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Trees and subsidence damage to property: a quick guide for local authorities

Following on from his article on tree risk management and the law in respect of catastrophic tree collapse in the Spring Local Authority Brief, Gabriel Fay now looks at the relevant law in relation to subsidence damage to property, caused by tree roots.

Background

A common figure put on the costs of subsidence to insurers is in the region of £400m per year.  Out of that figure, approximately 60 - 70% is considered to relate to subsidence caused by trees. Naturally, insurers who have settled the claim of an insured householder will investigate whether a recovery can be made from any neighbouring landowner.  Local authorities, being large landowners with extensive tree stock in residential and urban areas are often the obvious targets for such claims.

For the purposes of this enquiry, subsidence will be considered as the shrinkage of the soils beneath a structure causing loss of support and resulting in property damage.  In the context of tree root subsidence, this is caused by the extraction by trees of moisture in cohesive soils such as clay and silt, thus causing shrinkage.  Bearing in mind that a fully grown deciduous tree can extract up to 50,000 litres of water per year, one can see the potential for shrinkage of the clay subsoil, especially in particularly dry years.

There is a clear conflict between the law wishing to protect property owners’ rights to reasonably enjoy their property without interference, and the recognition of the aesthetic, social and practical utility of trees within the urban environment. However, it is clear through centuries of case law that the courts are sympathetic to the protection of private property rights and the protection of owners’ reasonable enjoyment of their land without interference.  In this regard a tree root subsidence claim is no different to any other type of nuisance action in that a claimant will need to prove causation, foreseeability and a failure on the part of the tree owner to exercise reasonable care. 

Causation: What will the claimant need to show?

In respect of causation, the claimant will need to demonstrate the existence of shrinkable subsoil underneath the structure, the plasticity (the potential for shrinkage) of the clay subsoil, the presence of tree roots and the desiccation of the subsoil. 

The claimant must show that the desiccation from the tree roots materially contributed to the damage, in which case it does not matter that there may be other contributing causes to the damage.  This approach has been confirmed in many decisions and it will take more than simply pointing to other possible causes if a case is going to be defended on causation.  In practical terms, actions such as monitoring of the movement of any structure may be key, specifically if the tree is removed or heavily pruned.  Should the tree be removed and the damage continues to occur, this would suggest that it was not the tree that was the cause.  However, the tree would have been lost at that stage.

It may be that the tree can be reduced to such an extent as to reduce the desiccation of the subsoil and again it will be important to monitor the building’s movement throughout the next year in order to assess whether it was indeed the tree which was causing the subsidence.

However, studies have shown that tree water use was reduced by crown-reduction but unaffected by crown-thinning in the year of pruning. For a crown reduction to be effective then it would need to be severe, in the region of 70-90% of crown volume would have to be applied. The study demonstrated that reductions of up to 50% were not consistently effective for decreasing soil drying. Such heavy reductions may of course be possible with certain species of tree, such as London Plane, through periodical pollarding but will not be with other species which are less resilient to such pruning.

Foreseeability arguments

Arguments in respect of foreseeability largely centre around knowledge of the nature of the subsoil in a particular area.  For example, if the general area on which a structure sits is made up of gravel subsoil but the structure happens to sit on a pocket of shrinkable clay, then it may be open to a local authority to argue that it was not reasonably foreseeable that their tree(s) would cause such damage, on the assumption that the subsoil in the general area is not made up of shrinkable clay.

Since the late 1970s, building regulations have been far more stringent with a requirement for foundations to be laid to a sufficient depth to avoid subsidence occurring. This may well assist in arguments pertaining to foreseeability should subsidence be suffered by newer properties. With newer premises there may be an assumption that the foundations are such that the property would be protected from desiccation of the subsoil by tree roots.  As in the case of Siddiqui v London Borough of Hillingdon, His Honour Judge Richard Seymour stated:

the Council had no reason reasonably to foresee the risk of any damage to the structure of the Claimant’s house from the roots of trees in the wood until such damage in fact occurred…modern standards of construction can be expected to take account of obvious hazards in the vicinity of the structure to be built, as the trees in the wood were in relation to any structure to be constructed on this plot”.

Furthermore, a local authority will not be expected to simply remove the trees in close proximity to a building unless there is a reasonable level of information presented to them that the trees are likely to be causing damage (see Berent v Family Mosaic Housing and London Borough of Islington, CA [2012]). The position on this point is not clear cut and will very much depend on the specific facts of the case.

Opportunity to mitigate

Should tree related subsidence be suspected, the owner of the property must give a local authority the opportunity to abate the nuisance.  It is not open to a householder to incur large underpinning costs to a property and then present that bill to the local authority without first putting the authority on notice that a particular tree on their land is suspected of causing the damage.

Joint Mitigation Protocol

This Protocol has a number of signatory local authorities in the London area, insurers and loss adjusters. Spearheaded by the London Tree Officers Association, it seeks to govern subsidence claims from an early stage in a non-adversarial way. In essence, it provides guidance for standards of evidence in relation to tree root subsidence. The standard of evidence will be determined by an assessment of the amenity value of the tree using the Capital Asset Value for Amenity Trees. (CAVAT). Interestingly, the CAVAT system was referred to in the case of Berent above, where the court balanced the foreseeable risk with the social amenity of the trees in question when Tomlinson LJ gave the view that the local authorities duties for the “preservation of such amenities as an established treed environment could not reasonably contemplate the desertification of such a neighbourhood by wholesale tree felling to avoid a possible risk of damage”.

Possible Controls/Mitigation

Removal

Evidently, removal of the tree will abate the nuisance but in some cases a valued community asset will be lost in so doing.

Pruning

As has been seen, this can be effective but to be so it will need to be severe pruning which in turn may cause a loss of the tree’s amenity value and will need to be carried out on a regular basis.

Root barriers

For tree roots to cause subsidence they need to be in very close proximity or underneath the structure’s foundations. Accordingly, it may be possible to sever those roots and place a sufficiently deep barrier between the roots and the structure. However, consideration will need to be given to whether such action will compromise the tree’s health and structural stability.

Underpinning

This is a process of increasing a structure’s foundations to a depth unaffected by the desiccation of the sub soil by tree roots. This is achieved by way of concrete filled pits, dug beneath the existing foundations. It is expensive but allows both structure and tree to co-exist without the more extreme measure of removing the tree or such heavy pruning.

The need for evidence

The approach taken by the courts is not particularly defendant friendly in respect of subsidence claims. There appears to be little appreciation or concern for the environmental benefit of trees when pitched against the right of a property owner not to have his/her reasonable enjoyment of that property interfered with. This is in fact very much in keeping with general principles surrounding the tort of nuisance.  That said, such claims are defendable but may require evidence involving tree root analysis, soil analysis, level monitoring and reports from both arborists and structural engineers. It may also be possible to abate the nuisance and retain the tree but this may also be costly.

Contact

For further information please contact Gabriel Fay on DD +44 207 645 4540 or at Gabriel.fay@dwf.co.uk

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Tree risk management and the law: a quick guide

By Gabriel Fay

This information is intended as a general discussion surrounding the topics covered and is for guidance purposes only. It does not constitute legal advice and should not be regarded as a substitute for taking legal advice. DWF is not responsible for any activity undertaken based on this information.

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