The LW Treecare Blog

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Trees and buildings

What is the problem with trees and buildings?

Structural damage caused by subsidence

This is generally only a problem on shrinkable clay soils. Buildings up to four storeys constructed before the 1950s are most at risk, as they frequently have comparatively shallow foundations

Roots may enter and block drains. If the drain leaks, it can lead to the formation of cavities where water flows into the soil. Older drains with poor seals and rigid joints are most susceptible

Branches can cause damage to roofs and guttering. Suckers and expanding roots can lift paving and lightweight structures, such as garages and sheds, but are incapable of lifting heavier structures

If there is a shrinkable clay soil, during prolonged periods of drought, trees can dry out the soil below the foundations causing the soil to shrink. This shrinking usually reaches a maximum in late summer, and is termed 'seasonal soil moisture deficit' because the soil normally has chance to rewet after winter rains. The movement in the soil can result in subsidence of the foundations and structural cracking. Cracks are most likely to develop around windows and doors (note: 5-10mm of movement is usually needed before cracks develop). Less commonly, a permanent soil moisture deficit can develop where the soil continues to get drier at depth year-on-year. In rare cases tree removal on such soils can lead to heave (i.e. upward movement of the foundations due to clay expansion rather than shrinkage) Tree roots are unlikely to directly penetrate sound footings Tree roots will proliferate where water is available, and this is what causes them to grow into drains. If the drains are watertight, then tree roots will not generally trouble them

Careful selection of trees for a site where buildings are close by is needed, but this does not mean large trees cannot be planted in a built-up area. The loss of street trees and garden trees in our towns and cities due to an overly cautious approach by insurers and councils is itself a worry, as urban areas need trees to maintain a healthy environment

It is hard to predict which trees will cause damage because even where conditions mean the risk is high, only a very small proportion of trees will go on to cause subsidence. Risk prediction systems are largely fallible so it is normally best to wait until a tree becomes a real risk (i.e. causes damage) before taking action.

Things to bear in mind if you're concerned about trees near houses and other buildings:

It must be noted that many trees grow near buildings and, in most cases, these will not cause any damage However, sometimes trees growing near buildings can cause major problems, especially after a long period of dry weather Subsidence is the main problem posed by trees, but there are also the physical threats caused by falling limbs or structural failure of the main trunk.

Careful selection of trees for a site where buildings are close by is needed, but this does not mean large trees cannot be planted in a built-up area. The loss of street trees and garden trees in our towns and cities due to an overly cautious approach by insurers and councils is itself a worry, as urban areas need trees to maintain a healthy environment

It is hard to predict which trees will cause damage because even where conditions mean the risk is high, only a very small proportion of trees will go on to cause subsidence. Risk prediction systems are largely fallible so it is normally best to wait until a tree becomes a real risk (i.e. causes damage) before taking action.

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