This article provides a general introduction to a broad topic, what is subsidence, while subsequent articles will look into more technical aspects. It aims to familiarise the average person with the concept, explaining what it is, where it commonly occurs, and offering suggestions on how to handle it.
What is subsidence
Subsidence is the general term used to describe the downward movement of the ground and consequently a building resting upon it. More specifically we normally refer to the consequent cracking or structural damage that results.
Subsidence causes vary, depending on the type of soil, trees, drains and type of foundations beneath the property.
Most subsidence takes the form of minor cracking which is easily repaired from time to time but in extreme cases more significant repairs are required. Sometimes the cause of subsidence can be dealt with but on other occasions it cannot realistically be completely eliminated in the future. See later.
Why do we worry about subsidence?
The term subsidence can sound overly dramatic as it conveys images of buildings sinking into deep mining holes are sliding off steep cliff faces and generally causing catastrophic damage to our valuable homes.
In reality the above is rarely the case and particularly in London and the Home Counties where the geology is relatively uniform and well known, albeit prone to perhaps more frequent but minor damage in most instances.
The term subsidence has repercussions depending on circumstance and status eg whether owning, buying, selling or renting property and some of the main concerns for each of these status are as follows;
- Repair costs might be significant if the damage is not covered by Building Insurance and more specifically Subsidence Insurance. The normal policy excess is typically £ 1500.
- Making a Subsidence claim can be detrimental to future insurance premiums. In some instances, Insurance companies withdraw Insurance cover after making a claim which is very harsh.
- If minor cracking damage reoccurs regularly but does not exceed the policy excess, year on year repair costs can mount up and reoccurring cracking is a nuisance visually.
- In extreme cases internal repairs or strengthening such as underpinning or piling would necessitate moving out of the family home for many months whilst repairs are carried out. Whilst this is extremely rare in practice, the potential of this risk is what triggers a natural fear of subsidence.
For those selling
- Subsidence damage may prevent or restrict the sale of a property in terms of market appeal and potentially lower the sale price. See below.
- It is important to obtain professional advice from a Chartered Structural Engineer prior to sale, which might be shared with a prospective buyer, in order to smooth the sale process.
- Many buyers will simply be put off buying a property with a current subsidence problem, for fear of costs of repairs and disruption to day to day living.
- It is difficult to obtain a mortgage, and Buildings Insurance, against a property with a current or ongoing subsidence problem even if the damage is minor, for fear of it becoming worse and expensive to resolve.
- A structural engineers eeport is almost bound to be required for any property showing recent signs of cracking, which might indicate a current or ongoing subsidence problem.
Estate Agent’s advice note
We always encourage Agents to recommend that a vendor obtains a Structural Engineers Report from a professional structural engineer when there are obvious cracks and signs of subsidence or historic settlement cracking, in order to either inform the owners repairing options pre marketing for sale, or alternatively, to share the report with prospective buyers on an open book basis so they can be informed at the earliest opportunity. This avoids wasted time and money later and can avoid disappointment all round.
Depending on the type of RICS report and on the experience and knowledge of the Surveyor, it is common for the General practice Surveyor to refer matters of significant structural cracking to a Chartered Structural Engineer and they will recommend a report by such an expert.
A brief description of subsidence
Subsidence is the general term used for “sinking” of the soil and the consequent movement and cracking of the building on that sinking soil mass. Whilst it might sound rather dramatic the day to day reality is that in most cases that amount of subsidence is measured in a few millimetres rather than centimetres or inches so damage is normally fairly modest structurally.
Subsidence causes vary, depending on many factors as above but in some 90% of cases in London and the Home Counties it is usually when a period house has shallow foundations resting on a shrinkable clay subsoil AND with medium to large trees nearby. There are exceptions however, which will be discussed in more detail in a subsequent paper.
A clay soil is a bit like a “blu tack” or plasticine and is mouldable by hand so is not very strong compared with bedrock, sands or gravels. More specifically, the clay comprises a large volume of tiny water molecules and it is the varying water content from one season to the next, or when sucked dry by tree roots, that causes a reduction in volume of that soil mass. This results in a downward movement of the soil together with that part of the building resting upon it.
In loose granular soils like sand, silts and fine gravel, if nearby drains are broken and fractured, the soil can fall into the drain pipes and be washed away, causing weaknesses in the soil beneath the foundations; a further but less common cause of subsidence.
Subsidence occurs regardless of any weight loading. For example, the ground under a garden path or lightweight conservatory might subside in periods of drought.
Mining subsidence and underground water course erosion of chalk and limestone rock, found elsewhere in the UK, is very unusual in the Home Counties due to the differing geology and is not dealt with in this article.
Settlement versus Subsidence
Subsidence is not the same as settlement and the distinction is very important.
Briefly, subsidence occurs when the soil mass below a house moves downwards of its own accord eg in dry summers, whereas settlement only occurs when weight loading is applied and squashes the soil downwards eg when building an extension or raising a house higher.
Settlement commonly occurred in most pre Second World War housing because foundation design was not understood and whilst many house were and are still very robust above ground, they have little or no foundations as we would know then today.
As a general rule of thumb most foundations in homes built before 1920 were typically the most shallow at some 20 to 40 cm below ground level. Post 1920s until the 1960s many house were founded at about 60cm deep
Only after the drought of 1976 did foundations routinely extend to 1m below ground and deeper in some instances, as the construction industry learnt of the dangers of tree roots causing subsidence.
