Subsidence: The Spring Effect

Subsidence: The Spring Effect

So, spring is on its way (although it appears that winter is not going to go too easily) and nature starts to come alive after its winter sleep.  With the start of the tree growing season not too far away (generally recognised as the end of April) tree roots will then start to absorb water. Moisture uptake varies cross the various species but the high water demand trees such as Willows, Poplars and Oaks can remove in excess of 50,000 litres of water a year!
In the last couple of blogs we discussed the main causes of subsidence (soil shrinkage).  Each cause usually has some external influence or trigger.  The largest trigger is the water absorption of trees and vegetation in clay and other cohesive soils — in fact over 60% of all subsidence claims are triggered by trees.
To understand whether a tree could have an effect on a property the property must sit within the zone of tree root influence, which is the area from which a tree absorbs moisture. The extent of the zone depends upon the type of tree and the location of other trees — as they compete for moisture so they send out further roots.  As mentioned earlier Oak, Poplar, Willow are regarded as the worst offenders.   A general rule of thumb to determine the zone of influence is to imagine cutting the tree at its base and laying it down — ‘imagine’ is the main word here as simply cutting down a tree can cause you significant legal and property issues (which we will go into further in my next blog)
In dry periods (drought or seasonal variance) the roots of the tree will spread in search of moisture from an extended area (indeed moisture removal of up to a depth of 6m can take place) and so although originally thought to be too remote to affect the building the tree can now do so.
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.
In the next blog we will explore the effect of trees and vegetation further – particularly tree management approaches and the impact of legislation and ownership.
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Andy Lucas Managing Director at Property Assure Ltd

Andy Lucas, Property Assure

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