Here, we put the problem of subsidence under the microscope. To find out all about it, dive in as we explain the following:
- What is subsidence?
- Signs of subsidence
- The causes of subsidence
- How to prevent subsidence
- How to fix subsidence
- The financial implications
What is subsidence?
Subsidence occurs when the ground beneath a house sinks due to unstable soil, causing the building’s foundations to move and potentially sustain structural damage. Subsidence can affect the structural integrity of a house, its safety and resale value, yet despite the seriousness of the problem, some misunderstanding exists over recognising the warning signs.
While it can be dealt with, subsidence damage can be costly to remedy and time-consuming, so early intervention is important to stop the problem worsening. While home 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.
Global Building Standards Director for the Royal Institution of Chartered Surveyors, Gary Strong, commented that while existing housing stock may experience problems with subsidence, upgraded building regulations and National House Building Council rules mean that deeper foundations are now dug, making modern homes less susceptible. Mr Strong, a chartered building surveyor, said:
“Victorian houses have shallower foundations and a dry summer or winter can trigger subsidence. Shrinkable clay soils are the worst cause and they are especially seen in the south east, following a line south of Bristol to The Wash: everything below that has an element of clay in it and London has very shrinkable clay.”
“Not all cracks in a building may be subsidence, they could be settlement cracks, thermal movement cracks or cavity wall tie failure which can cause corrosion, resulting in uplift in brickwork and horizontal cracking. A surveyor is needed to really identify the cause of cracking, however house buyers could consult a geological map to identify the soil type in the area.”
“In 80-90% of cases the probable cause of subsidence is large trees being too close to a house, in combination with shrinkable clay. A big oak, poplar or willow will suck in gallons of water from the subsoil and the roots will dry out the soil. If the roots are under the building, they will cause shrinkage of subsoil moisture.”
“To deal with this, options include cutting trees down to minimise moisture uptake, followed by monitoring the cracks in the building over time, and undertaking a soil analysis to see if this will allow moisture back into the subsoil. If this does not work, cracks may require resin bonding and stainless steel horizontal bars can be fitted to reinforce the brickwork structure, which creates a very strong house. It is rare that substantial foundations works are required today; technology has moved us on from the days of pouring new foundations.”
Warning 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, uneven surfaces 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 hair-line 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 swell as it absorbs water in wet weather and shrinks as it dries out, making the ground unstable. Tree roots can damage foundations and leaking drains and water mains can affect soil structure. Mining subsidence can occur in former coal industry areas and quarry sites can cause problems when material used to fill them decomposes.
Shallow foundations
Older homes may have shallow foundations making them vulnerable to subsidence cracks depending on ground conditions. The ground underneath buildings can move for reasons other than subsidence, including heave, which occurs when earth 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 ground subsidence
Trees and large shrubs are one of the main risk factors; 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 the surrounding soil. A tree surgeon will advise on how a tree may potentially affect a building’s foundations. Maintaining guttering, plumbing and pipework is important to avoid leaks and flooding which can wash away or soften soil. Using water butts to catch rainwater can also be helpful, and with clay soils, it is beneficial to keep the ground hydrated by laying grass or gravel to retain moisture and prevent shrinkage. Moisture levels in soil could potentially be affected by laying impermeable surfaces such as driveways and hardstanding areas.
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 full survey to confirm subsidence issues. A site may be monitored which usually entails digging a hole close to the building to assess the extent of any movement in the foundations: such monitoring for further movement 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 a property’s foundations and filled with concrete to create another 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 with a history of subsidence, but which has 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.
Insurers may charge higher premiums for specialist buildings insurance if the property has a history of structural issues or if subsidence claims have been made. 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 with a subsidence problem.
Have you dealt with subsidence?
If you have experienced an unusual case of subsidence or successfully dealt with the problem, let us know!
