Householders told that trees must be removed because of subsidence should insist that insurance companies provide proper evidence that the tree is to blame, the Dulwich Society's open meeting on trees and subsidence was told in September.

Over 100 people attended the meeting, organised by the Wildlife and Trees committees at the Old Library, Dulwich College, and heard criticisms from several experts that some insurers fail to obtain adequate evidence and often insist on trees and shrubs being removed as the cheapest, quickest and easiest solution.

The meeting was organised because of rising concern about the dangers posed to Dulwich's tree heritage from subsidence claims. It heard from four specialists in law, insurance, ecology and arboriculture as well as from representatives of the Dulwich Estate and Southwark Council and was chaired by His Honour Michael Rich QC, the society's president. The society plans to publish a record of the debate and also hopes to publish longer accounts in the Newsletter. The first of these, by Jim Smith, former chair of the London Tree Officers' Association's executive and London trees and woodlands framework manager for the Forestry Commission, is scheduled to appear in the next issue.

Mr. Smith told the conference he had spent two years helping to write the Institution of Structural Engineers' report Subsidence of low rise buildings, published in 2000, which involved the collaboration of the entire industry, from engineers and insurers to local authority tree officers, and remained the nationally recognised authority on resolving subsidence problems.

Unfortunately, he added, "many insurers' loss adjusters and arboricultural consultants do not actually follow the recommendations in that report. Frequently, they don't adopt a forensic approach in investigations. They fail to eliminate other potential causes of movement before identifying the tree as the potential cause."

Mr. Smith criticised what he called "loss adjusters' complacency in almost always assuming that the tree is the cause of the problem. My experience is that they commission limited tests to fit this hypothesis rather than to establish the real mechanism at work."

He said the Building Research Establishment had analysed the tests routinely used by insurance companies to establish the desiccation, or drying-out, of soils

and concluded that none of them was satisfactory. "By using tests that are less reliable and less accurate and then taking calculations off those, you're building into that process a margin of error that we, as tree officers, find unacceptable."

The Institute of Structural Engineers, the Building Research Establishment and the newly revised London Tree Officers' Association document A Risk Limitation Strategy for Tree Root Claims all advised using a set of tests appropriate to the soil type to eliminate other causes prior to establishing the tree as the cause of the movement, he added,. These test results required cross correlation with each other and with tests on ground conditions and the presence of leaking drains to create an accurate picture of the mechanism at work.

"Only by approaching this matter forensically and with due diligence can it be resolved to all parties' satisfaction. To date this has not been happening - indeed in many cases loss adjusters have actually withheld key test information, specifically, we believe, because they know it weakens their case."

Other highlights, quotations and points from the meeting include:

• "If a tree causes damage, it's very difficult for the local authority or the tree managers to defend the tree. It can cost a lot of money to deal with problems that trees cause, especially the cost of underpinning and often everyone thinks that the easiest and cheapest way to avoid that kind of liability is to chop down the tree." (Andrew Plunkett, lawyer)
• Tree root barriers can be an increasingly effective alternative to tree removal.
• "It's a nonsense to remove all trees within a certain distance of the house, and it must be stopped." (Paul Thompson, arboricultural and ecological consultant to the insurance industry)
• "If vegetation is involved [in subsidence claims], an arboricultural consultant will be appointed..An awful lot of flak is put at these consultants, often by me. Anyone who is assisting in a subsidence claim is acting on behalf of the building insurer. And that's quite important because the building insurer wants one thing - finality. And therefore, he wants the tree out." (Peter Osborne, chartered insurer and local authority adviser)
• "The current evidence regrettably is accepted by the courts, and therein lies the difficulty. If the courts accept it, why is the building insurer going to provide more? The last thing insurers per se want to do is pay out money - it doesn't matter whether it's to do an extra test or just pay you." (Peter Osborne)
• " [The courts say] you have got to manage your trees allowing for drought - buthhow should [local authorities] plan for drought and how could they avoid it? If you go on most urban streets you will find just about every street tree is within standing distance of a house, so its roots could reach under the foundations. But less than one per cent of tree stock causes damage. But again, which tree? So the only way they can avoid it is to fell all the trees . I don't think the law is working, therefore, in the way it should do." (Peter Osborne)
• "The best thing to do is to get [insurers] to re-define what damage is. At the moment, damage is a hairline crack. If damage was to be shown as [an] over 5-millimetre crack, then about 50 per cent of the claims would disappear and 50 per cent of the trees would probably be saved. (Peter Osborne).
• "We're a lone voice really - us and the residents who are very keen on trees. But everyone else is in the same pay bracket really from the insurers. They employ the arboricultural companies, the structural engineers, the loss adjusters - so if they don't sing from the same hymn sheet then they're no longer employed by them. So we're one voice against about 10." (Tony George, Dulwich Estate arboricultural adviser)
• Where buildings or extensions are built near existing trees and subsidence occurs subsequently, removal of the tree may cause "heave". The builder or engineer responsible may also be liable for inadequate foundations. From 2003, any new building built near a tree has to have an "engineered" foundation, not merely one that conforms to a certain depth.
• Householders who pave over their front gardens may be increasing their chances of subsidence because the paved areas could prevent rainfall penetrating and "rehydrating" the soil next to their houses.
• The presence of tree roots in an area of subsidence does not prove that the tree is responsible. "All you've proved [is] that you've found a tree root".(Jim Smith). Further investigations are needed - "a case should not be determined on the fact that roots were present."

David Nicholson-Lord, Wildlife and Trees Committees