Trees, roots and buildings

06 May 2011

If you are planning to build an extension, take special notice of nearby trees which, for all their beauty, are a potential source of future structural trouble.

Beautiful, mature trees are a most desirable feature of any property. It is no accident that so many modern housing developments are either built around existing trees or include newly landscaped areas. But old trees growing too close to buildings pose a very serious threat to foundations and drains. Remedial work in such cases is almost always difficult and expensive and you will certainly need professional advice.

Serious damage to foundations can be very costly to rectify, and even if your property is covered by insurance, there are often legal problems over ownership of the tree which is responsible for other liabilities. Ground subsidence is a particular problem during long, dry periods and sometimes it is difficult to tell whether this is the effect of long-term geological changes or ground movement due to tree roots.

Some settlement occurs in all buildings, especially those on clay soils. And it can be presumed that, in the case of a very old house and a nearby mature tree, any damage that has been done – and accommodated by settling, may have given the house a topsy-turvy character. Attempts at altering this balance by removing the tree or tampering with the tree roots could have potentially disastrous consequences.

But the most dramatic and damaging effect of root damage is the cracking of walls when foundations subside. All parts of the building are likely to be affected: cracks open up in exterior brickwork in the familiar step pattern; roofs cease to be waterproof as flashings crack; ridge pointing falls out or tiles are displaced. Frequently door and window frames will warp and bind. Interior walls may show large vertical cracks. Solid floors may become uneven. Damp-proof course may fail.

Settlement and its causes

Tree root damage and subsidence problems occur commonly in areas of clay soil, known as hazardous clays or firm shrinkable clays. Clay is composed of a net of very fine particles which enclose relatively large pockets of water. For this reason, it is not very compressible, unlike sand and gravel soils.

Structures built on sands quickly compress the subsoil and drive out excess water under their own weight. Clay can only be compressed very slowly because it is impervious structure tends to retain water. Buildings on a clay bed may take ten or more years to settle down; this is quite separate to the related effects of tree root growth.

Since water is such an important structural component of clay soils, they are greatly affected by seasonal changes in moisture content. In the rainy season, when the ground is saturated, clay expands both upwards and outwards. In the dry months, cracks appear in exposed clay and the ground level falls. Variation in ground level from season to season is likely to be at least 50mm, but can be more than double this. To counteract this effect, buildings on clay have foundations that extend below the level where soil is likely to be affected by seasonal expansion and contraction.

But any problems caused by these seasonal changes in ground level are accentuated by the presence of nearby root systems.  In winter, a tree’s fine root hairs are inactive, and in spring, the root hairs grow and seek out additional water to supply the tree’s new growth. In summer, a tree will reach its peak need for water. Clay that would have been depleted through evaporation is further drained by greedy roots. The quantities of water involved are not negligible: a large mature tree may take in upwards of 55 000 litres per day.

Local influences

In any situation where you suspect present or future damage directly or indirectly caused by tree roots, there are a number of other important points to consider. Generally, in towns the seasonal changes in ground level are likely to be less than in open countryside. The soil receives less rainwater because a good proportion is prevented from entering the soil by buildings and paved areas and is carried away through drains. If there are trees present, and they do have roots extending near or under buildings, their action will exaggerate seasonal moisture differences more disruptively than in rural area.

Another factor is wind direction – if there is a strong prevailing wind, tree roots are likely to favour the windward side, so consider the relative position of a tree and a building when making any assessment.

Size and spread of root systems

The root system of a tree tends to be quite shallow, particularly where water is available near the surface. In locations with a low water table, such as high on a hill, tree roots tend to plunge downwards. Quite a large tree near buildings in such a situation may pose no threat. As a general rule, tree roots can spread laterally to a distance equal to a tree’s mature height. When making assessments, it is important to consider what a tree’s final height will be, not its present size.

Areas likely to be affected

Problems of soil shrinkage and expansion occur all over the world, especially in areas where there are large seasonal fluctuations in seasonal rainfall. Although problems are often associated with firm, shrinkable clays, other types of soil are not always immune.  Do not assume that because a garden has well kept soil, broken down by years of cultivation, that there is no clay underneath. It may be lurking there, covered by as much as 2m of topsoil. Pockets of clay are found in areas of predominantly trouble-free soil. Sometimes these deposits are man-made: major excavations and mining operations can throw up troublesome deposits on which houses are often built. Another problem is an underlying layer of shallow impervious bedrock, which forces roots to keep to the surface in their search for nourishment.

Problems for old houses

Old houses often live happily in equilibrium with their trees, especially when the trees have grown to maturity. But many old houses have substantial areas of underground brickwork that is saturated and slowly decaying. Tree roots can invade cracks and cause further deterioration. As roots penetrate foundations, they will have a jacking effect on the wall, causing lateral cracks at its base. If the tree is much too close, large roots may actually have the strength to displace foundations. No trees should be within a third of its height, or less than 4m, from a building.

Drains may become clogged as fine root hairs invade cracks or the collar here sections join. And leaking drains may encourage the spread of tree roots in their direction. But in general, a well established site will tend to have a balanced cycle of soil moisture content. Old houses have already undergone their settlement. Unless new trees invade the area, or weather conditions change drastically (as in a long drought), there may be no cause for alarm about existing mature trees.

If you are buying an old house, the question of trees in the vicinity is one which your surveyor is sure to examine carefully. Be suspicious of houses close to trees that have been recently painted or plastered. It could conceal cracks, large and small. Subsidence existing at the time you purchase your house might cause difficulty when you come to claim insurance damage after the next dry spell.

Extensions

One of the greatest danger points for subsidence damage is an extension or garage attached to a house. Often these have shallower foundations that are differently affected by seasonal ground swell and contraction. When installed against old houses, their very presence may affect the balance of the soil moisture content in the vicinity. If you plan an extension, make sure that it will have adequate foundations and that the join between it and the house is flexible enough to resist some movement without allowing water to seep in, otherwise damage may be extensive.

Solutions

Every problem of this type is different. It may be the case that simply lopping a tree or removing a newly paved area will keep the situation under control. You may have to proceed by trial and error over a season or two to see if things worsen. In serious cases the remedy is underpinning, which is definitely not a job to be undertaken lightly. Underpinning involves the excavation of a series of small pits, individually but in sequence, to enable installation of new foundations or supports to replace damaged ones. The building is usually supported by jacks while underpinning is installed. – Antonella Desi