Subsidence occurs when the ground beneath a property begins to sink downwards. Subsidence can be caused by both environmental and property-related factors and can be extremely expensive to fix.
So, what causes subsidence in the first place?
Subsidence is commonly caused by one or more of the following issues:
- Soil type
- Nearby trees
- Water leaks
- Poor foundations
- Climate & weather
Although certain causes cannot be avoided, it is useful to know what can cause subsidence so you can look out for warning signs and catch the problem early. Prevention is better (and much cheaper!) than cure after all.
Let’s look at each cause in more detail and find out the best ways for you to reduce the risk of subsidence in the future…
1. Soil type
You’ve probably never given much thought to the type of soil your home is built on. However, soil type can be the biggest cause of your subsidence problem.
Soil types can be sorted into two categories:
- Cohesive soils, such as clay and silt.
- Non-cohesive soils, such as sand and gravel.
Each of these soil types can cause subsidence in different ways.
1.1 Cohesive soils
Cohesive soils, such as clay and silt, can absorb large quantities of water. This means the soil below your home will expand and shrink depending on the moisture content.
With cohesive soils, it is usual to have seasonal movement. Here the soil below a home will shrink in the summer when the weather tends to be dry and hot, and then swell again in the winter months when it tends to be wetter.
You may find that cracks in your home appear in the summer months and close again during the winter months.
"Shrink-swell" can become a subsidence problem when there are prolonged dry spells of weather. In this case, the soil remains contracted under the property during the unusually dry winter months and shrinks further when the warmer weather returns. This is known as soil shrinkage and can cause the foundations underneath your home to shift.
Soil shrinkage can be made worse when there are trees and large shrubs surrounding your home, which will suck all available moisture from the soil.
As well as subsidence, cohesive soils can also be the cause of “heave”. Heave is basically the opposite of subsidence where the ground levels below a property rise.
Cohesive soils may swell when they become waterlogged as a result of prolonged wet weather, flooding or burst water pipes. Heave can also be caused when a large nearby tree is removed as the moisture content is no longer being absorbed through the tree’s root system.
Cohesive soils are most common in the South East of England, particularly Greater London.
1.2 Non-cohesive soils
Non-cohesive soils, such as sand and gravel, are much more permeable meaning water will run through it rather than being absorbed.
Non-cohesive soils don’t shrink or swell dramatically like cohesive soils but they can be washed away (also known as leaching) if there’s a burst water pipe, leak or flooding below the foundations of the property.
1.3 What type of soil is my house built on?
If you want to find out what type of soil your home is built on, you can use the BGS’ Geology Viewer. Click on the search icon on the top right of the page, enter your postcode or location and then click on the map to bring up the bedrock geology.
We searched for Clapham Park in the above example, which has a bedrock geology of “London Clay Formation” made up of clay and silt.
Certain causes of subsidence are more prevalent in specific areas of the UK than others. Properties in the South East of England are more prone to subsidence movement than the rest of the UK due to the clay soil that is commonly found there and a warmer, drier climate.
2. Nearby trees
Trees are another common cause of subsidence in homes across the UK.
Not all trees will cause subsidence. If you do have a tree in your garden that you’re concerned about, you should consider the following factors that may increase the risk of subsidence:
- Type of tree - the trees most often involved in subsidence claims are oak, willow, ash, poplar, plane and sycamore trees.
- Distance planted from your home – what is considered a “safe distance” will depend on the type and size of the tree. We set out the “safe distances” for the most popular species below.
- Number of trees and large shrubs – more trees and shrubs equals more water absorbed from the soil beneath your foundations. If you have several large and thirsty trees planted near your property, your home may be at a higher risk of subsidence.
- Soil type – the risk of subsidence is increased if the tree is planted in clay soil, as this type of soil will shrink in volume when the surrounding tree roots absorb the available moisture from the ground.
- Climate – if there’s been a prolonged dry spell, trees will send out more roots to search for water and absorb any available moisture from the soil.
Certain trees have the capacity to grow larger and absorb more water than others. You must ensure that you plant any trees a “safe distance” from the property, which will vary depending on the species.
The Association of British Insurers have put together a useful table setting out the safe distances for each popular species of tree.
2.1 What should you do if a tree is causing subsidence?
If a specific tree has been found to be the main cause of subsidence, having it felled may be a suitable and permanent option to remove the risk of further damage in certain cases.
