Five tips about subsidence

A crack in the wall of a home. Photo: PA Photo/thinkstockphotos

A crack in the wall of a home. Photo: PA Photo/thinkstockphotos

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1. Subsidence is the shrinking and movement of the earth your home is built on, which results in the foundations moving and cracks appearing in the building.

There are several causes of subsidence, but one of the most common is trees. Tree roots can cause movement by growing under the foundations. Even if they’re not that invasive, the roots extract moisture from the soil, causing the ground to contract. Pruning or removing the tree is often the solution, but it can sometimes make the problem worse.

2. Clay soil is another common cause of subsidence. It usually holds a lot of water, but if the water table drops during dry weather, the clay will start to shrink. Hidden mine shafts or bell mines, defective drains, culverts and pipework, and sandy or gravelly soil can also cause subsidence. Leaking water can wash away unstable soil from around a building’s foundations, for example.

3. If you see cracks in plaster or brickwork, they’re not necessarily a sign of subsidence, as cracks can be caused by all sorts of things and are often nothing to worry about. However, new or expanding cracks wider than 3mm may be caused by subsidence, especially during a dry spell. Cracks that are wider at the top than the bottom, and that run diagonally, can be a sign of subsidence, too, as can a crack that’s on both the inside and outside of a wall.

4. If you think your home is suffering from subsidence, tell your buildings insurer, as they should get an expert to check it out. It’s often a case of monitoring the situation, which can be a lengthy process lasting months or even years - drain and geological surveys may be necessary. If you don’t have insurance, or don’t want to involve your insurer straightaway, get a chartered surveyor to assess the problem. In the worst cases, a structural engineer will also be needed.

5. Relatively easy solutions can often be found for subsidence, such as fixing a defective drain or water pipe. In other cases, metal piles can be used to shore up the foundations, but only if the foundations are strong enough. Jet grouting can sometimes be the answer, which involves filling underground gaps and making unstable soil, such as gravelly soil, solid. In a small number of cases, underpinning is required. This is where pits are dug under the foundations and filled with concrete - it’s an expensive process, but your home should be a lot sturdier afterwards.