When the heat is on - how heatwaves and drought can damage buildings

In the UK, we’re so used to worrying about rain and damp getting into our homes, but in hot, dry summers — like the repeated heatwaves we’ve seen this year — it’s the lack of water that can cause just as much damage. 

During drought years, the number of subsidence claims always goes up. Why? Because much of the UK sits on clay subsoil, and clay has a party trick: it swells when wet and shrinks when dry. Global warming is bringing with it hotter & drier summers, and wetter winters without the cold spells, which basically means that subsidence cases are expected to rise. Chartered Insurance Institute worryingly predict that over 1 million more homes will be at risk of subsidence by 2050.

Simply put, when prolonged heatwaves (following a heavy rainy period) dry out the ground, clay shrinks. So, if your foundations sit in that clay, they can move with it. And then, when autumn and winter rains arrive, the soil rehydrates and swells, sometimes pushing the building back into position…but not always perfectly. This process can cause differential settlement, where one part of a building moves more than another, and that’s when cracks appear.

Crack monitoring gauges can be used to assess whether movement is seasonal or progressive

More than just cosmetic cracks

Cracks in walls (when they get wider than over 3mm) aren’t just an aesthetic problem. Once they open up, moisture can get in, leading to freeze–thaw damage in winter (the kind you might remember from geography class but didn’t *actually* think you’d use in real life). However, over time, the problem accelerates, year on year.

In our work, we’ve seen everything from historic cottages to modern houses showing signs of movement, particularly after hot, dry spells. In one recent case, a Grade II listed property in Montacute developed significant cracks in its stone façade due to clay shrinkage, nearby shrubs, and hard landscaping that stopped rainwater soaking back into the ground. In another, a 1950s property in Littleton showed severe cracking and leaning walls linked to a mature Monterey Cypress tree drawing moisture from shrinkable clay.

If left alone, the subsidence could cause significant damage to the properties and potentially become dangerous for people using the buildings.

The Usual Suspects

From our inspections, there are some common contributing factors and we’ve found that subsidence is rarely down to just one cause. 

Often, it’s a combination of thirsty tree roots planted too close to buildings, leaky drains that wash away fine soil particles beneath foundations, and hard landscaping that stops rainwater soaking back into the ground. 

In older buildings, shallow foundations are sometimes another factor, as they often don’t meet modern depth standards and are more vulnerable to the seasonal shrink–swell cycle of clay soils we find here.

Hand dug trial holes can provide useful information on a building’s foundations and the underlying soil conditions.

Why insurers make you “wait and see”

If you’ve ever tried to make a subsidence claim, you may have been told to “monitor the cracks” for months or even years. This can be frustrating, but insurers often want evidence of ongoing movement before committing to repairs. 

Unfortunately, that delay can lead many homeowners to give up and fix it themselves…sometimes without addressing the root cause.

Emergency stabilisation can be required for buildings at risk

Ethical engineering

There’s also a growing conversation in engineering circles about the ethics of simply recommending tree removal. Because in many subsidence situations, especially modern housing, it’s not the trees that are causing issues, and sometimes “inspections” have failed to spot other causes due to nearby trees being assumed as the culprit.

And in some cases, like our current project at Fairfield House in Bath, trees have cultural or historical significance, meaning we have to find a way to protect both the building and the tree. 

What you can do

If you notice new or worsening cracks — particularly after hot, dry weather — it’s worth getting a professional inspection. At MESH Engineering, we carry out structural inspections, crack monitoring, and reporting to help clients and their insurers understand what’s happening and what to do next. 

Sometimes that means moving a shrub. Sometimes it means underpinning foundations. And sometimes it means finding a creative solution to preserve both he building and the landscape.

Better trees to plant

If you’re thinking about planting trees or shrubs near your property in your next foray into landscaping, choose less thirsty ones that take up less water from the soil. These ones are well-suited to planting on shrinkable clay soils: hazel, elder, hornbeam, holly or silver birch. Woodland Trust have some useful info on this.

Show us yer crack

We might joke about being “professional crack inspectors”, but subsidence is no laughing matter. The earlier you identify and address the cause, the more options you’ll have and the less it will cost in the long run. 

If you want us to check out your crack (pun totally intended). We can lend a hand with insurers by doing an inspection for you and a full report. Simply send us an email to find out more.