MET SURVEYORS

Why Can Hot Weather Increase the Risk of Subsidence?

 

The relationship between hot weather and subsidence is largely about moisture in the ground.

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During an extended dry period:

• Rainfall may be significantly reduced

• Water evaporates from the surface

• Vegetation continues extracting moisture

• Trees can remove substantial amounts of water from the soil

• Some clay soils shrink as their moisture content falls

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If the soil beneath or around foundations shrinks unevenly, part of the building may move.

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It is this differential movement, rather than hot weather itself, that can lead to structural cracking.

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Why Is London Particularly Susceptible?

 

Large areas of London are underlain by clay.

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Clay soils behave differently from many other soil types because their volume can change as moisture levels rise and fall.

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During wet conditions, clay can absorb moisture and expand.

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During prolonged dry conditions, it can lose moisture and shrink.

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Properties with relatively shallow foundations can sometimes be more susceptible to these changes, particularly where mature trees are also extracting moisture from the surrounding soil.

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This does not mean that every London property built on clay is at risk of subsidence.

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Factors such as foundation depth, building age, tree location, drainage and local ground conditions all need to be considered.

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Why Can Trees Make the Problem Worse?

 

Trees need substantial quantities of water, particularly during the growing season.

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Their roots draw moisture from surrounding soil.

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During normal weather this may cause no problem at all. During an unusually long dry period, however, the combination of reduced rainfall and continued moisture extraction by trees can cause some clay soils to dry and shrink more significantly.

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Factors that can be relevant include:

• Species of tree

• Size and maturity

• Distance from the building

• Root spread

• Type of soil

• Foundation depth

• Previous history of movement

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The presence of a large tree close to a house does not prove that the tree has caused subsidence.

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Its relationship with the building needs to be considered alongside the pattern of movement.

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What Cracks Might Appear After Dry Weather?

 

Possible signs of structural movement include:

• New diagonal cracks around windows or doors

• Existing cracks becoming wider

• Cracks visible internally and externally

• Tapered cracks that are wider at one end

• Previously repaired cracks reopening

• Cracking close to an extension

• Cracks developing on one part of the building rather than uniformly throughout it

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You may also notice changes that are not cracks.

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These can include:

• Doors beginning to stick

• Windows becoming difficult to open

• Gaps appearing around frames

• Slight distortion around openings

• Localised unevenness to floors

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None of these signs individually confirms subsidence.

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The overall pattern needs to be considered.

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Why Do Cracks Sometimes Appear Towards the End of Summer?

 

Ground movement does not necessarily become visible at the beginning of a hot spell.

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Moisture loss can accumulate over weeks or months.

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The effects may therefore become more noticeable later in the summer or into early autumn after a prolonged period of low rainfall and high temperatures.

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Where trees are involved, their demand for water throughout the growing season can contribute to continuing moisture loss.

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This is why homeowners may first notice cracking after the hottest weather has passed rather than during the first few warm days.

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Does a Dry Summer Mean My House Has Subsidence?

 

No.

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Most houses do not develop serious structural movement simply because there has been a hot summer.

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Cracks can appear for many unrelated reasons, including:

• Drying shrinkage within plaster

• Thermal movement

• Normal settlement

• Movement between different building materials

• Extension settlement

• Lintel movement

• Previous structural alterations

• Poor workmanship

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Timing is useful evidence, but it is only one part of the investigation.

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What About Cracks in Plaster?

 

Very fine cracks in plaster are particularly common during warm weather.

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Internal humidity and temperature changes can cause decorative finishes and building materials to expand or contract.

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Fine surface cracking may therefore have no connection with movement to the foundations.

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Greater attention may be justified where cracks:

• Extend through the masonry

• Are visible externally as well as internally

• Are noticeably widening

• Reappear following repair

• Are associated with distortion elsewhere

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Cracks Around Doors and Windows After Hot Weather

 

Door and window openings are common places for structural movement to become visible.

