Paul Vernon · Surveyor (retired)
You look at a building and think that it’s solid as a rock but all is not always as it seems. There may be hidden problems that have yet to manifest themselves, or the evidence may already be there for you to see when you view a house. Building subsidence occurs when the ground beneath a building sinks, pulling the property’s foundations down with it. I’ve written previously on what I have called Natural Subsidence where subsidence occurs as a result of the nature of the underlying rocks. What I’m discussing here is subsidence that can arise as a result of actions in the ground at the near surface. It usually occurs when the ground loses moisture and shrinks, which can be caused by prolonged dry spells, or when the ground gains moisture and swells, although, it can arise as a result of incompetent foundations.
Subsidence occurs when the ground underneath a building sinks and as the ground moves the building’s foundations can become misaligned. It is particularly problematic when the ground under your property is sinking at different rates. Subsidence is not the same as heave, which is where parts of the ground under your home shift upwards pushing the foundations higher. It is also different to landslip or landslide where the ground your home was built on moves down a slope or is washed away.
It is also worth noting the difference between subsidence and settlement. Subsidence is caused by the downward movement of the site on which a building stands – where the soil beneath the building’s foundations is unstable. Whereas settlement is a downward movement as a result of the soil being compressed by the weight of the building within ten years of construction. It is important to know the difference as your insurer may not cover settlement.
There is also the risk of ground movement at very shallow depths. Fine-grained, clay-rich soils can absorb large quantities of water after rainfall, becoming sticky and heavy. Conversely, they can also become very hard when dry, resulting in shrinking and cracking of the ground. This hardening and softening is known as shrink–swell behaviour and is the reason that buildings have foundations - to ensure that seasonal shrink and swell in the upper layers of the soil do not affect the integrity of the building. Early buildings were built on limited, if any, foundations - in many cases, the grass was simply scraped away and then the building works commenced. This means that older buildings are more at risk from seasonal shrink and swell. In contrast, modern houses that are built on good foundations should usually be less at risk from this effect.
For all practical purposes, whilst the root causes will be different, the impacts on a building will be similar, although the effects of shrink and swell from no / limited foundations may be seasonal and rectify themselves temporarily only to recur when conditions change.
How does it manifest itself?
As the ground moves it is possible that the foundations will be able to absorb any movement and you may not be aware of any changes. However, this is unlikely unless the foundations have been specifically designed to do so.
If a building has been affected, in most cases, you’ll notice a crack in the structure of the house, and a crack caused by subsidence or heave is likely to be:
- Wider than 3mm – that’s the width of a 10p coin.
- Visible on both the outside and the inside of the building.
- Diagonal and usually wider at the top than the bottom.
- Actually runs through individual bricks or stones.
- Located close to a door or window.
Cracks aren’t the only sign that you may have a subsidence problem. You should also look out for doors and windows that stick as this could be caused by the frames warping as part of your house sinks. Keep an eye on wallpapered rooms for signs of rippling at the wall and ceiling joints. You may also spot cracks where an extension has been joined to your main home.
What Causes it?
This can be a natural seasonal occurrence or can be enhanced by various means including:
- normal seasonal movements associated with changes in rainfall and vegetation growth
- enhanced seasonal movement associated with the planting, severe pruning or removal of trees or hedges
- changes to surface drainage and landscaping (including paving)
- short-term, unseasonal movements as a result of leaks from water supply pipes or drains
- long-term subsidence as a persistent water deficit develops, this may be from natural causes or as a result of water abstraction.
- long-term heave as a persistent water deficit dissipates.
To expand on this, there are a number of factors that can increase the chances of subsidence:
- Trees – If you have trees or large shrubs planted too close to your home they can cause subsidence as the plant drains the moisture from the soil causing it to dry out and sink. Estimates suggest that around 70% of all subsidence cases are a result of tree roots absorbing all the moisture out of soil.
- Clay – I’ve already touched on this. This type of soil changes a great deal with the weather. When it is hot and dry it can shrink, crack and shift which makes the ground unstable and there is a greater risk of it sinking.
- Drought – If you live somewhere that is prone to a drought then the soil could dry out which increases the chance of subsidence.
