Alastair Woodgate, Director of St Albans and Watford based Chartered Surveyors, Rumball Sedgwick considers the impact of trees on buildings.
“My colleague took a call yesterday, typical of many enquiries to the office: “A crack has appeared in the external wall of our house. Could the roots of the nearby oak tree be to blame?” A site visit will shed more light on the possible causes, but it’s worth noting here that while trees can cause problems to structures, many trees grow near buildings and, in most cases, never cause problems.
But, if you notice cracks appearing in house walls where there are nearby mature trees there is a possibility these are a factor in causing the damage. Understanding the soil type and depth of foundations will help determine the cause and what action is needed. If in doubt, get the opinion of a Chartered Surveyor.
Tree roots can grow far beyond the width of the canopy, spreading up to three times the height of the tree. But very little damage to buildings is caused by pressure exerted by tree roots – they’re not strong enough to push buildings around. But if there is a clay soil and prolonged periods of drought, trees can dry out the soil below foundations causing the soil to shrink and the movement in the soil can result in cracking.
Subsidence is generally only a problem on shrinkable clay soils. On non-clay soils (e.g. chalk or sand) you are unlikely to encounter problems.
Houses built before the 1950s are at greater risk, as they often have fairly shallow foundations. The Building Regulations came in during the 1970s so most houses built after 1980 will have deeper footings than those built earlier, making them less likely to experience damage.
If cracks appear in late summer, the cause is most likely vegetation related. Cracks appearing at other times of the year could be due to other factors such as an unstable ground, leaky drains, collapse of the soil structure following flooding on sandy soils, or, in extremes, as St Albans knows only too well, sink holes.
Tree roots will proliferate where water is available so if a drain leaks, roots can enter them, potentially causing blockages and cavities can form where water flows into the soil.
There is no direct link between damage to non-load-bearing structures such as garden walls and paving and load-bearing structures such as houses. Pavements and boundary walls with little or no foundations can be disturbed by large shallow roots. Most lifting occurs within a 2m radius of a tree’s trunk, so it is advisable not to pave within this area, which is also beneficial to the tree.
If you own a substantial tree it is well worth having it professionally surveyed every few years to assess its health and impact and to determine any necessary arboricultural work.
This doesn’t mean large trees shouldn’t be planted in a built-up area. Trees add a great deal of splendour and the loss of trees in our towns and cities is itself a worry, as urban areas need trees to maintain a healthy environment.”