Cracks are appearing... except in Aberdeen

Subsidence is the scourge of the property-owning classes.

If you live in an area prone to subsidence there is every chance the spring will see the arrival of new cracks in your walls and ceilings, putting on hold any plans you may have had for a spot of DIY, and certainly dashing any chance of a house move.

Mortgage lenders will simply not provide a mortgage for a property where there is a previous record of subsidence, nor for a house that has already been underpinned - even though common sense would suggest that an underpinned house is a better bet than many properties.

And following an incredibly dry start to the year, the conditions for subsidence are worsening.

January was the driest on record with 1.5cm of rain as opposed to the usual 8.8cm. February has also been dry and coupled with the dry summers of 1995 and 1996, home owners, especially in the south east, are faced with another round of subsidence and the costly repairs this can entail.

Subsidence is where the foundations of the house start to sink, very slowly, as the clay on which it is built dries out and shrinks.

Areas of the country which have stone or rock below the sub-soil are much better placed. Insurers never face claims from home owners in Aberdeen with its granite foundations.

So what can you do, if anything, to prevent subsidence? And once you have started to notice worrying cracks, how do you know that the problem is subsidence?

Subsidence occurs mainly where there is a clay sub-soil; where homes or estates have been built on old rubbish tips - landfill; or it can appear as heave, a form of subsidence in reverse. Heave is where an old tree is cut down, causing the water level underneath the house to rise and thereby forcing up the foundations. It creates a similar array of problems to normal subsidence - wall cracks, twisted door frames and contorted bay windows.

On the prevention front, things are bleak. If you are buying a new home you should check with the builders what lies underneath the foundations and what sort of foundations have been built. You should also check what guarantees they may offer for the property, and any specific clauses on subsidence.

Landfill need not be an insurmountable problem, but it depends on the sort of foundations that have been used.

If you are buying an older home, make sure the surveyor carries out a full check for any signs of subsidence. The small cracks you noticed may be just that - or they may be a warning sign of worse to come.

However, the only way to know for sure is by doing a full-scale investigation. This involves digging holes at the front and back of the house and drilling bore holes to take soil samples. These are then analysed to determine whether the clay is drying out. Many clays have a honeycomb-like structure which collapses when dry, allowing the clay to compact. Unfortunately there is no simple way of reversing this process.

Few buyers are going to embark on the expense of an investigation of every house on which they wish to make an offer. The most likely scenario, therefore, is that you will be confronted with the problem later on, and then you will have to turn to your buildings insurance.

Insurers are becoming better at paying out for subsidence claims and the old horror stories of home owners waiting months, even years before a claim was settled are hopefully a thing of the past. Insurance companies and loss adjusters have tried and tested ways of dealing with a subsidence claim these days. For example, if a problem emerges within eight weeks of you changing your policy, insurers are agreed that it is the previous insurer who will pay out. They also have an agreed formula for liability if you make a claim between eight weeks and one year of changing insurer.

How much an insurance company will pay out depends on the extent of any damage and the nature of the work required. The insurer may require that the situation be monitored for six months or even a year to see whether or not the problem stabilises. Once agreed, the repair work may consist of no more than immediate work on the cracks - rebuilding the brickwork, or filling in fractures with cement. Some homeowners will say this is all that they want but it won't necessarily be all that they get. The insurance company may insist that a full underpinning is essential.

Underpinning is usually a major operation. It can mean that your back garden and the front of the house have to be dug up and, in the worst cases, you may have to move out while the work is underway. Some insurance policies have a clause to provide alternative accommodation in these circumstances.

Underpinning means digging up the existing foundations and replacing them with concrete, laid to a depth which means they can support the weight of the house.

Before matters reach this stage, however, you need to know whether the hairline cracks that have appeared in your living room are, in fact, due to subsidence.

There are several guides available. The Subsidence Claims Advisory Bureau (Scab), set up five years ago to tackle the problems of homeowners faced with subsidence, has produced one which covers most aspects of cracking and slippage.

Scab also offers a specialist nationwide service through which you can obtain a survey for pounds 125 plus VAT to establish if you have subsidence or not. Encouraging news here is that four out of five owners find their cracks are due to something else. The problem is more likely to be thermal movement - where the brickwork or timber is expanding or contracting at different rates due to the weather.

Research from the Building Research Establishment in Watford concludes that 79 per cent of subsidence is caused by trees. "A willow tree drinks 40,000 gallons of water a year," says Chris Jordan, head of insurance at Scab.

There are various ways to manage trees to prevent subsidence. Pruning is one but it is wise to call in an arboriculturist to advise on any work. It may be that it is better to leave the tree standing - cutting it down entirely could cause the "heave" discussed earlier and just compound your problems.

A similar cause of movement problems and for similar reasons is a leaking drain.

Finally, if your house does have to be underpinned many insurers will, from then on, only offer cover on the basis that it excludes subsidence and land slip. That alone could leave the house virtually unsaleable. If this happens to you, contact Scab - it has an arrangement with an insurer in the US through which cover can be obtained.

Subsidence Claims Advisory Bureau, Charter House, 43 St Leonards Rd, Bexhill-on-Sea, East Sussex TN40 1JA. Tel: 01424 733727.