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  • A guide to subsidence

    A guide to subsidance

    What is subsidence?

    Subsidence affects almost as many people across the UK as flooding. It is known as the vertical downward movement of building foundations caused by the loss of support of the ground beneath. This movement can then in turn cause cracking within a building above and potentially substantial property damage.

    Causes of subsidence

    The main cause of subsidence is from soil shrinkage. Often clay soils are the most common cause of subsidence where they shrink and swell dramatically depending on how wet they are as they are extremely responsive to water. The structure of clay means that when mixed with water the spaces between the soil are pushed apart and cause a swell, whereas other soils work to simply allow the water to occupy the spaces in-between particles. Distribution of clay soils across the UK is particularly high in London with very limited instances in Scotland and therefore you can often find more cases of subsidence in the Greater London area. As the soil shrinks it pulls the foundations which can result in structural damage.

    In addition to clay soils, man-made soils can cause just as much damage on sites that have been in-filled where the composition of materials can degrade over time. Although in reality this is classed as settlement, it will still fall under subsidence when it comes to subsidence insurance.

    Trees and shrubs will also draw allot of water from soils causing shrinkage in the soil and often roots are pushed further under properties in search of more water. Subsidence from trees can occur especially when trees are planted in clay soil as trees draw thousands of litres of water from the soil each year. Clearly some species of tree affect the soil more than others, and Oak, Willow, Poplar and Eucalyptus are all able to root to a good depth even in clay soils causing the most damage. Each species has a recommended safe distance from a property dependant on how far their roots extend in search of moisture, so it is important to consider this when planting a tree or buying a property with a tree already close by.

    Another cause of subsidence can be leaking drains or water mains. In the event of a leak the surrounding ground is then softened and its load bearing capacity is compromised as well as non-cohesive soil being washed away.

    A few different man-made instances can also trigger subsidence including any excavations around the foundations, vibration from things such as mining or man-made water table changes such as in-filling a well or building a man-made lake.

    Heave and landslip, which can cause subsidence, are normally covered by standard buildings insurance.

    What you can do to reduce risk of subsidence

    It is important to be aware of the composition of the soil surrounding your property as well as the depth of foundations. This will be a good starting point in addressing your risk.

    Consider the ‘zone of influence’ on any trees close to your property. If necessary you could investigate relocating a tree with the professional help of a qualified tree surgeon. Alternatively, on trees that cannot be moved you could consider ‘Crown Thinning’ or ‘Crown Lifting’ which removes branches either throughout the tree or at the base to reduce the moisture uptake. In addition to considering the position and species of trees surrounding your property you should be aware of shrubs and in particular avoid wisteria, pyracantha or rose shrubs near to your house or if need be place them in containers.

    Regular checks should be made for blocked, leaking, or split drains with any build up cleared from gutters. This is particularly important for older properties which have drains made from clayware pipework, which is more prone to damage. Manhole covers should also be regularly inspected for drainage to reduce any risks of subsidence from leaking.

    Signs of subsidence

    Normally the evidence of subsidence will present itself in the form of cracks or expansion of cracks in the walls, ceilings or outside brickwork, rippling of wall paper, or sticking of doors and windows due to distortion of the building. Cracks occur in buildings as a natural part of settlement, however if your cracks appear larger than 3mm wide and are sudden then there may be more cause for concern. In particular you need to watch out for diagonal cracks that are mirrored both in the internal plaster and on the brickwork outside, and especially if these cracks widen at the top. Or if cracks occur after prolonged dry weather.

    What can you do if you do have subsidence?

    Firstly check that you have specialist subsidence insurance. If you are unsure then you can call your insurer and they will often send out a Structural Engineer or Building Surveyor who will be able to confirm if you are suffering from subsidence and advise on the likely cause.

    You may need to monitor the property over a period of time, reviewing the cracks and how they change and respond to weather conditions. This can now even be done using intelligent devices fitted to your property. In extreme cases the house may need to be underpinned, which involves re-work on the foundations. But this is very rare and you may just need some repairs carried out, ranging from plaster repair and repointing to major construction works to internal and external walls and realignment of windows and doors. Any cracks from 5mm width and above are considered more serious and will require some level of reconstruction.

    Getting insured after you have already made a claim…

    Many standard insurers will be wary and unwilling to insure on a property known for subsidence problems, and in these cases you are likely to need to contact a specialist insurance broker with expert experience. Their specialist knowledge will ensure that you are correctly covered moving forward and they will be able to advise on policy features that will be necessary.

    If you discover subsidence after switching provider then under an agreement drawn up by the Association of British Insurers (ABI) then the previous insurer is responsible for any claims identified within 8 weeks. If it is between 8 weeks and a year then both are equally responsible, and a year after the new insurer is completely responsible. New build properties are covered under their ten year NHBC warranty.

    Want to find out more about Bluedrop's Subsidence Insurance?
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