Now there is some good news for all those people living on shrinkable London clay: according to the experts, we have been over-reacting.
A series of building experts with professional and academic experience has come to the conclusion that a crack in the wall is unlikely to mean your house is falling down. Most cracks in houses can be filled in and papered over. The new accepted wisdom is that underpinning is rarely the right response.
North London suffers far more subsidence than south London, some of which is built on gravel, not clay. One of the hardest hit areas is around Harrow, from which the Royal Institution of Chartered Surveyors receives its highest numbers of subsidence inquiries.
Gordon Swindles, secretary of the RICS' building surveyors branch in the area, explained why attitudes had changed. 'Up till the mid-1970s no one worried about cracks. They papered over them and people accepted that, he said.
''In about 1975 underpinning became very popular, partly as a result of more people insuring their houses and partly because of surveyors being over-cautious about their professional indemnity insurance.
'Before that, when people had to pick up the bill themselves, they chose to live with the problem instead.
If one house in a terrace was underpinned, it tended to cause problems with neighbouring properties. 'The underpinning went down the road, said Mr Swindles.
Following the long hot summers and dry winters of 1989 and 1990, claims for subsidence rose from about pounds 70 million to nearly pounds 550 million. The insurers reacted by removing cover or imposing huge premiums on most properties with a high-risk postcode. Not surprisingly, the number of claims shrank back to pounds 270 million in 1992.
Now a large body of research has found that regardless of the insurance costs, underpinning is rarely the right structural answer to cracks. They say a small bit of movement in a house is perfectly normal and we should return to the more sensible attitudes of pre-1975.
The Building Research Establishment has produced a series of categories for house cracks: up to 0.1mm, which can barely be seen; up to 1mm; up to 5mm; 5mm to 15mm; 15mm to 25mm (an inch); and over 25mm Up to 15mm the cracks are hardly ever a problem and can be repaired by a builder using ordinary filler. Mr Swindles suggests that people take pictures of the cracks before they have them filled in, so that when they come to sell their house they can show future buyers' surveyors that they haven't covered up anything serious. It is when cracks get bigger than half an inch that problems arise.
Mr Swindles said: 'In 99 per cent of situations the first thing to do is to monitor the building. That way you can understand what is happening.
The latest book on subsidence from the Building Research Establishment, Has Your House Got Cracks?, says underpinning is a drastic, expensive solution. Instead, it suggests the structure of the building can be strengthened by 'corseting with tie-bars, or that the ground can be stabilised by the removal of nearby trees. The book includes some reassuring pictures showing houses before and after treatment and gives a guide to common problems and how to deal with them.
The four features which make a house most liable to subsidence are: if it is built on shrinkable clay; if the foundations are no deeper than 2ft 6in (this applies to most Edwardian and late Victorian houses); if the house is on a slope; and if there are large trees and shrubs nearby to suck moisture out of the soil.
It is when you get all four together and you have a heatwave, that you are most likely to have problems.
Although the past five weeks have seen almost unbroken sunshine, south-east England is unlikely to see the problems experienced in 1989-90. While the summer has been hot, the winter was wet. Many cracks which appear in summer will disappear again in winter.
The only rain recently has been from storms, which runs straight off into the gutters, rather than soaking into the soil. As the ground gets drier, the plants have to push their roots deeper to find moisture.
The BRE has set up an experimental site at Chattendon in north Kent to learn more about how weather and trees affect ground movement.
Mike Crilly, who is part of the research team, said: 'A lot of cracking doesn't matter from a technical or serviceable point of view, but there are vested interests which work against what is now established academic opinion.
Both he and Mr Swindles urge householders to get a building surveyor or structural engineer to investigate any cracks which are worrying them and to use this ammunition when negotiating with insurance companies.
Building Research Establishment 0923 894040 Gordon Swindles & Associates 081-421 5445.
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