Technical guidance: Building near trees

Trees, hedges and shrubs enhance the appearance and quality of a property but may cause you problems if you are considering the construction of new buildings/extensions close to them.

When considering the impact of trees on your building project you should think about the trees on adjacent properties as well as your own.

Damage to buildings

Damage may be caused to buildings by both direct and indirect forms.

Direct damage

Direct damage includes:

• Physical damage to foundations and services by expansion of the trunk and root growth.
• Damage to the superstructure by branches striking the building in high winds.

Indirect damage

Indirect damage is essentially caused by the removal of moisture from shrinkable clay subsoils by evaporation and transpiration through the leaves. It occurs when the subsoil is at its driest as the roots continue to withdraw moisture causing the ground to dry out and crack. The removal (or severe pruning) of trees can also be problematic as it causes clay soils to recover their moisture content and the ground can consequently swell and cause uplift which can result in long term, serious problems.

Foundation design

Foundations on shrinkable clay soils must be designed so that the superstructure will not be damaged by differential shrinkage or swelling. Before you start work it is important to:

• Establish whether the soil is shrinkable clay.
• Identify the species of any trees on or adjacent to the site and establish their likely maximum height. Include any mature trees that have been removed in the last ten years.

Once you have done this, use the table below to establish the “safe” distance for normal foundations to be suitable. If the distance is less than the “safe” distance, speak to your Building Control Surveyoror structural engineer as a specialist foundation design may be required - this will involve either a deep trench with a compressible layer or bored piles and ground beams. A suspended floor will also be required.

Root barriers

Root barriers may also be considered but are generally expensive and ineffective as tree roots can go around them if they are not designed properly.