Subsidence risk mitigation

In our last blog we looked at how trees and vegetation influence subsidence.  As we rapidly approach the key growing period we will look at risk mitigation and the impact of legislation and ownership.

Where there has been previous subsidence or the property is in a high subsidence risk area we need to consider the extent and type of trees where the building sits in their zone of influence.   If extensive or of concern it is advisable to consult an arboriculturalist.  Some key considerations are:

  • If the trees are too close and were planted after the building was built they should be relocated.  
  • If the trees are too close and were planted before the building was built they could be managed by pollarding and crown thinning.  They should not be removed without specialist advice as this could cause uplift of the ground (i.e. heave) which can result in structural damage.
If the property owner is considering planting young trees this needs planning as although they will not extract sufficient moisture initially to present a risk, this may readily change as it grows. It is a common myth that a new home cannot be affected by subsidence for this reason.  There are, however, further considerations. 
A tree with a Tree Preservation Order (TPO) or in a Conservation Area can never be removed or altered without consent from the relevant Local Authority.  It is recommended to check with the Local Authority and establish if there are any such restrictions during the purchase process — if this is the case it is pertinent to understand how the trees can be managed to mitigate risk.
In addition the property may be affected by trees that are not within the owner’s property boundaries.  This either involves a neighbour or the Local Authority.  There are a number of Case Studies (Russell vs. London Borough of Barnet 1984) which set a legal precedent – the key driver is tree owner’s awareness of nuisance.  Local Authorities, for example, are deemed to have the necessary technical knowledge regarding trees and their effect on property.  Even so mitigation in respect of Local Authority trees can be problematic as the removal of trees is contentious and residents regularly challenge their removal.  They require a considered response with an amount of information to be provided.
So the first step is to understand the subsidence risk and then take into consideration any trees within the zone of influence.  It is important that such risks are taken into consideration before the onset of any new growing season, so the risk (or associated costs of risk management and repair) is realised and managed from the outset.   If numerous trees exist on a property then specific arboricultural advice may be required.
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