As the weather continues to stay hot and very dry, the likelihood is that problems of subsidence will increase, writes David Lawrenson.

But, first, what exactly is subsidence?

Subsidence happens when the soil under a building gets drier and starts to shrink. It's a very common problem on clay soils and on gravel.

This shrinkage can cause a building to move.

There are a number of possible causes and the most common is the effect caused by large trees taking too much water out of the ground.

Other possible causes are leaking drains under a property, tunneling, local coal mining or building work in neighbouring areas or a change in the water table.

All houses move a little - and small movement of a millimeter or two are not usually a problem, so often there is no cause to worry.

The signs that you could have a subsidence problem are most obviously cracks in walls.

Cracks that are caused by subsidence are typically wider at the top than at the bottom and are diagonal.

However, not all cracks mean subsidence. Sometimes cracks will be seasonal and normal - tending to open and shut as the ground expands and contracts again.

In many older style houses (and some new extensions) cracks may be a feature that has been there for years. So, if you are looking to buy one of these it's worth asking the neighbours, especially if the house you are buying is in a terrace.

Chances are that the neighbours have lived with similar cracks for years.

In a new house, minor cracks may just be due to "settling." In fact, if cracks are linear and less than 3mm wide, there may not be any problem at all.

In many houses the crack will be seen at the point where an extension exists - as the extension moves away from the main part of the house a crack will appear. The same effect can lead to cracks around bay windows too.

Beside the crack then, other signs of subsidence include buckling wallpaper - especially where it happens close to the corners of a room. There may also be cracks in plaster and bricks.

If you have doors and windows that stick or wont close at all, this is also a sign that movement is happening.

What to do

If you are worried you have subsidence, call your buildings' insurer. They will normally send a surveyor around to assess the situation. He in turn may ask for a second opinion from a structural engineer.

Their investigations will be quite detailed. When I was worried about subsidence on one of my properties, the surveyor spent about three hours doing tests of the soil and drainage, so let your tenants know this in advance of how long they will be on site.

In the end, in my case, he concluded that subsidence was unlikely - but requested that I continue to monitor the small cracks that had appeared.

Where subsidence is found, the insurer will usually undertake to get the work done (though you should be given the option of using your own people).

Usually you are best off using the insurer's people to get the work done, unless, say, you happen to be a builder yourself.

In the more serious cases, you or your tenants will need to move out while the work gets done - and again this should be covered under the buildings insurance policy under loss of rent and alternative accommodation.

The main difference on landlords' policies compared to standard residential ones is that your loss of rent should always be covered too, in addition to the costs of arranging tenants' alternative accommodation.

Fixing Subsidence

How subsidence is fixed will depend on how bad it is.

If the subsidence is not too bad, and providing it is caught quickly enough, patching up of cracks may be all that is required, though it's likely that markers will also be used to monitor if any further movement occurs.

If the problem has been caused by leaking pipes underground you'll need to get those fixed. Also, the surveyor may recommend selective pruning of large trees nearby.

Again, if you have large trees nearby, and you think cracks may be due to these, it's still best to consult a surveyor as uncontrolled pruning could cause rapid movement the other way (called heave), as well as potentially reducing the value of the property by up to 20%.

In about 25% of subsidence cases, underpinning will be needed. This will involve builders digging under the property and pouring in concrete. For more serious cases steel piles may need to be inserted.

Neighbouring properties in terraces will need to be supported too.

Most insurance policies on domestic and let properties have a £1,000 excess and claims on subsidence can run to above £25,000.

If you have changed insurers in the last 12 months, an agreement between insurance companies will mean that they will share the cost of any claim made.

Useful contacts

Royal Institution of Chartered Surveyors (RICS) www.rics.org Tel 0870 333 1600

Association of British Insurers www.abi.org.uk Tel 020 7600 3333

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