ARTICLE

How to deal with subsidence

When dry weather, clay sub-soils and thirsty tree roots come together the results can be profound. Shifting, weakened foundations are the stuff of homeowner nightmares: learning that the ground beneath your feet isn’t supporting your house is inherently disconcerting.

The good news is that not all cracks mean your house is disappearing into the ground. In this article we look at the causes of subsidence, the symptoms, the treatment options and the treatment options available

How to deal with subsidence

How to deal with subsidence

Dealing with subsidence

  • According to Which? 70% of all subsidence cases are due to thirsty tree roots
  • Research has shown less than 10% of subsidence-affected properties require underpinning
  • When buying a property always commission a full structural survey to reveal any subsidence
  • Houses built before 1965 tend to have shallow foundations that are prone to subsidence

What is subsidence?
Subsidence is the shrinking and lowering of the ground under your house, which in turn leads to your home’s foundations moving. It can be caused by any of the following:

  • nearby trees with thirsty, growing roots which cause the ground to contract by removing moisture
  • clay sub-soils. In dry spells the water is sucked out of the soil causing it to shrink and foundations to destabilise. Clay soils are particularly vulnerable because they normally hold so much water in them.
  • previous mining activity, collapsing drains or culverts affecting the integrity of the foundations
  • water leaking into the soil and washing away the ground around your foundations (a common occurrence with sandy, gravelly and chalky soils)

    How do I tell if my house is subsiding?
    Tell-tale signs to look out for include:
  • new cracks appearing or existing cracks expanding wider than 3mm in plasterwork and brickwork, particularly after a spate of dry weather. Also watch out for cracks wider than the thickness of a 10p coin that are narrower at the bottom - it could be caused by ground movement.
  • doors and windows jamming, with no obvious cause. If your walls are moving, the frames may go out of square, causing them to stick.
  • wallpaper ripples not caused by damp. As interior walls move and warp, the wallpaper may start to lift from the wall, and not just rip with a crack

    It’s important to remember that not all cracks point to subsidence. New buildings in particular are prone to cracks emerging as the ground beneath settles and compacts and it’s not uncommon for builders’ contracts to require a return visit to make good. The drying out of building materials and the stresses in a building that accrue during construction also can contribute to minor cracks. In older buildings ‘summer cracks’ can also appear during dry weather, which subsequently close up in the wetter winter. Again, these are all usually harmless.

    What do I do if I suspect I have subsidence?
    The sooner subsidence is diagnosed the better. Contact your home insurance provider as soon as you suspect subsidence is present. They will arrange for an expert to inspect the damage and advise if it has been caused by subsidence and what action should be taken. You can also seek additional advice from a chartered surveyor registered with RICS (Royal Institute of Chartered Surveyors). A surveyor will diagnose if subsidence is present, a process which can take some time, particularly if cracks need to be monitored, or specialist geological surveys and drain inspections are involved.

    How is subsidence treated?
    If the subsidence is being caused by tree roots, the tree may be removed or cut back to reduce the moisture it removes from the soil. Similarly, if defective pipework is at fault, it’s usually a simple matter of repairs. If, however, your property has serious subsidence, underpinning – the strengthening and deepening of the building’s foundations - may be required. This is a costly and disruptive procedure requiring holes to be dug at intervals around your property so that your home’s foundations can be reinforced. It’s not rocket science but it is labour intensive. An alternative to under-pinning, using piles to shore up the foundations, is becoming the preferred method of dealing with subsidence.

    Underpinning: how much does it cost?
    Underpinning doesn’t come cheap. As a general rule it costs about £1,000 per metre, so a price of anywhere between £5,000 and £80,000 is possible.

    Are there any alternatives to underpinning?
    Jet grouting is sometimes used to fill voids or to inject chemicals that turn the soil into a solid aggregate. There are also various supportive systems involving piers, piles and jacks; the extent and location of subsidence will determine which is suitable for your property.

    Are there particular areas or types of houses most susceptible to subsidence?
    Subsidence mainly occurs south of a line between the Wash and the Severn with London being particularly prone due to houses being built on London clay. Houses with shallow constructions, notably Edwardian and Victorian homes, and homes with clay sub-soils are also vulnerable.

    The trouble with trees
    Trees are a cause of many subsidence issues. So when thinking of buying a property, planting a tree, or trying to find the cause of subsidence, keep an eye out for the position and species of tree. If you thinking of planting a tree near your house, you need to be aware of how big it will grow when it matures.