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    Are You Seeing a Harmless Hairline Crack or the First Sign of Subsidence?

    If you’re in the process of buying a property, it makes good financial sense to get the building professionally checked out before you commit to this major financial investment. 


    By way of illustrating this all-important point, let’s talk about what happens when you spot a crack in the wall. Would you know just by looking at it whether the crack is a minor cosmetic blemish that you can simply fill, paint over and forget about? Or, whether it is a serious symptom of structural failure that needs immediate attention to protect the integrity of the building? Of course you wouldn’t.

    Which experts should you consult?

    Instructing an experienced local surveyor to carry out an independent building survey on your behalf is a good idea as they can provide comprehensive knowledge of the condition of the property. This will allow you to make an informed purchase decision. Chartered surveyors are also worth every penny of their time as it’s their job to spot a whole host of building problems that we ‘mere mortals’ simply would not be able to see. 


    While most surface cracks are perfectly harmless, cracks that are the result of heave or subsidence – the upward or downward movement of the site on which a building stands and where the soil beneath the foundations is unstable – can be extremely serious. So let’s take a look at what exactly causes walls to crack and what course of action you should take.

    Why do surface cracks occur?

    Let’s start with some good news. If you find a crack in the wall of a house, rest assured that these are pretty common and usually nothing to worry about; they tend to appear as a result of natural settlement.


    Freshly plastered walls, for instance, can throw up hairline cracks as a by-product of the plaster drying process. Older properties may be susceptible to cracking in walls as a result of fluctuations in temperature and humidity which leads to slight expansion and contraction – again, completely normal.


    Cracking can also occur in properties situated near busy roads and experiencing constant traffic vibration. And, modern buildings – whether brand new houses or building extensions – may require time for their foundations to settle under their own weight, with hairline cracks being a minor inconvenience.

    How are subsidence cracks different?

    Structural cracks are another matter altogether and should never be ignored or literally ‘papered over’. Pay close attention to the shape and size of an individual crack to see whether it could be an indicator of subsidence.


    Structural cracks tend to be wider than three milimetres and may have appeared suddenly, especially around windows and doorways. They often follow a diagonal pattern and typically widen out at the bottom. Also, check to see if the crack is visible from both inside and outside.


    Below are the four main factors causing subsidence cracks in a building:

    1. Tree roots

    Large trees planted close to the building are one of the worst culprits for subsidence, especially in houses built on clay soil. Tree roots can find their way under the buildings where they absorb moisture from the soil, which can destabilise the foundations and cause the structure to move. Large, thirsty species including oak, willow, elm, ash and poplar should be planted at least 20 metres from the building and ideally more than 35 metres away. The infographic below provides more detail.

    2. Damaged or blocked drains

    Defective drains and underground pipes that lay undetected for long periods of time can lead to major problems too. “If a drain springs a leak, or indeed an entire water pipe has burst, the flow of water can have a seriously adverse effect on your property as the water washes away the soil and creates an unstable, waterlogged surface,” explains one drainage expert. If the ground is unable to support the property on it, structural movement and damage to the building can occur as a result. Unfortunately, by the time subsidence cracks appear, the issue may be well advanced and require extensive remedial action.

    3. Heatwaves and hot summers

    According to recent figures from LV=GI, subsidence insurance claims increased by a staggering 49% in the last year, largely as a result of significant heatwaves. Prior to that, the last major spike was in 2018 – a 51% increase from the year before due to the long hot summer of 2018. Prolonged dry spells are particularly worrisome for properties built on clay soil, which has the tendency to shrink and harden when dry, pulling away from the foundations. 

    4. Heavy rainfall

    The same goes for the opposite weather condition – heavy rain and flooding – which we are also likely to see more of as the climate changes. Homes built on clay-rich soil are particularly at risk here. This type of soil is able to absorb a huge amount of water after rainfall, becoming sticky and heavy as it expands. In the UK, most cases of subsidence occur as a result of seasonal weather patterns, with soil shrinkage in the dry summer weather followed by soil swelling during the wetter winter season. This poses a danger to the stability of building foundations.


    If you are at all concerned about any cracks in the walls of a property you are about to buy, you should seek professional advice without delay. It may turn out to be nothing of any consequence, in which case you will have the peace of mind of knowing that the building is structurally sound. However, if your surveyor does detect evidence of subsidence, this is vital information that may well impact your purchase, the value of the property and your financing options.

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