What Is Subsidence?

Subsidence is when the ground beneath a property sinks. This can destabilise the building, cause cracks in walls and floors, and lead to ill-fitting doors and windows. Subsidence is more common when houses are built on clay soils which expand and contract seasonally. The typical scenario is that clays expand in winter when the weather is wet and shrink in summer when the weather is drier.  It is the seasonal shrinking and swelling of the soil that causes movement and damage to buildings.

A History Of Subsidence

We have probably all known old terraced houses in which doors did not fit and floors were uneven and it goes without saying that subsidence will have existed as long as buildings have been built. However it is easy to put a date on when subsidence first became a problem and that was in 1971 when insurance companies started to add subsidence cover to household insurance policies.

The rise in home ownership in the 1970’s was, to some extent, driven by mortgage lenders who saw adding subsidence to insurance policies as an additional safeguard to their investment, exposing them to little additional risk! That changed after the drought of 1976 when the extensive shrinkage of clay subsoil resulted in a considerable number of claims. At that time the policy of many insurers was to carry out quite major repairs and underpinning to properties that displayed even minor damage and this obviously resulted in substantial expenditure.

This then created a situation where insurers became very reluctant to offer cover to new policyholders on properties that had shown any signs of previous movement, as they had become in today’s parlance ‘risk adverse’. As mortgage lenders also wished to protect their position, they would be equally reluctant to lend on a property where it might be difficult, if not impossible, to obtain insurance against subsidence.

These concerns were in turn passed onto Valuation Surveyors who, mindful of their professional indemnity insurance, tended to report even minor cracks as potential subsidence issues. Thus in a few years the situation changed from one in which no one worried too much about a few cracks, to a situation where everyone became paranoid about even a hairline crack in case it would render their property uninsurable and hence unsaleable.

brick crack

Subsidence Claims Today

Claims normally begin when properties are re-decorated or put on the market. At this point a homeowner may worry about cracks that they had not really noticed before or a mortgage lender for a prospective purchaser may send a ‘Valuation Surveyor’ who reports evidence of crack damage or previous movement.

On receiving notice of a potential claim insurers will usually appoint a Loss Adjuster to investigate on their behalf.  The Loss Adjuster will visit the property and prepare a report recommending what further action needs to be taken. It is very difficult to arrive at any firm conclusions from one visit, particularly if the damage is relatively slight, so it will be quite normal for the Loss Adjuster to recommend that further investigations are carried out. Typically the investigations would involve establishing the depth of the foundations, recording the type of subsoil, recording any trees or major shrubs on the site and analysing soil samples and any roots recovered from the investigations.

It would also normally be recommended that the cracks are monitored. This involves taking detailed measurements at set periods to establish whether the cracks are opening, closing or stabilising.  It may also be suggested that a ‘level monitoring’ exercise is carried out.  This would establish whether the building is rising, falling or staying in the same position relative to a fixed point. With modern measuring equipment crack and level monitoring is carried out to a very fine degree, often talking about movements of a fraction of a millimetre. This level of investigation takes a considerable period of time, months or even years, and the costs can run into several thousand pounds, often outweighing the cost of the eventual repairs.

If the trees are on adjoining land, the insurers will have to prove that they are implicated and ask the adjoining landowner to remove them.  This request is often accompanied by the threat of action to recover any losses if the landowner does not remove the trees and sometimes this can be quite intimidating.  Most homeowners, receiving a legal letter of this sort will, unfortunately, feel obliged to comply without challenging it.

More often than not the tree thought to be causing the subsidence is a street tree owned by the council. This makes the council liable for any damage and if the council refuses to fell the tree the insurers will threaten to take the council to court. It only takes a small bit of tree root found under a property for the insurers to win such a court case, thus rendering the council liable for all the costs: the extensive monitoring, underpinning, re-homing of residents while the work is being done and the legal costs incurred. These sums can amount to hundreds of thousands of pounds. It is not surprising then that councils usually go ahead with such tree-felling without challenging the Insurers. It is sad to see that the regular excessive pollarding of street trees which we see today in so many towns is also done as a preventative measure, to make sure that, by reducing the foliage, the trees’ demand for water will also be reduced and so will the demands of insurance companies.

