So I’ve been approached by a number of media outlets over the past few months about subsidence and how it is impacting houses in the UK. I thought I would summarise many of the types of questions I’ve been asked, and answers I’ve given in one place here. I hope this is of help!

Q&A / Briefing notes on subsidence

 

What is subsidence?

Subsidence is most commonly recognised as diagonal cracks in houses, and is caused by the downward movement of soil under part of the home.

The most common cause of subsidence in the UK is clay-related subsidence, which occurs when clay soils dry out, and the soil shrinks. Soils will dry out in sustained hot and dry summers, and can be dried out even more where trees are present, as their roots extract moisture from the soil

 

When and why does subsidence happen? Is there a “perfect storm” for subsidence?

The “perfect storm” is actually perfect summer weather! Long, hot and dry days, starting before Wimbledon and lasting through till the kids get ready to return to school. This year (2018) is an ideal example of the type of weather that is likely to result in a lot of subsidence damage.

To understand clay related subsidence it is helpful to know the answer to 3 key questions:

1: is the soil shrinkable (clays are shrinkable, sands are not.)

2: is the summer hot and dry? (like this year)

3: is there a tree in close proximity to the building?

 

Are there going to be more subsidence claims in 2018 than normal?

There is a very, very high likelihood of 2018 having lots of subsidence claims. The weather has been hot and dry for many months and already loss adjusters and arboriculturalists (tree-surveyors) are investigating many insurance claims for subsidence as far north as Leeds. As the summer progresses and the soils dry out even more, the expectation is that there will be even more subsidence damage.

As well as damaging houses, subsidence also can break pipes in the ground. In recent weeks, some water companies are reporting more than twice as many burst pipes compared to the same period last year. Combined with the lack of rainfall to fill the reservoirs, understanding subsidence’s role in pipe failures is key for maintaining resilient water supplies.

 

Which areas of the UK are normally are prone to subsidence?

The South-East of England is well known for its subsidence problems. Areas of North and Western London as well as Essex, Kent and Hampshire and Dorset commonly experience subsidence issues.

The map below shows area commonly affected by subsidence in a normal summer.

subsidence hazard in a normal year (c) Cranfield University 2018

Figure 1: Areas of the UK most at risk of subsidence in a normal summer.

(find out more at our Natural Perils Directory Page)

 

 

Are there any areas of the country which are normally safe, but in hot year, may be affected by subsidence?

Yes, in a hotter and drier year, homes in other parts of the country become more at risk of subsidence. The next map shows the subsidence hazard rating for the hottest summer we would expect every decade or so. As you can see, the threat of subsidence spreads across the Midlands, to the North and West. This exposes properties in Manchester, Birmingham, Bristol, West Sussex, Kent and South London to an increased risk of subsidence damage. Even some properties in the normally “damp” North-East can see an increase in the chance of suffering subsidence.

As our climate changes to one with hotter, drier summers and warmer, wetter winters, people in these areas are more and more likely to find subsidence an issue that they need to be aware of.

subsidence hazard in an extreme year (c) Cranfield University 2018

Figure 2: Areas of the UK most at risk of subsidence in a hotter and drier summer (1 in 15 year event)

(find out more at our Natural Perils Directory Page)

If you want to see our paper on future climate risks from subsidence, it is here:

https://link.springer.com/article/10.1007/s10584-015-1486-z

 

 

What kind of alternations can make a property more susceptible to subsidence?

Because subsidence most commonly occurs as a result of differential settlement of the property (that is, one part, or corner, of the house is subsiding down at a faster rate than the rest of the house) older houses that have been extended can face an increased risk of subsidence.

Typically, older houses have very shallow foundations (less than 30 cm). If you build an extension, modern building regulations state that the foundations need to be at least 70 cm deep, and often deeper in shrinkable soils. In this situation, the extension is more stable than the rest of the house, the rest of the house will move more than the extension. What this means is that cracks will form on the join between the older and newer parts of the property.

The same effect can be seen where terraced houses have had basement extensions. This is, in essence, building a deeper foundation (so more stable house) for part of the terrace, and cracks may well occur on the join between the homes with basement extensions, and those without. Surveyors like Michael Lawson of Property Risk Inspection have experience of these types of extensions and how subsidence impacts on these types of properties.

 

I’m an insurance company – how can I get access to these maps?

Head over to our Natural Perils Directory website or send me an email.


0 Comments

Leave a Reply