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11. Damage to property and drains

The perceived threat of damage by tree roots is probably the biggest worry people have about trees near to buildings. Much of this concern is completely unwarranted, but trees may cause damage in some circumstances.

There are two types of damage - direct and indirect.

Direct damage results from the pressure that may be exerted by tree roots or trunks. This may affect lightly loaded structures such as garden walls, driveways and patios, for example, but it rarely affects heavily loaded structures such as houses.

Indirect damage is unrelated to the force exerted by roots or trunk, but occurs when tree roots take moisture from shrinkable soils, usually clay. As they do this the soil shrinks and if the foundations of a building are not deep enough, this may result in subsidence and cracks appearing in the structure. Shrinking of clay is a reversible process. A clay that swells as it rewets can exert pressure which may lift a building and cause damage, therefore it is important that before carrying out works to a tree, consideration should be given to the possibility of heave.

In the case of shrinkable soils new structures should be designed and constructed to withstand or accommodate the potential effects of movement which are likely to increase with climate change regardless of the presence of trees.

It is often asserted that roots damage underground services (particularly pipes).

Although roots may be found sheathing a pipe or growing inside it, they are generally not the cause of damage - they can neither detect water in a sound pipe nor exert sufficient pressure to break into a sound pipe to gain access to the water it may contain.

However, if a pipe is already damaged and leaking, tree roots may be attracted by the moisture, enter the pipe, proliferate and ultimately cause a blockage. Pipes with clay-packed joints are most prone to invasion by roots, particularly as the joints may fail. Modern plastic pipes made of PVC plastic and fibreglass and those using rubber sealing rings are resistant if constructed properly.

Where evidence is provided which demonstrates a council owned trees is the cause of damage, the council will take appropriate action, which may result in the pruning or removal of the tree, particularly where no other alternative solution exists.