signs of subsidence are cracks appearing; usually diagonal, wider at the top and often near windows and doors. All buildings expand and contract to a degree and not all cracks indicate subsidence. Newly built properties and extensions often crack as they ‘settle’ and fine cracks frequently appear on a recently plastered wall as it dries out.
If you think that your home is at risk of subsidence, keep an eye on old and new cracks – especially those that suddenly appear, are wider than a 10 pence coin and go through brickwork or stone. If you’re concerned about subsidence on your property it's advisable to contact a surveyor for an assessment.
Should you worry about cracks in plaster?
Innocuous cracks are common in houses, particularly above doorways and in plasterwork. "Such cracks, particularly after a hot dry summer, could be merely thermal shrinkage rather than subsidence," says Chartered Surveyor John Conlin. "Unless they start to extend or widen significantly observe them over a full cycle of seasons and you may find that they open and close in response to temperature and humidity. If this is the case they are providing structural stress relief and can be made good with a flexible filler."
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Underpinning your property
In the worst cases of subsidence, your property may have to be underpinned – often a complex, lengthy and disruptive process. Underpinning is a way of increasing the depth of your home's foundations to prevent it from sinking further. Be aware that buying or selling a house with a history of subsidence and underpinning becomes more problematic when it comes to getting insurance cover.
"Never jump to conclusions," says John Conlin. "Too many houses are unnecessarily underpinned and suffer a reduction in value as a result. No proper diagnosis can be made without detailed professional monitoring over a full cycle of seasons. Don't rely on the opinion of an underpinning contractor - that is like asking a fox to count chickens. Seek a chartered building surveyor or a structural engineer. Do not alert your insurers until you have had a professional assessment because the insurer will log it as an ‘open claim’ and underwriters may load your premium on renewal even if the claim is not pursued."
Meanwhile, regular home maintenance is important, particularly if your house is built on clay. Leaks from drains and pipes will saturate the ground causing ‘heave’ or wash away the soil so keep gutters and drains clear and fix any leaks.
Steps you can take to reduce the risk of subsidence
There are steps you can take to prevent your soil drying out during hot, dry weather.
Trees close to the house will take moisture from the soil. If your home’s at risk, you may need to prune or even remove mature trees and certainly consider where you plant new ones. If in doubt, get in touch with a tree specialist or surveyor.
Help retain moisture in the garden by planting less thirsty plants like lavender and buddleia that thrive in hot, dry conditions.
Mulch, bark chips and gravel help prevent evaporation.
Think twice about increasing patios, decking and paved areas in the garden that actually prevent water getting into the soil.
Catch rain water in butts and buckets and use household ‘grey water’ from the bath, washing up, boiling vegetables and so forth for watering the garden.
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