MET SURVEYORS

Why Do Extensions Move Differently From the Main House?

 

An extension may behave differently from the original building because it may have:

• Different foundation depths

• Different foundation types

• Different wall construction

• Different materials

• Different loading

• Different ground conditions beneath it

• Different levels of exposure to heat and weather

• Been built decades after the original property

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Even where an extension is well constructed, some difference in movement can occur.

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The key issue is whether that movement is minor and stable or whether it is continuing.

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Is Cracking Between an Extension and a House Common?

 

Yes.

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Cracking at the junction between old and new construction is relatively common.

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This is particularly true where:

• The extension is fairly recent

• The original house is much older

• Different materials meet

• The new structure has settled slightly

• Movement joints were not provided

• The extension foundations differ from those of the house

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Fine cracking does not automatically mean there is a serious structural problem.

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Wider or progressive cracking deserves more attention.

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What Is Differential Movement?

 

Differential movement occurs when one part of a building moves differently from another.

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For example, the original house may have been stable for many decades while a newer extension is still adjusting to the ground beneath it.

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If the two sections move by different amounts, stress can develop where they meet.

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Cracking may then appear:

• Internally

• Externally

• At ceiling level

• Along vertical wall junctions

• Around openings close to the extension

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This type of cracking can sometimes be relatively minor, but the extent of movement needs to be considered.

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Could It Just Be Settlement?

 

Yes.

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A newer extension can experience some settlement after construction.

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This may result in minor cracking, particularly during the early years.

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Settlement cracking may be associated with:

• New foundations bedding into the ground

• Drying shrinkage

• Changes in building materials

• Minor movement between old and new construction

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Where cracking remains fine and stable, it may not indicate a serious defect.

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If cracks continue to widen or reopen after repair, further investigation may be required.

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Could It Be Subsidence?

 

Yes, but subsidence is only one possible explanation.

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Ground movement beneath an extension can sometimes cause cracking at the junction with the original house.

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Possible contributory factors include:

• Shrinkable clay soil

• Mature trees nearby

• Prolonged dry weather

• Leaking drains

• Inadequate foundation depth

• Poorly compacted ground

• Previous disturbance to the site

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The pattern of cracking, age of the extension and condition of the surrounding structure all need to be considered.

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What Type of Cracks Should You Watch?

 

More attention may be justified where cracks are:

• Becoming wider

• Wider at one end

• Visible internally and externally

• Reopening after repair

• Running diagonally from openings

• Associated with distortion

• Appearing in several connected areas

• Developing suddenly

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The presence of one of these signs does not prove serious structural movement, but it may justify closer inspection.

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Vertical Cracks Where the Extension Meets the House

 

A vertical crack along the junction between the extension and the original building is very common.

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This may be caused by:

• Differential settlement

• Poor bonding

• Thermal movement

• Different materials

• Lack of a movement joint

• Structural movement

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A fine, stable crack may be relatively minor.

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A crack that is widening, reopening or associated with other movement may require professional advice.

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Diagonal Cracks Near the Extension

 

Diagonal cracking can indicate differential movement.

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This may appear:

• Above windows

• Above doors

• Near the corner of the extension

• At the junction between old and new walls

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The crack pattern alone does not establish the cause.

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The condition of the foundations, openings, walls and nearby ground all need to be considered.

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What If the Crack Is Only Inside?

 

Internal cracking may sometimes be limited to plaster or finishes.

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Possible causes include:

• Shrinkage

• Decoration

• Plasterboard movement

• Minor movement at junctions

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However, if the crack is also visible externally, this can indicate movement through the wall construction rather than just the finish.

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Cracking Around Patio Doors or Bifold Doors

 

Large openings are common in extensions.

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Where patio doors, French doors or bifold doors have been installed, the structure above the opening must be adequately supported.

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Cracking can occur where:

• The lintel is inadequate

• Bearing is insufficient

• The opening has been altered

• Support has moved

• The extension itself has settled

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Where cracking is concentrated around a large opening, lintel movement should be considered alongside foundation movement.

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Could the Foundations Be Different?

 

Very often, yes.

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An older house may have relatively shallow foundations by modern standards.

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A newer extension may have deeper concrete foundations.

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Alternatively, the extension foundations may themselves be inadequate for the local soil conditions.

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Differences in foundation depth and construction can contribute to different rates of movement.

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This is why cracking at the junction is common.

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Cracks After a Hot Summer

 

Dry weather can sometimes make existing movement more visible.

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This can be relevant where:

• The property is built on clay soil

• Mature trees are nearby

• Cracks appear after prolonged dry weather

• The extension has relatively shallow foundations

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However, hot weather alone does not prove subsidence.

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The timing is simply one factor to consider.

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Trees Near an Extension

 

Trees can influence ground conditions, particularly in clay soils.

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Factors that may be relevant include:

• Tree species

• Size

• Distance from the extension

• Foundation depth

• Local soil conditions

• Location of cracking

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A nearby tree does not automatically mean tree related subsidence.

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The relationship between the tree, soil and structure needs to be considered.

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Can Drainage Problems Affect an Extension?

 

Yes.

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Extensions are often built close to drains, gullies and manholes.

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Leaking underground drainage can affect the ground supporting the foundations.

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Where cracking appears near drainage runs, further investigation may be appropriate.

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This can include:

• CCTV drain inspection

• Drain testing

• Further structural investigation

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Could the Extension Have Been Poorly Built?

 

Yes.

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Cracking can sometimes be caused by construction defects rather than ground movement.

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Possible issues include:

• Inadequate foundations

• Poor masonry bonding

• Insufficient wall ties

• Inadequate lintels

• Poor structural detailing

• Lack of movement joints

• Defective previous alterations

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A proper inspection considers the construction as well as the crack pattern.

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What If the Crack Has Been Repaired Before?

 

A crack that has been repaired and later reopens can be significant.

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This may indicate that the underlying movement has continued.

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Useful information includes:

• When the repair was carried out

• How long the crack remained closed

• Whether it reopened in the same place

• Whether it has become wider

• Whether other cracks have appeared nearby

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Previous repair history can provide useful evidence.

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Should You Monitor the Crack?

 

In some cases, yes.

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Monitoring can help establish whether movement is continuing.

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This may involve:

• Dated photographs

• Recording crack width

• Formal crack monitoring equipment

• Periodic inspection

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However, monitoring is not always the first step.

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Where cracking is substantial or accompanied by distortion, a structural inspection may be more appropriate before deciding how monitoring should be carried out.

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When Should You Be Concerned?

 

Professional advice should be considered where:

• Cracks are widening

• Cracks are reopening

• Movement is visible inside and outside

• Several areas are affected

• Doors or windows are sticking

• Floors appear distorted

• Cracks appeared suddenly

• The extension appears to be moving away from the house

• There is concern about subsidence

• Previous repairs have failed

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When Is a Specific Issue Survey Appropriate?

 

A Specific Issue Survey may be suitable where the concern is limited to one clearly defined area.

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Examples include:

• One vertical crack at the extension junction

• One area of diagonal cracking

• Cracking around one opening

• One localised area of movement

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The survey focuses on the reported issue and provides a written assessment with recommendations.

Specific Defect Survey From £695 + VAT

Available across London and the Home Counties.