Have you received a letter from a firm acting for an insurer of an adjoining property asking you to remove trees? We can advise – and perhaps save your trees.
We have investigated on behalf of insurance companies, structural engineers, loss adjusters, surveyors and private clients, over 1500 cases of risk of subsidence or suspected subsidence in relation to trees in the last 30 years.
If you are a loss adjuster we are happy to engage on your behalf in the, sometimes delicate, process of liaison with local authorities to agree measures in connection with council-owned ‘street trees’.
What is tree-related subsidence?
Most people are aware that trees can cause damage to buildings but many are not clear at all on the process by which various types of damage occur.
What causes subsidence? Often, not trees. However, in some cases subsidence – most visibly presenting as cracks in a building wall – can be justly attributed to the activity of trees.
Tree-related subsidence is contingent on a number of factors, including weather conditions, the species of tree, and the type of soil. To understand the subsidence of buildings requires some discussion of the process of transpiration.
Transpiration is the process by which water is lost to the atmosphere from living plants – in this case, a tree. Water is drawn from the soil into the roots, from where it passes into the vessels of the tree, is conducted upwards, and is finally lost to the tree mainly through pores in its leaves.
On certain soil types – in particular clays, such as those frequently found across London – this loss of water causes the soil to shrink. This allows foundations resting on it to sink or move. There is a higher risk of this happening in very low rainfall periods. The buildings constructed on those footings may then crack.
What about heave?
Heave can be viewed as the opposite of subsidence. As far as tree–building relationships are concerned, heave is the movement – usually upward – of structures founded on dessicated clay soils. Such desiccation can cause problems if trees that have caused the desiccation are removed. When such dry soil absorbs moisture, swelling of the subsoil can occur, forcing structures upward.
However, heave can only occur in certain fairly precise circumstances, and we have found it to be much rarer than is popularly imagined. Heave is not caused by large roots decaying and leaving voids in the soil! We can inspect your trees and determine, usually quickly, whether a threat of heave applies and if it does, whether the risk is high or low.
What can be done about subsidence?
It is imperative to first judge accurately whether trees are actually responsible. The mere presence of a nearby tree does not automatically mean it has caused the damage.
We draw our our extensive knowledge to interpret the raw data – for example, monitored crack levels and root identification from external, impartial professionals – to provide an answer to this question.
If it has been established that the subsidence is likely to have been caused by a particular tree or trees, their removal almost always arrests further cracking. After this the previously dried clay will, usually fairly rapidly, return to its normal proportions by the natural action of rainfall. This will consequently lift the footings back to the position they were in prior to the damage, thus closing or nearly closing the cracks. Internal redecoration is often all that is required.
We have the right approach to this sometimes emotive area of practice: minimising tree loss without inappropriate tree retention or treatment. When tree removal is unavoidable, we provide advice on choice and siting of appropriate replacement trees.