Spotting Subsidence

Subsidence….the very word can send shivers down the back of any homeowner or developer since it can prove to be pretty fatal. It can potentially affect a house’s structural safety and value.
Here I explain what subsidence is, how to avoid it, how to spot it and what your options are if your home or one you are hoping to buy has it.

What is Subsidence?

Subsidence occurs when the ground underneath your house sinks. As the ground moves, it lowers the foundations and your house can become misaligned. It is particularly problematic when the ground under your property is sinking at different rates.
Subsidence is not the same as heave, which is where parts of the ground under your home shift upwards pushing the foundations higher. It is also different to landslip or landslide where the ground your home was built on moves down a slope or is washed away.
There is a difference between subsidence and settlement:
Subsidence is caused by the ‘downward movement of the site on which a building stands – where the soil beneath the building’s foundations is unstable’.
Settlement is “downward movement as a result of the soil being compressed by the weight of the building within ten years of construction”. It is important to know the difference as most insurers don’t cover settlement.

Hover over to the image to see the change!

Are cracks in my house a sign of subsidence?

One crack in a wall is not a sure sign of subsidence. It is far more likely that the crack is a result of the walls or ceilings of your home swelling and shrinking over time due to temperature changes. Also, a new home – or one that has had some major plastering – may develop some cracks as the plaster dries out or the structure settles onto its foundations.

Signs of Subsidence

The obvious signs of subsidence in a property, is if there are a series of cracks generally around one part of the building, e.g. bay window, corner or additional structure such as a porch or annex. Similarly, if repaired cracks reappear or doors or windows begin to stick, then these are all signs that there is movement to the walls which may be due to subsidence, heave or landslip and requires further investigation.
Generally speaking a crack caused by subsidence is likely to be:

  • Wider than 3mm
  • Visible on both the outside and the inside of your home
  • Diagonal and usually wider at the top than the bottom
  • Located close to a door or window
If a property is found to have subsidence it needed necessarily be a complete write off. In structural terms, damage caused by subsidence rarely causes the building to become unstable overnight. As a general guide for more modern properties, the crack width/movement needs to be about 15mm or more to warrant a ‘severe damage’ classification. Even with such movement, the building may not be unstable.

If you suspect that a property has subsidence

The first step is to contact your buildings insurer – like many problems with homes, the sooner you take action to fix it, the better. The insurance company will arrange for a surveyor to inspect your home and confirm if it is subsidence. It may be that the surveyor decides your home needs to be monitored before they can determine whether the ground is sinking. This can take up to 12 months.

Should remedial work need to be undertaken, they will also oversee and co-ordinate repair work. You should also notify your buildings insurers as soon as possible, as they may wish to appoint a loss adjuster to help with any insurance issues.

Which houses are most at risk of subsidence?

Some homes have a greater chance of sinkage than others due to a variety of factors from the type of soil they sit on to the local climate.
Here are the main risk factors that increase the chances of subsidence:

  • Trees – If you have trees or large shrubs planted too close to your home they can cause subsidence as the plant drains the moisture from the soil causing it to dry out and sink. Estimates suggest that around 70% of all subsidence cases are a result of tree roots absorbing all the moisture out of soil.

  • Clay – This type of soil changes a great deal with the weather. When it is hot and dry it can shrink, crack and shift which makes the ground unstable and there is a greater risk of it sinking.

  • Drought – If you live somewhere that is prone to a drought then the soil could dry out which increases the chance of subsidence.

  • Leaks – A leaking drain or water main can soften the soil, or wash it away, causing sinkage.

  • Age and construction – If you live in a period property there may be a greater risk of subsidence as your house may have shallower foundations than a more recently-built home. However, the flipside of this is that older properties tend to be built from bricks and lime mortar which could make them more flexible and less likely to be damaged by the ground shifting beneath them.

  • Mining – This is one of the more well-known causes of subsidence. If your house has been built near to a former quarry or pit, then it could be unstable as the material used to fill the site will shift as it decomposes. Your house could also be affected if mining activity occurs close by. You can purchase a subsidence claims report from the Coal Authority to see if your property is affected.

About 20-30% of properties suffering from subsidence need some form of underpinning but in most instances, remedial measures such as removal of trees or repairs to leaking drains are sufficient for the building to regain stability and the repairs are no more disruptive than you needing to redecorate the property.

How to Prevent Subsidence

If you live in a house that is at risk of subsidence, then there are a couple of steps you can take to reduce the chances of your property sinking.
Firstly, keep the trees at a safe distance. Don’t plant any trees within 10 metres of your home and particularly large trees should be no closer than 40m.

Martin’s Word of Warning
If your subsidence is being caused by tree roots then the easiest solution is to remove the tree, but you should only do so after consulting a surveyor and with the help of a tree surgeon. If you get it wrong, you could make the situation worse and cause more instability.

Buying a Property with Subsidence

If you are just about to buy a property that you know has serious cracks and are applying for a mortgage, it’s likely that the mortgage company will insist on a Structural Engineers Report. They may also request a drain survey. Depending on what the report states, you can expect the mortgage company to put a ‘Retention’ on the mortgage. This in effect means that they hold back part of the loan pending the work being carried out to fix the problem.

If repairs for subsidence have taken place in the past your conveyancing solicitor should get legal documents from the vendor to verify that the repairs were done to the standard set by the Building Research Establishment. These documents should include a formal Completion Certificate, which is issued by the council if the property has been underpinned, and a Certificate of Structural Adequacy, which should have been created by a building surveyor if the repairs were part of an insurance claim. Many repairs come with guarantees which you will want to have passed to you.