Rising temperatures due to global warming will result in hotter, drier summers.. Even when it rains quite heavily, if the ground is dry, the water tends to run off so that less is absorbed into the subsoil below. In areas where the subsoil is a shrinkable clay this can result in subsidence damage to buildings supported on the clay subsoil. So we might expect to find more claims for subsidence damage to houses and flats in dry summers in the years ahead. 

Subsidence damage to buildings on clay subsoils is associated particularly with the action of tree roots and surveyors undertaking mortgage valuation inspections will be looking critically at any trees close to the proposed mortgage security. 

You do not have to be a tree-hugger to appreciate trees especially in an urban environment. Watching trees through the seasons is good for the soul, trees absorb CO2 and give us oxygen and many species absorb contamination from the air. There is lots of anecdotal evidence to suggest that property values are higher in tree-lined roads or close to open spaces with trees. 

The downside is that trees can damage buildings and surveyors undertaking mortgage valuation inspections are therefore required to note any trees within the influencing distance of the foundations both because of potential risk to the property value but also because the surveyor is advising on potential insurance issues. 

If a tree is very close to a building with shallow foundations the roots can actually move the building but this is quite exceptional and rare. More common is the subsidence damage to buildings caused by tree roots extracting moisture from the shrinkable clay subsoil under foundations. 

Surveyors with their local knowledge and armed with the Geological Survey Map for the area will know which properties are built on shrinkable clay. Much of the London area and many other parts of the UK have this type of subsoil. The tap roots of trees extend into the clay and remove moisture so the clay becomes desiccated and shrinks. When the rains return the clay swells often resulting in an upward heave pressure on foundations. Damage from clay swell causing heave can often be as severe, or even worse, than damage caused by downward subsidence. Heave damage often occurs following the removal of trees close to buildings. 

The surveyor’s valuation report, generally now completed on-line, will typically include a section to comment on trees and specifically any trees within the influencing distance of the foundations.  Many modern houses and flats built on clay will have special deep foundations and this applies particularly to developments undertaken since 1976. The severe drought in 1976 caused lots of subsidence damage and prompted a reappraisal of Building Regulations resulting in special deep foundations being specified for use in shrinkable clays, sometimes using piles which can be supported 6 metres or more below ground – well below the level of potential desiccation. 

Older buildings erected prior to 1976 typically have very shallow foundations consisting of a concrete strip with brick footings or similar typically less than a metre below the surface. Victorian and earlies houses, especially in rural areas, sometimes have no concrete foundation at all with the walls just resting on a bedding of rubble. 

Having established that the subsoil is shrinkable clay and that the proposed mortgage security is likely to have shallow foundations the surveyor then has to consider whether any trees present are likely to cause a problem. As a general rule no trees should be permitted or planted closer to the main walls than their mature height. Certain very thirsty trees such as Poplars are best avoided altogether in a garden setting since Poplar roots can extend for up to one-and-a-half times the height of the tree and even twice the height if Poplars are planted in rows when competition for moisture forces the roots to extend sideways. 

When a potential problem with trees is identified the normal procedure is for the surveyor to request a specialist report from an Arboriculturist and this will typically make recommendations regarding the potential threat posed by the tree and any tree-management actions that need to be taken. Because of the risk of upward heave damage the immediate removal of offending trees may be unwise. 

Some specialists advise the use of root barriers between the tree and the house, these being deep trenches filled with concrete intended to prevent the roots reaching the foundations but tree roots can be very crafty and find their way around root barriers if the barriers themselves are not very well designed. 

Most household insurance policies include subsidence, landslip and heave as insured perils so if you buy a house or flat with the normal insurance cover you will be insured for any new incident however you will not be insured for a defect which already existed before the insurers came on risk. If the property exhibited signs of structural movement when you bought it you will not be covered if the new damage is simply a continuation of the existing problem. 

Not all trees will be within the garden of the property under review and tree roots do not respect property boundaries. Potentially damaging trees may be in a neighbour’s garden, may stand on the highway verge or sit on a railway embankment. There is good precedent to indicate that the owner of a tree can be legally liable for damage to adjoining properties due to action of its roots. The surveyor undertaking a mortgage valuation will therefore look at trees on adjoining land as well as any in the garden of the proposed mortgage security, 

The Arboriculturist’s report will often provide the property owner with the necessary ammunition to approach a neighbouring landowner with recommended tree-management advice including lopping, crowning or pollarding to reduce leaf area (and therefore water intake). Any such actions typically have to be undertaken on a fairly regular basis however because a healthy tree will quickly replace lost foliage. 

When the specialist’s report is received the surveyor can review the advice given and, assuming the result is satisfactory, may then go on the complete the mortgage valuation and confirm that the property is suitable mortgage security. 

Peter Glover is a surveyor and author of Building Surveys and Buying a House or Flat