How to prevent subsidence

Could subsidence be a problem for your block? Richard Ayton-Robinson explains what to look out for and how to lower your risk

Insurers are speculating that we may see an increase in subsidence claims this autumn after one of the driest summers in years. A combination of hot weather and dry ground means that properties are more likely to suffer movement, especially where homes are built on soils that are susceptible to shrinkage. 

All buildings suffer movement over the years as a result of bedding down, thermal and moisture changes within construction materials and ageing. This often results in cracks in the internal or external surfaces. It normally has no structural significance and can be dealt with by normal maintenance. Some buildings are at greater risk of subsidence than others, but there are simple measures that can be taken to minimise or avoid the risk.

Medium and high rise blocks present the lowest risk as their construction will incorporate a steel or concrete frame and the foundations are likely to be designed and engineered to suit the site conditions.  Low rise blocks of up to five floors are more likely to be built of traditional methods and with traditional foundations. These buildings are at greatest risk from subsidence particularly in those parts of the country such as London and the South East where the subsoil is clay.

Complex geotechnical failures such as landslip, mining, and loss of support, due to excavations or retaining wall failures are thankfully rare. The most common cause of subsidence is trees growing too close to buildings in clay subsoils, or leaking drains.

Both these problems can be avoided by developing a maintenance programme which includes periodic inspection and cleaning of drains and tree management. Of course not all trees will be within the ownership of the freehold and gaining the cooperation of neighbours can be problematic. Many trees will also be protected by preservation orders. However, managing the vegetation near the building and within the boundary of the block will reduce the risk considerably.

Identifying a potential problem at an early stage can avoid an expensive bill if left to develop. Here are some simple things that leaseholders or block managers can do to minimise the risk:

Inspect the outside of the building annually, paying particular attention to any cracks in the brickwork or rendering. Take photographs to provide a reference point if repairs are not deemed necessary at the time. Where the subsoil is known to be clay, the best time to do an inspection is during September.

Prepare a sketch of the principal trees and vegetation and note their height and distance from the building. Use this as a reference point for vegetation management.

Make a visual inspection of the drains around the building by lifting manhole covers and ensure the drains are free running. Pay particular attention to gulleys and the base of soil stacks, where these are adjacent to the building, to ensure they are not cracked and leaking.

Encourage tenants to promptly notify the building manager or RMC of any new cracks in wall and ceiling surfaces. Deal promptly with leaking overflows. Where water supplies are metered, promptly investigate any abnormal consumption which might indicate an underground leak. Where works such as replacement windows are planned, make a detailed inspection prior to the works to record any external cracks. Movement in brick lintels, following the works, may appear to be a subsidence problem. Inspect following the replacement or laying of underground services near the building.

If you are in doubt as to the cause of damage, instruct a competent surveyor or structural engineer as soon as possible. If subsidence is suspected, let your insurer know straight away and they will investigate.

Richard Ayton-Robinson Product Development Director Cunningham Lindsey

Cunningham Lindsey’s Specialist Subsidence Team can be contacted on 01473 716123 or e-mail  sst-hub@cl-uk.com