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Trees, Houses and Subsidence  

Andrew Cowan of Tree Craft Ltd explains the problems associated with trees near houses - and dispels the myth, that all trees cause subsidence.

Subsidence occurs when the (clay) soil under the building foundations shrinks; thereby leaving the walls without proper support. The building will then drop, and cracks may appear in the walls.

The ‘shrinking’ and ‘heaving’ of the soil may be a result of the seasonal variations in ground water levels. Extremely dry summers often make the problem of house subsidence worse.

Clay soil is the only type of soil that will cause problems of subsidence to houses  in this way. Such a soil will shrink and expand according to the amount of water retained within its structure. Other soils, such as sand or gravel soils do not vary in volume – regardless of their water content.

The Robinia frisia often sold as a small tree - but can tower over the top of houseIf your house is suffering from subsidence on a clay soil, then trees may well be the problem

The water demands of large trees can be regulated by efficient tree surgery. In particular, by reducing the crown size (and therefore the leaf area) of a tree, its uptake of water can be reduced.

It is important to find out if the tree in question, is covered by a Tree Preservation Order (T.P.O.), or whether it is in a Conservation Area, before any work is carried out. There are severe legal implications if you do not!

Roots have two main functions:

  • To support the tree and hold it in place. The large roots often near the surface, and near to the trunk, act as anchors, to keep the tree upright.
  • To provide nutrition for the tree. The small fibrous roots – further away from the trunk – draw up the water and minerals from the soil to feed the tree. These roots will grow away from the tree in order to find water – sometimes to distances which are several times the height of the tree - A recipe for subsidence and many other problems! These roots can get into drains and soakaways, which both provide a good and readily available source of water.

Soil Heaving (On clay soils only). can occur when an old tree dies - or is felled. The water content of the surrounding soil increases, thereby swelling the clay soils. This can cause subsidence to surrounding buildings, by applying pressure to walls and solid floors, or even lifting the foundations. ‘Heave’ is only a problem if the tree had been established for a significant time before the building was constructed.

Drains that have weak pipe joints and therefore leak, provide a source of water for tree roots which will be gratefully received. The roots will of course get greedy and invade the weak drains. They will grow – making the problem worse – and may even completely block the faulty drains.

The roots often block up the crack that they entered by, and the leak may only be noticed if the roots die back after a tree is felled or pruned.

It is important to consult a qualified arboriculturalist, when you are considering planting, pruning or felling trees near building. Future subsidence to houses and other buildings can often be envisaged and prevented.

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