Why do we all find much more movement and cracking specifically within the rear addition/projection compared to the front elevation, when considering mid terraced Victorian houses in London.

Any discrepancy between the amount of trees to front and rear can be ignored. Bay window projections are also excluded for discussion purposes.

The answer

The answer, prior to the explanation, is simply due to the relative lack of lateral restraint or buttressing of a rear addition compared to the continuous nature of the front elevation.

The explanation

To be read with the sketch Sk13 (a to d) below.

Diagram (a) shows a front elevation of four houses with the subject house coloured green.

Diagram (b) shows its plan form with continuity of the front elevation, with rear projection adjoining its neighbour to the right. The letter “J” denotes a joint between abutting walls, often where the party walls abut the front elevation, where the flank wall abuts the main rear wall and also often within the party wall where there is sometimes a joint or gap (Used for the old lead pipes for the water tank) but more commonly poor bonding of brickwork.

Diagram (c) shows a simple isometric demonstrating how the rear projection is free to tilt away from the main house and to a lesser extent away from its neighbour.

Diagram (d) replicates the previous question 1 for ease of reference where we explored the subsidence to the right hand side of the frontage with tapered diagonal cracks emanating from the free edge at the top flank corner and gradually propagating downwards to the pivot point near the front door.

Overview

If we step back from our detailed site surveys and Structural Engineers reports for a moment we can consider the structural “form” of different building types whether they are framed steel or concrete office blocks or, in this instance, very simple plain brickwork housing stock.

In their purest forms, houses are a series of boxes.When a terrace is formed, with one long elevation at the front and a number of rear projections at the back, we have a number of boxes but connected together to become stronger, at least as far as the front and main rear elevation are concerned.

The rear projections are normally pairs of boxes twinned with the neighbour in respect of its rear wall but remain a single box structure with regard to the flank wall.

Unfortunately, whilst a box structure is normally strong when the corners are rigidly connected, many Victorian houses have gaps or joints between abutting walls, and in particular with regards to the rear projections. This is because they were invariably built by different teams of people, a number of weeks after the main house was built and with little regard, in many instances, to connecting the two structures together.

Whereas, the front elevation of a mid terraced house is sandwiched in between other houses and forms part of a much longer and continuous wall structure, a rear projection. As the name infers it is a “projection” from the general building line and, therefore, lacks any lateral support by comparison.

The enclosed diagram indicates the compressive or lateral restraint of this elevation if it tries to tilt within its vertical plane as per the detached or end of terrace house, as explained in a previous article.

The plan diagram (b) and simple isometric diagram (c) shows how the flank wall of the rear projection and the rear wall, to a lesser extent, is free to rotate when subsidence of the ground occurs. NB subsidence magnitude is not uniform and occurs greatest nearer a tree.

In Quiz question 1 we explored how unrestrained walls, such as those of a detached or an end of terrace, tend to tilt and rotate as a consequence of foundation movement. See diagram (d) on SK13. The rear projection flank walls behave in the same way, as do bay window projections.

There are multiple reasons why a rear projection might subside, many mentioned in the various comments thank you. It is the shape and form which allows the rear addition to move, assuming all other factors are broadly equal.

By way of general observations the most common causes of rear addition subsidence (Modern day movement), compared to the front elevation, tend to be;

1. Located nearest mature and or more frequent trees and large shrubs.
2. Drainage passes parallel and often below flank wall footings.
3. Lack of bonding or tying-in to main rear wall or house party wall.
4. Shallower foundations than the main house, particularly when the cellar is under the hallway and stairs.

Other common defects found within rear projections but which do not particularly contribute to the foundation movement discussion include;

• Poor brickwork quality generally, compared with frontage
• Poor bonding with the use of snapped headers.
• Built in timbers including backing lintels to brick arches
• Lack of lateral restraint perpendicular to floor span.
• Overloading of first floor joists from heady partition loads and roof props.
• Mono pitched roofs often undersized. Often no loft access to assess.

Watch this space and on Linkedin for further quiz questions and discussions.

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• RICS general surveys used to be called structural surveys.
• The industry term ‘survey’ is too broad without clarification.
• Lack of public knowledge about the work of Structural Engineers
• Incorrect use of the terms by lenders, insurers, agents and brokers
• Only a few structural engineers carry out Structural Engineers’ Reports.

It’s important for you to understand the difference between structural surveys and structural engineers’ reports, so you can be confident your building is structurally sound and safe.

What is a structural survey?

A structural survey should be a survey of the skeletal structure of a property, which is a Chartered Structural Engineer’s area of expertise.

A structural survey or report is still a misused term and can be used to mean a general building survey such as the Level 2 RICS Home Survey or Level RICS Building Survey.

So, be careful to clarify with your adviser whether they mean a surveyor’s report or a specialist Structural Engineers Report. If a specialist report is required, it’s important to hire a fully qualified Chartered Structural Engineer.

A general survey is normally sufficient for routine property purchases unless there are structural problems. As a rule, the RICS recommend a level 2 surveyors report for well-maintained modern apartments and smaller houses, and a level 3 survey for older properties, especially if unmodernised or in poor condition.

A surveyor’s report will examine the whole house including roof coverings, joinery, plumbing, plasterwork, gas services and more. A surveyor’s structural knowledge will vary, and some will report decisively while others will refer any concerns to specialist structural engineers like us at Pole.

A structural engineer’s report is a specialist report which inspects the structural integrity of the building only. This is basically the load-bearing components of the building, such as the roof, walls, floors, beams and associated defects like cracks, subsidence, or non-compliant building work.

In the absence of industry guidance for Structural Engineer’s Reports, we have defined our own two types of report:

Structural Engineers Defect Inspection Report for specific localised defects

Structural Engineers General Movement Report for multiple cracks or movement

A Structural Engineers Report is commonly requested when:

• A mortgage valuation surveyor has concerns such as cracks or movement
• A Surveyor carrying out a level 2 or level 3 survey (see above) has structural concerns.
• A buyer is concerned about a defect and might not have commissioned a surveyor’s report, although we recommend this.
• A vendor or estate agent becomes aware of a potential defect during the purchasing process and wants to understand the issue.
• A homeowner (not selling) is concerned about cracks or other defects in their property.

A Structural Engineers Report should really be called a Chartered Structural Engineers Report as almost all lenders and insurers expect such reports to be prepared by fully qualified MEMBERS or FELLOWS of the Institution of Structural Engineers. See FAQ 2.

Be aware there are numerous partially qualified Structural Engineers, graduates and associate levels of membership and building industry professional bodies with a range of qualifications. But many of these are not recognised by lenders and insurers.

Pole’s referrals come from the surveyors, solicitors, and estate agents we have worked with for over forty years in the industry, as well as social media recommendations.

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