An owner contacted SERL, having found us via an online web search and based on our good Google reviews.

The client sought our expertise in structural engineers’ reports, especially within the South-West London area.

SERL’s structural reporting service

In response to the client’s requirements our qualified Chartered Structural Engineer recommended a General Structural Inspection (GSI) report.

SERL offers two types of structural reports:

1)       A localised, specific, and limited Specific Structural Inspection (SSI) report or a Structural Engineers Defect Report.

2)       A broader General Structural Inspection Report (GSI) or Structural Engineers General Movement Report which focuses only on matters of cracking and movement.

Our Chartered Structural Engineer’s report for the South West London property

The owner, who purchased the house the previous year, had become increasingly aware of various internal cracks in numerous rooms and was concerned they might indicate serious issues. Before proceeding with a major refurbishment, the owner wished to understand the significance of these cracks.

This property is a typical 1920s semi-detached house, constructed with solid brick walls, suspended timber floors, and roof framing.

Situated on the well-known London Clay geology, the property is near several large trees, especially at the rear.

SERL’s findings

The living room, master bedroom, and stairwell exhibited several cracks. However, we assured the owner they were merely superficial plaster imperfections, nothing that was structurally concerning. Our examination of the building’s exterior found no evidence of any contemporary structural movement.

SERL’s conclusions and recommendations

We were satisfied that none of the cracking was serious which relieved the owner and allowed them to move ahead and plan the internal refurbishment without concern.

We did make various recommendations with regard to removing a tree in the front garden and pruning trees at the rear, but because the latter were on neighbouring land addressing this issue always proves more complicated.

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Calling all industry professionals!

Has anyone in the industry had the chance to see/use/benefit from the above data which has just been published by the British Geological Survey? It sounds very useful indeed if it brings together not only the geological maps we are all used to, but in addition, information on building types and trees in particular in the case of classical clay subsidence.

The GBS promotional material lists the following considerations below. These are similar to those used in our own risk assessments that we use for our pre purchase Structural Engineers Reports but obviously brings together huge amounts of data which we cannot match individually:

• Geology including clay thicknesses/depths
• Building type and number of storeys
• Building age
• Drainage
• Tree proximity (the key one for clay soils)

All of the above apparently produces a combined hazard score.

Insurance companies and conveyancing professionals are likely to find the information invaluable. However, we engineers and surveyors will be quick to point out that there’s no substitute for a property-specific survey. Such surveys consider all factors, including internal alterations and condition, which even advanced tools like Google Maps and AI haven’t fully matched yet.

Any thoughts and experiences from fellow professionals would be most welcome once the data is in use.

If anyone would like to share knowledge on this subject please e mail Simon.pole@Ser-l.co.uk or comment on the SERL Linkedin page

The post The Property Subsidence Assessment (PSA) appeared first on Structural Engineers Report.

When purchasing a property in Greater London, contacting the Local Authority as part of your searches will provide guidance on the risks of subsidence, landslip, and heave. This information is based on statistical factors such as local geology and insurance company data on subsidence claims in your postcode.

Subsidence claims are most frequent where London Clay is near the ground surface and large mature trees are nearby. Consequently, the risk to a specific property depends on various factors, though some general tips and guidance can be offered.

While this advice is intended to inform, it is not meant to alarm potential buyers or sellers or suggest that all properties in a particular London area, town, or postcode are affected by these risks. On the contrary, due to proactive measures by most Local Authorities to maintain trees, these risks have decreased in recent years.

The following general guidance may be of interest:

1. Geological Map of London, the South East, and the UK

The British Geological Society (BGS) offers a free website and app that provides likely geological details for any postcode. This resource needs careful use but can help establish whether a property is located on clay soil.

The map uses two distinct layers to show the deep underlying bedrock, i.e. the soil very deep below the ground, together with the superficial geology which is the most important soil type as this is closest to ground level. This is likely to be the soil type found immediately below most foundations in period houses built before the war and even before 1976.

If no superficial geology information is recorded for a postcode, this indicates that the surface geology is the same as the bedrock – generally London Clay in most of Greater London, or chalk near the North Downs in Surrey, for example.

General soil types and associated risks:

– London Clay: High risk of subsidence especially with large nearby trees.

– Head: Intermediate risk with nearby trees.

– Alluvium: Soft soil with a risk of historic settlement.

– Langley Silts: Clay with silt.