Make sure you consult with a tree surgeon before grabbing your chainsaw though…
Sometimes removing a tree can actually do more harm than good for your property. When a large tree is removed from an area, the soil below it can become saturated over time with excess water that was previously absorbed by the tree’s root system. If you live in an area with clay soil, this can expand and cause the foundations of the house to rise in places (aka “heave”).
An alternative solution recommended by the Royal Horticultural Society would be to manage the problem by reducing the tree’s size. This is done by pruning the crown volume. RHS states that to be effective, “pruning needs to reduce the crown volume of the tree by at least 70% and be repeated on a regular basis such as every three years”.
A tree surgeon will be able to recommend the best course of action for your particular case.
3. Water leaks
Another cause of subsidence is burst pipes, leaking drains and gutters.
Subsidence can be caused by water leaks in one of two ways:
- Water leaks can wash away fine particles in the soil underneath the foundations of your property if your home is built on non-cohesive soil.
- Water leaks can saturate the soil under your house’s foundations if it’s built on cohesive soil. Foundations can either give way and subside or rise and cause another type of ground movement called heave.
With this in mind, it’s worth clearing out your gutters every autumn to prevent build-up and maintaining pipes and drains surrounding your home. If you suspect a leak, make sure you act fast to avoid any long-term damage to the foundations of your home.
4. Poor foundations
Older buildings can be more susceptible to subsidence, especially if they are built on clay soil. This is because homes built in the Victorian and Edwardian eras tend to have simple shallow foundations.
The most significant changes to the soil’s moisture levels and temperature happen at the surface. If the property’s foundations are shallow, the soil below the foundation of the property is more likely to suffer from more significant shrinkage or swelling when the soil contains clay.
However, older properties also tend to be built with more flexible materials and so can deal with a certain level of seasonal movement.
Subsidence issues can also arise when a person decides to add an extension to their older property. The original property will have a shallow foundation but flexible building materials. Newer extensions will have deeper foundations to comply with the Building Regulations introduced in the mid-1960s but will be built with more rigid materials.
When there is a lack of consistency in foundation depth, the two parts of the home can move at different rates and therefore cause damage and cracking.
Peter Barry from Peter Barry Chartered Surveyors recommends introducing “a properly formed expansion joint between the two different sections of the building”. This will allow each part of the building to move at different rates without causing any damage to the structure.
Historic mining can cause subsidence when the earth beneath or surrounding your home’s foundations has been weakened by mining works or from underground mines that have collapsed.
If you are buying a home, you should consider getting a Coal Mining Search if the property is located in a former or current coal mining area, such as Cornwall. This report will reveal if the area has a known history of mining and will flag any possible hazards. It will also tell you if there have been any subsidence claims for neighbouring properties within 50 metres of the property boundary.
6. Climate & weather
The weather and climate can cause subsidence if a property is built on clay soils, such as those found in Greater London.
Dry weather and high temperatures will cause the clay soil beneath the foundations of a property to contract and crack. This is made worse if there are trees surrounding the property, as they will be searching the soil for any available moisture through their roots.
If the soil is contracted for an extended period of time, the foundations of the property can become unstable and signs of subsidence can begin to emerge.
As we have seen from the heatwaves in recent times, our climate is becoming more unpredictable. Prolonged periods of hot weather and drought may start to become a common occurrence which will have a knock-on effect on the number of subsidence issues and claims made.
My advice on how to reduce the risk of subsidence
Whilst you cannot avoid certain causes of subsidence, there are things you can do to reduce the risk of subsidence.
- Do not plant trees or shrubs near to the property.
- Regularly prune trees and large shrubs with the advice and guidance of a tree surgeon.
- Regularly maintain pipes and clear debris from gutters.
- Act fast if you spot or suspect a leak.
- When adding extensions, ensure the soil type and foundation depths are considered.
If you suspect your home may have subsidence, you should contact your insurer as soon as possible.
Can you sell a property with subsidence?
If you’re trying to sell a property with subsidence, don’t panic! It can be done…
You can sell a house with ongoing or historic subsidence, but it can be trickier and take a lot longer than selling a problem-free property.
For a lot of people, your best option will be selling by auction as it can offer a faster and more certain sale, and potentially a better selling price. If you think this could be a good option for you, hit the red button below to learn more.
Finally, if you want to read more about selling your property with subsidence by auction, read our Auction Case Study: Finding out your property has subsidence…when you’re 9,000 miles away.
By Matthew Cooper, Co-Founder of Home Selling Expert