As a buyer, there’s nothing worse than having your heart set on a property and finding out later down the line that it has experienced issues with subsidence and has been underpinned.
And on the flip side, subsidence is one of those things that most homeowners keep their fingers crossed about and hope they never have to deal with — especially when it comes to putting their property on the market.
But regardless of whether you are a buyer or seller, it’s important to understand exactly what underpinning and subsidence really are and what they entail. Although the terms usually incite dread in those who hear them, they are not always detrimental to plans around buying or selling a property.
In this article, we’re going to walk you through everything you need to know about these terms, from what they mean to the impact they have. We’ll explore the different techniques used for underpinning a property and why in some cases, it’s necessary. We’ll also discuss whether you should still go ahead with plans to purchase a property if you find out it has been underpinned. So, let’s get started!
What is an underpinned house?
An underpinned house is a house that has had its foundations reconstructed or strengthened, usually due to subsidence — this is where the ground underneath a property shifts or sinks. Underpinning is a technique that corrects this issue (or similar ones) and stabilises the building.
Underpinning requires the careful removal of any loose soil that surrounds the property’s foundations. Once this has been done, new materials are used to replace the soil and stabilise the property. In this article, we’ll explore the different ways that a property can be underpinned.
Why is underpinning necessary?
Underpinning is necessary to stabilise a building that has been affected by subsidence and is continuing to sink. It is a preventative measure to ensure that the property does not sink further, potentially causing more damage to areas such as the foundations, walls, and floor.
The worst-case scenario is that subsidence causes irreparable damage, which leaves the property structurally unsound, and, therefore, uninhabitable. This is why it is crucial to underpin a property that is sinking before it is too late to rectify the situation.
While this may sound like a nightmare for any homeowner, it is important to remember that underpinning is typically a last resort when all other attempts to stabilise a property have failed. In fact, the Royal Institution of Chartered Surveyors (RICS) estimates that less than 10% of properties affected by subsidence require underpinning. This is usually because the issue can be resolved using other less time-consuming and expensive techniques.
But if you’re still concerned about your property and potential costs, it’s also worth noting that having a valid home insurance policy can help cover the costs incurred as a result of subsidence.
What causes subsidence?
Subsidence occurs when the ground beneath a building sinks or moves, moving the foundations of the building with it. This then destabilises the property. There are a number of situations that can cause subsidence, with the most typical ones being the following:
- Clay shrinkage: the most common cause of subsidence. This happens when the clay in the soil beneath a property dries out and shrinks. Typically, clay has a water content of around 35%, so long periods of hot weather can cause the volume of the clay soil to decrease dramatically.
- Nearby trees: large trees nearby property can sap water out of the ground and cause the soil to dry up and decrease in volume. This is also referred to as transpiration.
- Water leaks or floods: larger volumes of water from leaks or floods can physically wash away the soil beneath a building and make it unstable. These can also soften the surrounding soil, again causing it to shift.
- Soil features: soil features occur when the soil beneath a building erodes and causes an underground cavern. This then collapses, taking the foundations of the building down with it.
- Compaction of soil: all properties “settle” for a period of time after being built as the soil beneath them compacts. However, soil compaction can be more detrimental to a property and its stability due to aspects such as poor-quality soil or unsuitable materials being used to fill the ground beneath a building. These can cause the base of the property to sink too far and damage its overall structure.
- Decomposition of organic fill: organic fill in a property’s foundations can decompose and lose volume, causing the building to sink.
- Collapsed mines: although not as common, subsidence can be caused by the collapsing of a mine underground — even ones that have long been out of use.
How are houses underpinned?
There are a few methods used to underpin a building and stabilise it. However, they all aim to strengthen the existing foundations of a property to stop it from sinking or moving further.
1. Soil strengthening
Soil strengthening involves the careful removal of loose soil from under the property and the injection of a special resin to replace it. Once injected, the resin expands in the empty space around the foundations of the property and hardens.
This is one of the less expensive and disruptive methods of underpinning a property. It is also less labour-intensive, making it a popular technique to use.
2. Mass concrete
A traditional method of underpinning, mass concrete is precisely what the name suggests. The old foundations of a building are extended until they reach a stable layer of rock — known as a stratum. To achieve this, pits are dug underneath the existing foundations and then filled with concrete to strengthen them and provide a secondary layer of foundations. For this reason, this technique is suitable for buildings with shallow foundations.
Aside from being more cost-effective than many of the other methods of underpinning, one of the biggest benefits of mass concrete underpinning is that the property can still be used while the work takes place.
3. Beam and base
Also sometimes referred to as pier and beam underpinning, this technique is a progression of mass concrete underpinning. The reason is that mass concrete underpinning is not as effective for properties with deep foundations.
The beam and base method requires reinforced concrete beams to be placed beneath the building’s existing footing. This then transfers the load of the building onto strategically placed concrete piers or bases to stabilise the property.