– Gravel Soils (e.g. Kempton Park, Taplow, Lynch Hill, Hackney) – Generally more strong and stable than clays and other soils listed above.

2. Trees

Most significant subsidence cases on clay soils involve trees or mature shrubs. The risk of subsidence depends on the location and size of the tree if the superficial or outcropping bedrock is clay-based. Conversely, stable soils like sand, gravel, or chalk reduce this risk. However, large trees can disrupt drains, potentially leading to subsidence due to soil softening or erosion.

Useful guidance regarding trees:

a) Consider the proximity of the tree to the house. If a tree’s height exceeds its distance from the house, root systems likely extend beneath the property.

b) Determine if the tree is within your control (i.e. within your property boundary) or on neighbouring or local authority land.

c) Check if the tree is protected by a preservation order or if the property is within a conservation area, requiring local authority approval for any alterations.

d) If a tree is suspected to cause subsidence damage, the tree owner may need proof, such as soil and root testing, to address the issue.

3. Drainage Issues and Loose Soils

Period houses often rest on manmade materials known as fill, covering London’s natural geology, whether gravels or clays. As a general guide, if loose soils are near the surface and below the house foundations, softening and erosion may occur, especially with poorly maintained drains located beneath or adjacent to the house. Subsidence can result from drains even when sands and gravels are close to the surface, not just clay soils. Given that drain surveys are relatively inexpensive, it is prudent to include these in early maintenance plans when purchasing a property. Requests to test drains pre-purchase can delay transactions, with some vendors reluctant to undertake such testing. Fortunately, drainage repairs are typically affordable, often costing a few thousand pounds rather than tens of thousands.

4. Foundation Depths

Subsidence risk is heavily influenced by foundation depth. Shallow foundations are more susceptible to soil drying out and shrinking during summer, tree root intrusion, and proximity to drains. Deeper foundations are generally more stable, with reduced susceptibility to seasonal soil volume changes and tree root impact. Modern properties typically feature deeper and more robust foundations, especially following the design improvements post-1976 after the significant summer drought.

Houses with cellars benefit from naturally deeper and more stable foundations. Houses with lower ground floors are usually better protected by having most of the property and foundations significantly below street level where trees are located. This might not apply to trees in rear gardens, however, where the external ground level is often close to lower ground floor level.

Foundation Depth Guide:

– Pre-1920: 30 cm to 45 cm

– 1920-1930s: 45 cm to 60 cm

– 1950-1975: 60 cm to 1 m

– Post-1976: 1 m or deeper

5. Building Regulations Approved or Engineer-Designed Foundations

Modern foundations are generally more robust, particularly if approved by the Local Authority Building Control Department or designed by a Chartered Structural or Civil Engineer based on soil testing and considering the mature height of nearby trees.

Note: that self-contained conservatories with external doors to the main house often do not comply with Building Regulations.

Local Authority approvals are based on current site conditions, not future tree heights, and the Authority holds no design liability. Therefore, consulting a reputable firm of chartered Structural Engineers for foundation designs of extensions and new constructions is advisable.

The National House Building Council (NHBC) offers extensive guidance to designers and developers for building near trees on clay soils, considering the soil’s shrinkage characteristics determined by testing or assumed worst-case scenarios. Tall trees, such as large oaks, London Planes, and Poplars, can necessitate foundation depths or underpinning up to 3.5 meters.

For new foundations exceeding about 2 meters, alternative methods like bored piles, concrete stilts drilled deep into the ground with a reinforced slab or a series of linking beams on top, are often more economical and practical.

Conclusions

This article serves as a general overview and should not be used in isolation without obtaining location-specific advice from relevant professionals, such as a local chartered structural engineer. It aims to provide valuable insights for property owners and non-engineering professionals, including solicitors, general practice surveyors, and estate agents, on broad geological and structural matters.

The paper introduces the geology of the London area and provides guidance on how to use the British Geology viewer map or app to identify the most likely soil type for a specific address or postcode. It explains the distinction between bedrock and superficial geology, as well as the differences between clay soils and gravel/sandy soils. Guidance is also provided on typical foundation depths for buildings of different ages.

This article highlights the importance of considering trees on clay-based soil, and the potential mature height of a tree if it isn’t controlled. This is crucial when designing foundations for extensions or new buildings. While it aims to be informative, we recommended you consult with a chartered structural engineer for more specific advice.

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