Subsidence: Identifying and Resolving

Here, we put the problem of subsidence under the microscope and examine the following:

What is subsidence

Signs of subsidence

The causes of subsidence

How to prevent subsidence

How to deal with the problem

The financial implications

What is subsidence?

Subsidence occurs when the ground beneath a house sinks due to unstable soil, causing the property to move and potentially sustain damage.  Subsidence can affect both the safety of a house and its resale value, yet despite the seriousness of the problem, some misunderstanding exists over recognising the indications. While it can be dealt with, subsidence can be costly to remedy and time-consuming, so early intervention is important to stop the problem worsening. While building insurance may cover subsidence, it is worth checking individual policies; claims escalated in 2020 and hotter summers are predicted, increasing the risk of soil drying out in susceptible areas: making regular checks of a property is good practice.

Signs of subsidence

• Large cracks (larger than 3mm) appear in internal and external walls

• Diagonal cracks at the edges of windows and doors, usually wider at the top than the bottom

• Sinking foundations or sloping floors

• Paths or driveways sink or dip

• Problems opening doors or windows 

• Cracks where an extension meets the main house

• Wallpaper crinkling or tearing (not caused by damp) at wall or ceiling joints

• Small cracks may be mistaken as signs of subsidence, but they are often caused by natural shrinkage from temperature and humidity change. New homes with fresh plaster usually develop small cracks as the plaster dries out.

What causes subsidence?

Seasonal factors are relevant; drought-prone areas are susceptible as the soil is more likely to dry out. Geological factors also play a part, for example clay soil beneath a house can shrink and swell, making the ground unstable. Tree roots can damage foundations and leaking drains and water mains can affect soil structure. Local mining activity can cause subsidence and former quarry sites can cause problems when material used to fill them decomposes. Older homes may have shallow foundations making them vulnerable to subsidence depending on ground conditions. Buildings can also move for reasons other than subsidence, including heave, when the ground moves upwards forcing foundations with it; landslip, when ground shifts down a slope or is washed away in a storm; and settlement, which is caused by soil compression from the weight of a building.

How to prevent subsidence

As trees can be an issue, it is best to avoid planting them close to a house and choose carefully as some species absorb considerable amounts of water and dry out surrounding soil. Maintaining guttering and pipework is important to avoid flooding and using water butts to catch rainwater can be helpful.

What to do if subsidence is suspected

The problem is best dealt with promptly; a structural engineer, chartered surveyor or building surveyor can undertake a survey to confirm evidence of subsidence. The site may be monitored which usually entails digging a hole close to the property to assess the extent of any movement: such monitoring may take many months. If, in the worst-case scenario a property needs structural support or underpinning to strengthen its foundations, Building Regulations approval is required.

Methods of dealing with subsidence 

• Underpinning – this involves excavating the weakened soil and replacing it with a stronger material. Deeper footings are made into firmer ground beneath the property to stabilise the foundations.

• Soil strengthening – this can be a less costly method, involving injecting a resin into the ground to replace eroded soil.

• Mass concrete – pits are dug below the foundations and filled with concrete to create another foundation layer beneath the existing foundations.

• Beam and base – a concrete beam is placed beneath existing footings that distributes the building’s weight onto concrete bases.

• Screw piles and brackets – screw piles are inserted deep in the ground as anchor points; supporting brackets are then attached under the foundations to level off the structure.

• Jet grouting – high pressure jets mix the existing soil with grout to make a stronger base. 

• Cantilever needle beam – this external solution is less disruptive but requires some space around the property.

Financial implications

Purchasing a property which has suffered from subsidence and been underpinned should not affect a buyer’s ability to obtain a mortgage, providing that a full structural survey reveals no further issues with the structure of the property: any history of underpinning must be declared by a seller or estate agent. Higher premiums may be charged for specialist buildings insurance if the property has a history of structural issues. Subsidence will affect the value of a property depending on the extent of the issue: general estimates vary between 20%-25%. It is important to check all available information and undertake a full structural survey to assess any further risk, when considering buying a property which has experienced subsidence.

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