• Trees: subsidence assessments and amenity tree valuation

Trees: subsidence assessments and amenity tree valuation

Thomson Ecology arboricultural consultants offer experienced and practical advice on tree-related subsidence issues.

Subsidence and trees

Building damage from tree-related subsidence is a problem in parts of the country where the underlying soil type is shrinkable clay. Sometimes subsidence is caused by seasonal water removal by nearby mature trees. Whether soil movement is causing building damage is usually determined by crack monitoring over time. This is carried out by a structural engineer.

An arboriculturist may be called in when subsidence has been identified and it is suspected that trees are responsible for causing the ground to move by removing water from the soil. We survey the surrounding trees, noting their age, water demand potential and proximity to the damaged building and examine the soil to determine whether there are any tree roots within the potential ‘damage zone’. We then identify the species of any tree roots found. This allows us to determine which trees may be the cause of the problem.

If subsidence is confirmed as being due to the presence of trees, we will recommend works that could reduce the tree’s water usage. Practically, this usually means complete removal or extensive crown reduction.
At this point it is important to establish whether the tree is protected by a Tree Preservation Order or Conservation Area designation. Under the Town and Country Planning Act 1990, permission is usually required from the local planning authority before works can be carried out on protected trees. If this is required, we liaise with the local planning authority on your behalf and put together an application for works to a protected tree.

If the tree is not owned by the resident of the damaged building,  we can help to negotiate with the  tree owner and the insurance company to decide on a course of action that is agreeable to all parties.

Amenity tree valuation

Often a value needs to be attached to trees involved in subsidence cases to allow the insurance company to weigh up the relative costs and implications of removing or pruning the offending tree versus structurally reinforcing the damaged building.

We are experienced in using both the traditional Visual Amenity Valuation of Trees and Woodlands System (more commonly known as the Helliwell system) and the more recently developed Capital Asset Value for Amenity Trees (CAVAT) system for  assigning a monetary value to trees. Both systems have been tested in court, and are recognised throughout the industry as being the benchmarks for amenity tree valuation.