What is Building Subsidence?

A structure is only as stable as the ground on which it’s built. Subsidence is the naturally occurring downward movement of the ground that supports the structure.

What causes Subsidence?

Older homes with shallow foundations and homes built on clay soils are more prone to subsidence. Either the underlying water table drops due to a prolonged dry spell, or water is sucked out of the soil by surrounding trees and plant life. As the clay contracts due to the lack of water, it pulls on the foundations of the house which can cause structural damage.

Subsidence can also occur when water leaks into the soil from sources such as broken pipes or leaking drains. These can wash away the soil from the foundations and typically occurs in soil that has a high sand or gravel content.

How do I avoid Subsidence?

There are some simple measures you can take to minimise the risk of subsidence. First, drains and pipes should be checked regularly to ensure there are no blockages or leaks. Second, trees should be pruned occasionally to reduce water uptake, and any new trees or shrubs should be planted at an appropriate distance from your property.

Identifying Subsidence

The underlying issue with subsidence is that the movement in the supporting ground is rarely uniform throughout the building. The tell-tale signs of subsidence include new or growing cracks in plasterwork, cracking to exterior brickwork, doors and windows that become hard to open or are sticking for no obvious reason, and wallpaper that starts to rip or bulge but there’s no evidence of dampness.

While cracking is the most obvious sign of subsidence, finding a crack is not proof that you have a problem. In fact most buildings will experience cracking at some time during their lives, particularly new homes and in new extensions or additions, as they continue to settle under their own weight. However, the cracks that should immediately raise an alarm are those that appear suddenly, especially after a long, dry spell. Usually these cracks will be wider that 3mm to 5mm and, in most cases, will be wider at the top than at the bottom.

In these situations you should seek immediate professional help to confirm if you’re experiencing subsidence and, if so, what’s causing it.

Rectifying Subsidence

The good news is that, in most cases, the problem of subsidence can be fixed. However, you should carefully check your insurance cover as not all aspects of subsidence may be included. Most policies will cover repairs to damage caused by subsidence but you may not be covered for remedial work to halt the subsidence.

Some of the most common methods for rectifying subsidence are:

Repairing Pipework and Drains – Repairing or replacing leaking pipes and drains may be all it takes when the problem is caused by soil being washed away.

Pruning and Removing Surrounding Trees – if the cause is moisture being extracted from the soil by nearby tree root systems trees can be pruned to reduce their moisture demand or, in more serious cases, they can be removed. Consult an arborist to find out the best course of action.

Underpinning the Foundations – In approximately 20 to 30 percent of cases the structure will require underpinning to stop the foundations from moving any further. Call SA Underpinning for a free inspection and quote.

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