It would be wrong to assume that all trees growing close to buildings are potentially hazardous. Many other factors can be responsible for structural damage and so detailed site assessments by qualified professionals are required to determine the exact cause.
Trees are normally the largest living structure in the vicinity of buildings and can cause significant problems - especially after a long period of dry weather.
Subsidence immediately springs to mind but the physical threat, caused by falling limbs or structural failure of the main trunk, can be significant. If you do have a substantial tree within ‘range’ of a building or public highway it is well worth having it professionally surveyed every few years to assess its overall health and to determine any pruning or felling that may be needed. Ensure that you keep these reports in a safe place, as they may be useful in any negotiations with insurance companies or public bodies.
During prolonged periods of drought, trees can further dry out the soil to the extent that clayey soils will shrink which can result in structural cracking, particularly around windows and doors. However, the removal of a large tree suspected of causing subsidence can result in swelling or soil heave which can be as damaging as subsidence.
Subsidence can be prevented by not planting larger, more vigorous trees such as poplars, oaks and willows near buildings - but be aware that the extent and spread of tree roots is extremely variable and are unlikely to grow in a uniformly radial pattern. A useful guideline is that roots can commonly extend a distance equivalent to two-and-a-half times the height of the tree.
Tree roots are unlikely to directly penetrate sound footings or pipework but can exploit any cracks or faults (perhaps caused by soil shrinkage or heave) thereby compounding the problem as they extend and expand. Root barriers can be used when planting new trees but if these are deemed necessary than it is probably better to select a smaller or less vigorous specimen!
Pollarding may help reduce the potential impact but before taking steps to remove or reduce in size any tree thought to pose a risk, make sure that it is not protected by a Tree Preservation Order (TPO), which is issued by the Local Planning Authority. Their permission must be obtained before any protected tree is pruned or felled and similar constraints apply in a conservation area.
Finally be circumspect about removing a specimen that is presumed to be causing a problem. Unless there is an imminent danger from structural failure, precipitous action could cause more extensive damage in the future. It is nearly always worth seeking independent advice from a qualified aboriculturalist as well as a building surveyor.
Possible problems and high risk areas
Structural damage
Generally limited to shrinkable clay soils. Trees taking moisture out of these soils exaggerate soil shrinkage. This results in shifting foundations, which cause structural cracking. Conversely removing large trees from clay soils can cause the ground to swell, again leading to structural displacement. Buildings up to four storeys constructed before the 1950s are most at risk, as they frequently have foundations only 50cm (20in) deep.
Drain damage
Roots may block drains, which burst as a result. This can lead to the formation of cavities as the water from the drain flows into the soil. Older drains with poor seals and rigid joints are most susceptible.
Physical damage
Branches can cause damage to roofs and guttering, suckers can disturb paving and stems can rub against walls. Light buildings, such as garages and sheds, are most at risk.
Root spread
Many factors, both of the site and tree, can affect root spread. Roots often extend for a radius wider than the tree height. If unsure always seek professional advice before planting.
Tree legislation
A tree is the property and responsibility of the land owner, who may be liable for any damage caused. Always check with the Local Planning Authority whether a Tree Preservation Order is in place before working on a tree.
Further information
Arboricultural Advice and Information Service (Tree Advice Trust), Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH. Tel. 01420 22022.
The Arboricultural Association's Directory of Consultants and Contractors. Tel. 01794 368717 or visit www.trees.org.uk
Building Research Station, Bucknells Lane, Garston, Watford, Herts WD2 7JR. Tel. 01923 894040.
Royal Institution of Chartered Surveyors, 12 Great George Street, London SW1P 3AE. Tel. 0207 222 7000. www.rics.org.uk