There are a number of urban myths and misconceptions about subsidence which this article seeks to shed light on.
Disclaimer; as always we are not offering legal advice and circumstances all differ and should be considered individually.
1. “It’s not subsidence – it’s just a bit of normal seasonal / differential movement”;
Any foundation movement (however small or normal) due to clay shrinkage beneath the foundation is “subsidence”. Buildings built on clay typically have foundations which are deep enough to avoid movement due to clay shrinkage (unless roots grow beneath them) and have been so for many decades. Most buildings on clay successfully exist without structural cracks ever developing. Some (particularly much older ones on shallower foundations) do undergo some seasonal clay shrinkage movement which causes minor cracks to open and close seasonally, and this is on the rise with climate change. Very often it is exacerbated by minor vegetation growing roots beneath the property, but in rare cases it can happen without any vegetation being involved simply due to the effects of surface-evaporation. This is still “subsidence”.
Differential movement (as opposed to uniform movement) just means that one part of a building foundation moves differently from another – usually resulting in a crack developing between the two. The phrase just describes the way a building is behaving, it does not seek to address the question of why it is behaving that way. “Differential settlement” for example describes where foundation settlement (see below definition of that) occurs at a different rate or magnitude in one area of a building (such as an extension for example) from another. This is usually not subsidence – though confusingly it can be in some circumstances – such as where the ground is weakened by a water leak long after initial bedding-down settlement is complete, and the building re-settles (or subsides) into the newly weakened ground.
Differential movement can also describe structural behaviour that is not associated with foundation movement at all – such as thermal movement in parts of the property built from different materials (i.e. different types of brick or block).
So differential movement doesn’t tell you what the cause is – it just describes the symptom.
Seasonal movement just means cracks which open and close seasonally – again describing the pattern of movement rather than the underlying cause. It is often associated with clay shrinkage, but not always. It could be seasonal thermal movement in masonry for example.
2. “It’s not the trees – it’s the clay / weather”;
Clay shrinkage subsidence is (in the vast majority of cases) due to the combination of roots (from any significant vegetation – not just trees) beneath foundations, in clay and during hot weather. There are some (very few) cases where foundations are exceptionally shallow and you get a small amount of shrinkage beneath even with no vegetation present, but these are very rare – though on the increase with climate change. We also see cases where the root growth or clay shrinkability is sufficiently low that cracks only occur (or widen) during very hot weather and not in cool / wet summers. But in the vast majority of cases it’s due to roots from something or other growing beneath the foundations and causing the clay to shrink – which would not happen if the roots weren’t there (or were dead). Hence vegetation control is the normal and usually cheapest solution.
3. “Its not subsidence, it’s settlement”
Subsidence is the downward movement of the ground on which the buildings stands – not arising through the loads imposed by the building. Settlement is the “squashing” of the ground under a building due to loads imposed on it not being spread onto a sufficiently wide base.
Clay shrinkage is therefore always subsidence if it is sufficient to cause cracking or noticeable distortions. If a building moves uniformly due to seasonal clay shrinkage without suffering any damage, then it would not generally be regarded as subsiding in our view.
The line between settlement and subsidence is blurred when it comes to a subsoil being weakened (for example by water entering the soil later) so part of the building then settles a bit more into the weakened ground. That is regarded as subsidence as although it is directly associated with the imposed load, it was brought about through an independent external factor (i.e. a leaking drain) and was not inevitable from the moment of construction.
4. “You need a Structural Engineer to deal with a subsidence issue”.
Structural engineers are not routinely trained in subsidence. It is not part of the core competency framework of the Institution of Structural Engineers or indeed of any professional body. Structural engineers are trained in structural design and analysis and the vast majority of them have no experience with subsidence.
Building Surveyors are trained in building pathology and surveying defects – and should be able to identify the likely cause of cracks but beyond that most would not routinely be involved in dealing with investigating and resolving subsidence.
Subsidence is a niche skill which some building professionals pick up and specialise in. Most are either structural engineers or building surveyors (sometimes both) but the list is by no means limited to those backgrounds. Subsidence experts can come from many other professions such as;
Construction
Architecture
Other areas of engineering (i.e. geotechnical, civil etc.)
Other areas of surveying (quantity surveying, general practice etc.)
Insurance (i.e. specialist subsidence loss adjusting)
The background is less important than the years of relevant experience specialising in the field of subsidence.
Chartership in a particular profession is proof of a level of professional training and experience but does not prove competence in subsidence matters – far from it.
Membership of a professional body does usually require adherence to a Code of Conduct set down by that body – who might well also be willing to investigate complaints against a member and apply sanctions in appropriate circumstances.
Professional indemnity insurance is of course important and the subsidence expert you select should be able to show you proof of current P.I. insurance.
5. “Subsidence cracks are tapered”, or “it’s not subsidence unless you can fit a pound coin in the crack”, etc.
There are so many such “rules of thumb” written on line about this – none of which are reliable. Some have some general truth about them but there are always exceptions and it is risky to make decisions or assumptions based on them. Crack diagnosis is a specialised skill which takes years of experience to become adept at. See following link for more details;
All big cracks start as small ones. Some small cracks stay small and never progress, others get bigger over time. A trained professional should be able to identify whether a small crack is likely to become bigger and / or present a problem either structurally or legally (ie with insurance, finance or a property sale / property value).
Some people prefer to wait and see if a crack develops before spending money getting it professionally assessed – which is a completely valid approach, but not such a good idea if it needs to be disclosed to an insurer or might present a legal problem. In this day and age of heightened anxiety about cracks (amongst insurers, lenders and buyers in particular), it is often more sensible to get an early professional diagnosis which can be relied on.
6. “If it’s not subsidence, you dont need to tell your insurer”
Some policies only require you to notify them if your cracks are due to subsidence, but many require you to notify them of any cracks you see in structural or load-bearing or external walls, some insurers require you to notify them of any cracks at all. Insurance wordings vary considerably (and are forever changing) so it depends who you are with and what their precise wording says at the relevant time. See the following article for more details;
Should I disclose that minor plaster crack to my buildings insurer?
7. “Removal of the tree will make it worse by causing heave”
In the vast majority of cases, no it won’t. Heave is a problem which can occur in very specific circumstances which hardly ever apply these days. Basically it’s when you build a building on top of the root system of a big old tree in highly desiccated clay, then the tree dies or gets removed (or is removed just before construction). It was more of an issue in the 1970s and ’80s when there wasn’t so much known about it (yet there was much building on previously-wooded clay sites) but since then foundation design standards have been such that it has been practically designed-out of existence – with some uncommon exceptions. Anything built before then would have suffered problems long ago.
“Heave” is often confused with “recovery” in clay soils. Both are exactly the same phenomenon – re-swelling of desiccated clay causing expansion and upward lift of foundations, but we call it “recovery” when it can’t lift the building back beyond its stable starting-point, and term it “heave” when it is so extensive that it is capable of forcing part of the building up higher than it was when it was first built. Even when heave is a risk, tree removal might still be the best option in some cases. A heave potential analysis can be carried out on the soil which estimates the amount of uplift that could be expected in the clay at a particular point (measured in mm) so you can see whether or not the building is likely to be able to tolerate that without undue distortion taking place.