4. Screw piles and brackets
This underpinning method is used when other more traditional alternatives are unsuitable. Underpinning a building using this method requires a trench to be dug around the foundations and screw piles to be driven into the ground to act as anchor points to support the property. Then, support brackets are attached to the screw piles and used to lift the building back into a stable position.
5. Jet grouting
A similar technique to soil strengthening, jet grouting is a method of underpinning where a stabilising fluid is injected into the soil beneath a property using high-pressure jets. The fluid then hardens within the soil and supports the property’s structure.
6. Cantilever needle beam
Another method of underpinning a property is through the use of a needle beam. This technique requires the building to have a strong interior column and space to deepen the foundation.
If this is the case, a pit is dug beneath the exterior wall of the property. A needle beam is then inserted and attached to join an exterior wall and an existing interior column. The property is stabilised with a hydraulic jack supported on Fulcrum, which pushes the needle beam.
Does underpinning affect my insurance?
Yes, if your house has been underpinned, your home insurance will be affected. As a result of underpinning, you’re likely to find it more difficult to get standard buildings cover. The reason is that insurance companies consider properties that have a history of issues with subsidence riskier to insure. This is because underpinning is usually the last resort for a property that has been damaged due to subsidence, but there there is no guarantee that the issue has been fully rectified.
However, it is still possible to insure an underpinned property. If you can’t get cover for your home under a standard policy, there are some insurance providers that offer specialist cover. When you take out a new insurance policy for your home, it’s important that you are honest about the work that has been done to the property and provide as much information about it as possible. Your insurance provider will usually want to know the following, where applicable:
- What caused the subsidence
- What work was carried out, and when
- Details of any previous claims made
- Structural issues
- Whether there has been any more ground movement
When you take out a home insurance policy, ensure the information you provide to your insurer is accurate. Omitting important information or lying on your insurance application will invalidate your policy and leave you footing the bill for issues later down the line.
Should you buy an underpinned house?
This will depend on the details of the property you are trying to buy. It is best to weigh up the situation before automatically rejecting a property that has been underpinned.
Before you decide on anything, you should get a complete structural survey carried out by a professional surveyor. They will be able to advise you on the extent of the issue and whether it is likely to persist in the future. Aside from this, you should also consider the following:
- Work carried out on the property: ensure that any work carried out to underpin the property had been done legitimately and by professionals. Ask for paperwork as well, where necessary.
- Cause of underpinning: some causes of subsidence can be resolved, meaning the house may have settled years ago. If this is the case, there’s no reason why you should reject a property.
- Selling the property on: underpinning can make it more difficult to sell your home. If you are planning on living on your property for a considerable amount of time, this may not be such an issue. But, if you think you will only be in it for a couple of years, you may consider alternative properties.
- Cost of insurance: insurance premiums for a property that has been underpinned will be slightly more expensive, but it is possible to get specialist insurance. It’s worth comparing quotes to find the best policy for your needs.
It is always worth doing your research on a property before you purchase — and even more so when it comes to buying an underpinned property. But so long as you have done the right checks on it and you have buildings insurance in place from the second you take over ownership, there’s no reason why you shouldn’t purchase it.
How do you know if your house needs to be underpinned?
In extreme circumstances, subsidence causes buildings to lean to one side as a result of the ground moving. But more often, the signs that a property needs underpinning are less obvious. Other indicators of subsidence may include:
- Cracks in brickwork or plaster: small cracks in your home are common and are usually nothing to worry about. However, do keep an eye on any cracks to ensure they aren’t getting wider, as this is a sign that the property is continually moving. Finally, large cracks (ones that are thicker than 2mm) visible externally and internally are often a sign of subsidence and should not be ignored.
- Torn or rippled wallpaper: this can be a key indicator of subsidence as wallpaper may rip or ripple due to the walls of a property moving. This can be noticeable around joints in the property too.
- Windows and doors that stick or won’t close: subsidence can cause door and window frames to warp and move out of alignment, making them stick or unable to be closed altogether.
Be aware that these signs are not always an indicator of subsidence. However, if you spot anything from the list above, it’s still always a good idea to seek professional advice from a structural engineer or qualified building surveyor. They will be able to establish whether your property has subsidence and needs underpinning or whether any other work needs to be carried out to resolve another issue found.
Summary
When looking to purchase a new home, you don’t need to automatically reject a property that has previously experienced issues with subsidence or been underpinned.
If you have been told by an estate agent that the property you are interested in has a history of subsidence issues, it’s important to have a full structural survey done and ensure that the property is unlikely to continue moving. If this is the case, there’s no reason why you shouldn’t be able to consider purchasing. But for extra peace of mind, it is always worth making sure you have suitable home insurance in place from the start of your ownership. This can help cover the cost of work needed to be carried out if you make a subsidence claim.
