Trees and wall

We have all heard people voice concern about the possible effects of tree roots near a house or wall. Surely they will damage the foundations and destabilise the property?

Let’s look at the facts. There are two types of damage caused to the built environment by roots – direct and indirect. Direct damage is a result of roots exerting pressure on a structure: if the force is greater than the resisting force, damage will occur. We often see this in towns where tree roots have lifted paviors or cracked Tarmac, although this is preventable by using planting kit and techniques designed to allow trees to thrive in an urban environment.

Although it is possible for tree roots to affect freestanding walls, they are far less likely to cause direct damage to house foundations as the resisting force is so much greater than anything the root can exert. When roots meet a solid object they tend to grow laterally, rather than downwards. Roots will also seek out and follow moisture from any source, including the condensation naturally created on the outside walls of a building. A few years ago JP Associates supervised the demolition of a derelict Victorian house that had trees growing close to it on one side and when the foundations were exposed there was a bank of tree roots following the line of the old house wall. The house had not been damaged by this. Roots can also grow into drains - again seeking moisture - but only if the pipework is damaged.

Subsidence and heave

Then we move on to indirect damage, otherwise known as subsidence and heave. Although both can be caused by factors other than trees – such as underground excavations and re-wetting of drought affected soil - they are probably what most people would see as the main concerns when trees grow close to buildings.

For tree-related subsidence to occur, three factors need to be in place:

1) a structure

2) shrinkable clay soil

3) trees or woody vegetation

This kind of subsidence usually happens when a prolonged dry spell is exacerbated by a tree drawing further moisture out of the soil. The ground level sinks due to volumetric change, thereby potentially causing structural damage to nearby properties.

Subsidence is a complex issue – and can be costly to deal with: underpinning may be carried out to damaged buildings but this is a disruptive and expensive option. NHBC-backed houses built over the past few decades have foundations designed to resist the effects of subsidence and heave, although older houses - albeit with shallower foundations - are more ‘flexible’ structures than modern builds and may therefore be less affected.

There are many factors to take into consideration during subsidence investigations: if a tree is suspected of being the cause, it is not necessarily simply a case of removing it as this may not eliminate all the problems and taking out a large tree could lead to heave (more on that shortly). There may also be amenity issues to consider with the tree, which may be the subject of a tree preservation order (TPO). You have to look at existing damage to the property, its age, the type of soil present and its plasticity index (in other words, how shrinkable it is), the species of vegetation, distance from the property, the age of vegetation, its vitality/vigour and water demand.

If remedial action to the tree is deemed necessary it could include canopy reduction to decrease water requirement, although there are various schools of thought as to the type of pruning that gives better results. Certainly larger, more vigorous and thirsty trees such as oak and willow should not be planted near buildings – nor houses built near to existing specimens.

Heave occurs when a shrinkable clay soil swells upwards due to rewetting.  If a property is built on a desiccated shrinkable clay soil with a mature tree present and the tree is subsequently removed, heave could occur.  However, unless the house is on shrinkable clay the likelihood of it being damaged is low.

Many trees grow near buildings and cause no damage. However if you are concerned about the presence of trees close to buildings you should seek professional, independent advice before any remedial work is carried out.

Further reading

Tree Root Damage to Buildings, Dr Giles Biddle
ISBN for two-volume set: 0 9533086 0 X

The British Geological Survey website contains lots of useful Information on shrinkable soil areas in the UK

For products and guides to planting trees in the urban environment