What is the link between trees and subsidence?

Subsidence in buildings is the continuous downward movement of the structure caused by the shrinkage or downward movement of the underlying soil. Subsidence caused by tree roots can only occur on soils containing clay. Clay is just soil made up of microscopically fine particles below a certain size.

A unique property of clay is that it shrinks and expands depending on the amount of water suspended between the particles. The more clay is in a soil, the more water it can hold and the more it can shrink and expand. Because tree roots remove moisture from the soil, they can cause the soil to shrink which in turn can fail to support the structure of a building, leading to cracks in the building. Many other things can cause cracks in buildings that have nothing to do with trees such as 'settlement', leaking water and sewage pipes and climate.

Most of the District of South Northamptonshire is on soil containing clay, but on those of low to moderate shrinkage potential. This means that it is possible that buildings in the District may suffer from tree root induced subsidence.

There has been much conjecture in the media over the years where articles have been published that claim to name and shame certain tree species and state at what distance from your house they are likely to cause damage. Inevitably, to capture popular attention, these articles have usually vastly oversimplified what is an incredibly complex interaction of many variables and there are no 'rules of thumb'. Variables include:

In fact, there is no limit to how close any tree can be to a structure provided that the foundations have been constructed to resist the influences of the tree roots. This applies to all tree species and all types of clay soil, however shrinkable. This usually entails constructing foundations so that the footing extends to a greater depth than the influence of the tree roots.

The variable factor that is not possible to control is climate. Climate is arguably the most highly influential factor in the process, because it is essentially the weather that dictates the amount of water that is available for the soil to maintain equilibrium, or a volume that is capable of continuing to support foundations.

If we experience one or more summers of exceptionally low rainfall, the amount of moisture leaving the soil can exceed the amount of water being taken up by the soil. This can be exacerbated by tree roots. Insurance companies refer to these periods as 'event years', because they are events that lead to an unusually large number of claims.

In summary trees are not the only things that cause cracks in buildings. They are capable of it, but so are other factors. Subsidence only occurs on clay soils.

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