Tree root subsidence is caused by the extraction by trees of moisture in cohesive soils such as clay and silt, causing shrinkage. The issue is currently topical due to this year's drought and previously in 2018. Insurers have indicated an increase of subsidence claims in the region of 400% when compared to years of 2021 and 2017, respectively.
Insurers who have settled the claim of an insured householder will investigate whether a recovery can be made from any neighbouring landowner. As large landowners, councils with extensive tree stock in residential and urban areas, are often targets for these claims, if there is an indication that a tree is involved.
A tree root subsidence claim is no different to any other: a claimant will need to prove breach of duty, foreseeability and causation. However, determination tends to require investigation in reverse, for example, causation, foreseeability, then breach of duty.
Causation: what will a claimant need to show?
A claimant will need to demonstrate the existence of shrinkable subsoil such as clay underneath the structure; the plasticity (potential for shrinkage) of the clay subsoil; the presence of tree roots and desiccation of the subsoil. A claimant must show that desiccation from tree roots has materially contributed to the damage, in which case it does not necessarily matter if there are other contributing causes of the damage, such as other trees on the claimant's property. It will take more than simply pointing to other possible causes if a case is to be defended on causation.
Structure movement monitoring is key. Tree root induced subsidence has a unique presentation, which is cyclical in nature. Deciduous trees extract moisture during the growing season with little to no moisture extraction once the leaves fall in the autumn. This presents on the monitoring graphs, with downward movement between May and October and upward movement between November and April.
Monitoring may assist in defending the case on causation if:
- There is no cyclical presentation
- The tree is removed and movement continues, suggesting the tree was not the cause.
The tree may be able to be reduced to such an extent as to reduce desiccation of the subsoil. The building’s movement will have to be monitored throughout the following year to assess whether the tree was the cause.
Foreseeability
The question is whether the tree posed a 'real risk' rather than a potential risk to the affected property.
Each case will turn on its own facts and relevant factors to consider will be as follows:
- Soil classification
- The species of the tree(s)
- The age of the tree(s)
- The distance of the tree(s) from the risk address
- The claims history for the area
- The historic maintenance regime for the tree(s).
If there are very few claims in the location and none previously in relation to the risk address then it may be argued that the likelihood of damage was not reasonably foreseeable until such time as sufficient evidence was provided implicating a tree.
To what extent extreme weather such as 2022 and 2018 drought should be anticipated has yet to be litigated. Our view is that maintenance regimes cannot be informed by the possible eventuality of a dry year. It is clearly unrealistic to expect a council to manage its tree stock in anticipation of such extreme conditions. The duty of care upon a council to prevent subsidence related damage could not reasonably extend to the prevention of such damage in an event year the like of 2018 and 2022.
Breach of duty
A claimant will need to demonstrate that a defendant did not do what they reasonably should to prevent the tree from damaging the property. Providing the defendant can evidence a cyclical inspection and pruning regime it may be able to demonstrate the duty was discharged. Other considerations will be the level and frequency of pruning. The reasonableness of the maintenance regime can only be determined by assessment of the perceived level of risk.
The HortLINK study found that biennial reduction of crown volume by between 70% and 90% is required to adequately reduce water uptake in particular species.
Early reports from an arborist and structural engineer go a long way to make an early determination on the risk of litigation. A robust tree inspection and maintenance regime is essential. Irrespective of the HortLINK study regarding pruning, recent cases have found a defendant to have fulfilled their duty with pruning to a lesser degree than the biennial 70-90% recommended.
An implicated tree may be within a third-party property and covered by protection of a preservation area or subject to a tree preservation order (TPO). Failure to consent to an application to work on a protected tree implicated in subsidence may be actionable under Section 24 of the Town and Country Planning (Tree Preservation Order) (England) Regulations 2012.
Opportunity to mitigate
If causation is established by way of the technical evidence, the owner of the property must give a council landowner the opportunity to abate the nuisance. A householder cannot incur large underpinning costs to a property then bill the council. The council must be put on notice that a tree on their land is suspected of causing the damage and the complainant must provide evidence implicating the tree.
