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The second in our series of NBS Shortcuts examines the effects of tree-root systems on building design. Foundation design needs to take account of the water-absorbing capacity of nearby trees, as well as potential groundswell caused by trees that have been removed. This guide highlights the dangers to structures that may be caused by the proximity of certain species of tree.
Most trees in the UK have a significant radial root system, extending one to one-and-a-half times the height of the tree. Severing just one of a tree's major roots during careless excavation for construction or services can cause the loss of up to 20 per cent of the root system; this undermines the tree's ability to absorb water and also leaves it unstable in high winds. Therefore, when laying service pipes, it is advisable to tunnel under the root system if possible, running the pipes under the middle of the tree (on plan) if necessary.
In general, 80-90 per cent of all tree roots are found in the top 600mm of soil and almost 99 per cent of the tree's total root length occurs within the topmost 1m of soil, with some variations depending on soil porosity. The undoubted nuisance that fine root systems create for the development of specific sites has to be weighed against the importance that they play in soil stabilisation on sloping ground (acting in a similar way to geotextile matting).
Only a few mature species, such as oak, pine and fir, have significant central tap roots - the main, central roots from which the others spread - and, in most instances, even these extend downwards by only about 2m. So it is the radial tree roots that extend outwards that are of primary concern here; these can influence soil conditions well beyond the circumference of the tree's leaf canopy. With around £400 million worth of tree-related insurance claims made every year in the UK, it is worth considering what precautions are needed to build near them.
It is always recommended that construction takes place as far away from trees and established vegetation as possible, because the distance at which tree roots can detrimentally affect a building is quite significant. Damage directly caused by proximate tree roots, such as roots...