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Because openings interrupt the continuity of a wall, cracking can develop from their corners if movement takes place.

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Diagonal cracks may therefore require closer consideration, particularly if they:

• Appeared recently

• Are becoming wider

• Are visible externally

• Occur alongside sticking doors or windows

• Form part of a wider pattern of cracking

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However, cracking around an opening can have several other causes and should not automatically be described as subsidence.

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What If an Extension Starts Cracking?

 

Extensions can behave differently from the original building.

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They may have been built:

• Decades later

• On different foundations

• At a different depth

• Using different construction materials

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Some movement between the extension and the original property may therefore occur.

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Dry weather can potentially make existing differences in foundation movement more noticeable.

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Where a crack opens at the junction between an extension and the original house, the extent and pattern of movement should be considered before deciding whether subsidence is involved.

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Can the Ground Recover When It Rains?

 

Clay soil can regain moisture when wetter weather returns.

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However, that does not necessarily mean that any movement to the building will simply reverse without consequence.

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Ground conditions can be complex and repeated seasonal movement may occur.

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Where significant structural movement is suspected, it can sometimes be necessary to monitor the property over a period of time to understand whether movement is continuing or seasonal.

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Should I Water the Ground Around My House?

 

Trying to manage suspected subsidence by deliberately watering the ground is generally not an appropriate substitute for investigating the cause.

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Introducing water unevenly can make ground conditions more complicated.

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Where significant movement is suspected, professional assessment should be obtained rather than attempting to stabilise the soil yourself.

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Should I Remove a Tree?

 

Not simply because cracks have appeared.

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Removing or heavily reducing a mature tree without understanding the circumstances can potentially create other ground movement issues.

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Before significant tree work is undertaken in connection with suspected subsidence, the relationship between the tree, soil and property should be properly considered.

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Planning restrictions may also apply to protected trees or trees within conservation areas.

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Where appropriate, arboricultural advice may be required.

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Could Drains Be Responsible Instead?

 

Yes.

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Hot weather and trees are not the only possible explanations for cracking.

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Leaking underground drains can alter ground conditions around foundations and may contribute to localised movement.

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If the cracking occurs close to drainage runs or inspection chambers, further investigation may be appropriate.

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This can include a CCTV drain survey.

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What Should I Do If New Cracks Appear After Hot Weather?

 

Start by recording what you can see.

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It can be useful to note:

• When the crack was first noticed

• Whether it appears to be widening

• Where it is located

• Whether it can also be seen externally

• Whether nearby doors or windows have changed

• Whether there are mature trees nearby

• Whether you know of previous drainage problems

• Whether the crack has previously been repaired

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Take clear, dated photographs.

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This information can be useful when deciding what type of structural inspection is required.

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Should I Monitor the Cracks?

 

In some circumstances crack monitoring can provide useful evidence about whether movement is continuing.

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However, it should not be assumed that monitoring is always the first step.

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Where cracking is substantial, widespread or accompanied by obvious distortion, a structural assessment may be more appropriate before deciding how the movement should be monitored.

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When Is a Structural Survey Appropriate?

 

A broader Structural Survey may be appropriate where:

• Cracking affects several areas

• Several elevations are involved

• There is significant distortion

• Movement appears widespread

• Cracks are increasing

• There are multiple signs of possible subsidence

• Previous repairs have reopened

• You are particularly concerned about the overall structure

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A wider inspection allows the building and the pattern of movement to be considered as a whole.

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When Is a Specific Issue Survey Appropriate?

 

If there is one clearly defined area of concern, our Specific Issue Survey may be more suitable.

Examples include:

• One area of new cracking

• A diagonal crack above a window

• Cracking to one wall

• Cracking around a bay

• Movement at the junction of an extension

• A localised concern following a period of dry weather

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The survey concentrates on that particular issue and includes a written report setting out our observations and recommendations.

Specific Defect Survey From £695 + VAT

Available across London and the Home Counties.