Trees and Hedges

Trees and hedges are important to all of us; however, they can cause problems when they are too close to buildings. Regular maintenance is required to clear leaves and debris from drains and gutters to prevent blockages but structural damage can also occur if buildings are constructed on shrinkable clay.

The closer trees are to buildings the more likely they are to affect them. The severity of that effect depends upon the species of the tree and the nature of the subsoil. A group of trees generally has a greater effect than a single tree and even a low level hedge can cause major structural damage to a building.

Some high water demand trees such as oaks, willows, poplars, plane and eucalyptus can have an effect when up to 30 metres away and conifers up to 18 metres away. Where no trees are present a foundation depth of 1 metre is normally acceptable (subject to depth of drains and undisturbed ground) as seasonal movement does not normally occur at this depth.

Trees take up moisture causing clay soils to dry out and shrink. As the soil below the building shrinks the weight of the building will cause the soil to compact and settlement will occur. Where no trees are present a foundation depth of 1 metre is normally acceptable (subject to depth of drains and undisturbed ground).

If trees are present, the foundations and ground floor construction must be designed to prevent unacceptable movement to the structure.

Trees often have a zone of influence that extends beyond the extent of the root system and this must also be taken into account. When the moisture content of a soil is reduced this condition is known as desiccation.

Removing trees has the opposite effect to shrinkage and will cause the clay to swell which can also affect the foundations. This is known as clay heave and it can also occur when roots of remaining trees have been severed by the digging of foundations.

Foundation depths and floor constructions for new building work should be designed at an early stage to prevent the problems detailed above. Advice from a structural engineer and soil specialist should be sought.

Care must be taken when planting and landscaping around existing buildings to prevent possible future damage. Advice regarding the mature height, water demand and growth rate should be obtained from a specialist to ensure that they are suitable for the proposed location.

Advice on the likely foundation depths required for proposed building works within the Borough can be obtained from building control on 01992 785566 during normal office hours.