In extreme cases when building near very large oak or poplar trees, new foundations were on occasions built to 2.5m deep or alternatively are now commonly supported on concrete stilts called piles instead, which extend much deeper into the soil.
Settlement takes place fairly immediately on granular soils like sand and gravel, usually within a year or so, and within the first five years or so on clay soils. For period houses, settlement is therefore by definition historic.
Settlement only rarely occurs in modern extensions and new houses due to the demands of modern building Regulations and design office Eurocodes used by Structural Engineering designers for their calculations of beams and foundation sizes.
We do see some settlement in poorly constructed ground bearing slabs for extensions when the ground is not sufficiently well compacted.
All this explains the importance of obtaining proper “design” advice from a structural engineer together with achieving Building Regulations Approval, including the completion certificate when works are completed.
What you should do if you think you have subsidence
There are two general options but these might be influenced by a number of factors mentioned further below;
- Notify your Insurance Company and hope they deal with it professionally.
- Obtain an independent report from a Chartered Structural Engineer
Historically subsidence claims were dealt with via a panel of independent Structural Engineers overseen by Loss Adjusters on behalf of Insurance Companies. This has all changed and almost all cases are now dealt with “in house” by Insurance Companies and “term contractors” with mixed success as far as the homeowner is concerned.
As a house owner your probably have Subsidence Insurance as part of your Building Insurance policy. If you are a leaseholder, the Insurance is probably via the freeholder.
Most Insurance policies have an excess to pay upwards of £ 1500.
If you start with your insurance company you will not incur any initial costs but may or may not receive the correct technical advice as Chartered Structural Engineers will not be involved in most instances.
Alternatively, you might initially seek the advice of an independent Chartered Structural Engineer with professional fee costs of the order of £ 1500, paid by yourself.
Some Insurance Companies now ask for a report from a Chartered Structural Engineer before they will visit or deal with a claim in order to establish whether there is likely to be a subsidence case to deal with or not.
Note not any damage or any cracks or movement are automatically dealt with by Insurance Companies. They do not deal with either pre existing, pre policy inception date, damage nor maintenance and wear and tear issues.
You are obliged to notify your insurers if your property suffers significant damage but you may or may not elect to make a formal claim in order for them to consider covering repair costs. Some people would rather fund modest repair costs and maintain a “ clean” insurance policy much like a “no claims” car insurance policy. This is particularly useful if you are intending to sell the property in the near future as a prospective buyer might be put off by news of recent insurance claims. In our experience Solicitors advising buyers are very nervous about recent Insurance claims.
A good independent Chartered Structural Engineers report will make clear whether you have a current subsidence issue or not and what order of costs might be involved in dealing with repairs. This will hopefully help inform you whether to make a formal insurance claim or not.
What Insurance claims and repairs do NOT do however is “improve” the property nor prevent similar damage occurring again in the future. Ie Insurance is about repairs rather than improvement. For example, the repair of cracks does not mean they will not reoccur one day. If, however, an offending tree can be easily removed or reduced in size this will undoubtably reduce the risk of a future re-occurrence at marginal cost.
In the past properties were frequently underpinned, deeper foundations were created by pouring concrete under the existing foundations, as part of a repair but this rarely occurs today as it is deemed an “improvement” to the house and is not often needed solely to “repair” damaged foundations. Only in very rare circumstances where a very large tree on neighbouring land or with a tree protection order placed upon it and where year on year recurrent damage is eminently foreseeable, might underpinning be considered as part of a repair.
Without going into detail now, in the majority of cases, cracks and damaged decoration are easily repaired and rooms redecorated.
Routine tree pruning or pollarding together with any drainage repairs are commonplace.
Cracks masonry must be repaired properly with either resin ( for fine cracks) or Cementitious grout for larger cracks. In instances where a wall has been substantially weakened and would benefit from additional tensile strength, some cracks are reinforced by stainless steel helibar rod reinforcement but only when specified by the structural engineer.
Repairs must be carried out by specialist repair contractors and approved helibar installers not general tradesman or builders ideally.
Most crack repairs are relatively inexpensive, certainly in comparison with very high property prices. As a rule of thumb, one or two cracks can be repaired for a £ 1,000 to £ 2,000 and some 20 or 30 cracks dotted around a large house could be repaired for less than £ 20,000 excluding redecorations, so the figures are by no means astronomical. Smaller cracks can be repaired within the normal Insurance policy excess, typically £ 1500 and whilst Insurance companies should always be notified of damage, there is often no financial reason to make an Insurance claim.
We are frequently surprised that lending institutions are so risk averse, at the slightest mention of subsidence, given how rare expensive underpinning is and how marginal most crack repairs are.
Most of us know, anecdotally, that many of London’s older housing stock has had some sort of minor subsidence in the past and either an insurance claim is on record or there are a few cracks in need of repair from time to time.
Unfortunately there is a big disconnect between the perception of subsidence and what it really means and this creates significant tensions during the buying and selling process which is related more to public perception and lending/ insuring implications rather than issues of structural engineering.
Our role as Chartered Structural Engineers in this arena is therefore as much about human perceptions and expectations as it is about structural engineering. A good Structural Engineer can hopefully help you with both.
This article has introduced the broad subject of subsidence and settlement and what it means for you whether owning, buying or selling property. It provides guidance on what you might do if you think you have subsidence and gives food for thought regarding the issues surrounding selling a property at this time.
A future article will shortly provide some information and guidance on what subsidence cracks might look like and when you might become concerned, if at all.