- Leaks – A leaking drain, water main, sewer or culverted watercourse can soften the soil, or wash it away, causing sinkage.
- Age and construction – If you live in a period property there may be a greater risk of subsidence as your house may have shallower foundations than a more recently-built home. However, the flip side of this is that older properties tend to be built from bricks and lime mortar which could make them more flexible and less likely to be damaged by the ground shifting beneath them.
- Mining – I’ve discussed this in another briefing note and it is one of the more well-known causes of subsidence. If your house has been built near to a former quarry or pit, then it could be unstable as the material used to fill the site will shift as it settles or decomposes. Your house could also be affected if mining activity occurs close by. You can purchase a subsidence claims report from the Coal Authority to see if your property is affected by coal mining.
Where does shrink–swell happen?
The rock formations most susceptible to shrink–swell behaviour are found mainly in the south-east of Britain. Here many of the clay formations are too young to have been changed into stronger mudrocks, leaving them still able to absorb and lose moisture. Clay rocks elsewhere in the country are older and have been hardened by burial deep in the Earth and are less able to absorb water. In some areas (e.g. around The Wash and under the Lancashire Plain) they are deeply buried beneath other soils that are not susceptible to shrink–swell behaviour and so are not considered an issue. Superficial deposits such as alluvium, peat and laminated clays can also be susceptible to soil subsidence and heave (e.g. in the Vale of York and the Cheshire Basin).
What can you do to prevent subsidence?
If you live in a house that is at risk of subsidence, then there are a couple of steps you can take to reduce the chances of your property sinking. Firstly, keep the trees at a safe distance. Don’t plant any trees within 10 metres of your home and particularly large trees should be no closer than 40m. The Association of British Insurers (ABI) has this guide to how far away different popular tree species should be from your home:
Secondly, try to catch excess water to avoid your soil from becoming waterlogged by using water butts to collect rainfall. Also, keep your guttering, pipes and plumbing well-maintained to avoid leaks.
What can you do when looking for a house?
When you are viewing properties keep an eye out for the signs of subsidence. Ask the seller and estate agent directly if the house has suffered from subsidence. If you are interested in buying a property that you suspect may have subsidence get a full buildings survey done, this should inform you if there is any risk of subsidence. Your mortgage lender’s valuation is not a survey into the condition or structure of the property so will not highlight subsidence; you will need to get a full survey done yourself.
If repairs for subsidence have taken place in the past, your conveyancing solicitor should get legal documents from the vendor to verify that the repairs were done to the standard set by the Building Research Establishment. These documents should include a formal Completion Certificate, which is issued by the council if the property has been underpinned, and a Certificate of Structural Adequacy, which should have been created by a building surveyor if the repairs were part of an insurance claim. Many repairs come with guarantees which you will want to have passed to you. Your conveyancer will be able to find out who the property is currently insured with and for how much. You should shop around to see what the options are as your choices may be limited or policies more expensive. Remember, that you will need to insure your property going forward and if the insurers consider that there is an increased risk, then their premiums will reflect that.
It’s not always doom and gloom.
There may be instances where you can see some of the indicators that I’ve mentioned, but where there is no cause for alarm. It’s not unusual for hairline cracks to form in plaster as it dries out. If the walls are not papered, then you may never get rid of these, and they are nothing to worry about. You might also experience something similar in brickwork and rendering. .
Would I buy such a house?
Well that’s the $64,000 question. It depends.
If the house was showing signs of subsidence that hadn’t been properly and successfully rectified, then almost certainly not. Until you’ve fixed it, you won’t know the extent of the work required to put right the damage and stop it happening again.
If all you can see are signs that the property has historically experienced subsidence but you can be assured that it’s all been properly rectified then perhaps. But I’d want my surveyor to be able to assure me that the chances of recurrence were very low. My solicitor would also need to be happy that relevant receipts and warranties were up to scratch.
Your lender will require you to have buildings insurance in place, and if the property has suffered subsidence then the premium may well be loaded. In fact, you may find it prohibitive, and this will be an ongoing cost, so you should get firm quotes. Remember, if you are thinking twice about whether you should buy a property for technical reasons, then when the time comes to sell, your potential buyers may too. You could end up with a property that you can’t sell.