Insurance companies are businesses that are, quite understandably, looking to thrive and make a profit. Obviously, they will look to minimise the amount that they have to pay out under any particular claim.  To a large extent, this is why the removal of trees is often suggested, as it may cure the problem and has no cost implications for insurers.  Some insurance companies even skip the costly investigations, demanding that trees are removed as a first step to see if this remedies the problem.

It does not matter to Insurers that the removal of trees is always somewhat experimental. There is no guarantee that removing trees or shrubs will have the desired effect and even if the property does stabilise after tree removal there is no way of proving that the building would not have stabilised of its own accord anyway. When looking at clay subsidence there are so many variable issues, such as the type of clay, the depth of foundations, climatic conditions, underground water sources and slope of site that it is never going to be possible to say conclusively that removal of trees alone is the problem. All these factors should be checked before a tree is needlessly felled.

Under current legislation buildings have all the legal protection and trees have none. Insurance companies currently have no obligation to look at the wider ecological issues of tree removal; they have only to consider their interests and those of their shareholders and they have to keep their costs down to be competitive. It will only be through powerful and strongly worded legislation that we will see a SHIFT in procedures relating to trees and subsidence.

tree roots exposed

What Role Do Trees Play In Subsidence?

Trees need to take in moisture through their roots to replace the moisture that evaporates through the foliage.  Some trees have higher water demands than others, for example Willows, Poplars and Oaks.

Tree roots can spread many metres in search of moisture and there is really no hard and fast rule as to how far they will spread.  It used to be generally accepted that the spread of tree roots would be about equal to the height of the tree, but cases have been encountered where the root spread has been 1 ½ or even 2 times the height of the tree.

However this does not mean that the roots are a problem. In many cases buildings and trees have happily co-existed for many years without any damage being caused.  If trees have been in place before buildings were constructed it is likely that the subsoil would have been in equilibrium at the time of building.  In these cases removing trees would possibly do more harm than good because if the trees are removed their roots will no longer extract moisture from the soil and it is possible that ‘heave’ could occur.

Heave is likely to be progressive, as the clay will continue to take up moisture following tree removal until it finds a new equilibrium. This can result in upward movement of foundations over a number of years with the damage resulting to the buildings often being substantially more severe than that resulting from subsidence.

Removing trees then is no simple solution and should not be undertaken lightly. Each case has to be treated on its merits. And other factors should also be taken into account:

  • The nature and depth of the building foundations
  • Is the site flat or sloping?
  • Is there evidence of ground water?
  • Is the nature of the subsoil consistent across the site?
  • Variations in climatic conditions
  • Is the ground around the building pervious or impervious?
  • Are the foundations of extensions, porches, outbuildings, etc different from those of the main building?

How To Deal With Subsidence

A helpful Blog from Peter Barry, surveyors, explains that it is neither necessary nor practical to simply remove all trees and shrubs and underpin all houses. They explain that whilst we have to treat each case on its merits, we have to bear in mind that most subsidence is just cosmetic and may have no significant effect on the structural integrity of the building or its function. Redecoration may be all that is needed! Read the full post here >>

Also, this type of movement tends to be cyclical and long-term monitoring will often show that the problem has resolved itself without any drastic action being taken. In some cases, where different parts of a house have different foundations, it may be appropriate to simply introduce expansion joints into the structure to prevent future damage.

Armed with this information then, you should always challenge Insurers who insist on ‘implicated’ trees being removed. You should point out that removing trees may help or it may not and could even make the situation worse if heave results. Quote Peter Barry, a well-established firm of Chartered Surveyors with decades of residential surveying experience, who state clearly that tree removal is always somewhat experimental and you can’t guarantee what effect it will have. They say they would always prefer to retain mature trees, even if this involves underpinning a property affected by subsidence, rather than remove the trees and run the risk of long-term heave affecting potentially several adjoining properties.


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