Guide to Conversion of Traditional Buildings

Section 1: Structure - Application of the structure standards

1.1 The Building Regulations

The conversion of existing buildings is covered by regulation 12 and is considered further in section 3.9 of the Building Standards Procedural Handbook, particularly clauses 3.9.5 – 3.9.11, where the topic is expanded to include listed and traditional buildings.

The direction provided in both sources is the same. From a structural point of view, the basic requirements can be summarised as follows: “every conversion shall meet the requirements of the regulations in so far as is reasonably practicable, and in no case be worse than before the conversion.”

Whilst the requirements of the regulations give apparent licence to not fully comply with the regulations, any decision to do so must be carefully considered and based upon a defensible structural logic. This should include a full understanding of the implications of any nonconformity, and a mitigation strategy must be put in place. Equally, any intervention deemed necessary to address a perceived shortcoming must be carefully considered and based upon a defensible structural logic. This is particularly true for historic assets, where as much as possible of the existing fabric should be retained in situ.

Any exemptions that are deemed necessary should be agreed at the outset of the project with the verifier, whether the SER Certification approval route is being adopted (SER Certification is discussed in more detail in 1.4.8).

This guidance sets out how decisions that relate to the building regulations should be made when working within the existing and historic built environment.

1.2 The Building Standards

The building standards provide the mechanism by which the requirements of the building regulations are met.

The building standards consider two aspects of building design from a structural point of view. These are the actual design assessment of the structure, and the management of the risk of disproportionate collapse of the building should something untoward occur. The second matter essentially concerns the robustness of the building. The building standards identify and define ‘mandatory standards’ that must be met as part of any building project.

The two mandatory standards for structure that are applicable to both domestic and non-domestic buildings are as follows.

1.1 Structure

Mandatory Standard

Standard 1.1

Every building must be designed and constructed in such a way that the loadings that are liable to act on it, taking into account the nature of the ground, will not lead to:

a) the collapse of the whole or part of the building

b) deformations which would make the building unfit for its intended use, unsafe, or cause damage to other parts of the building or to fittings or to installed equipment, or

c) impairment of the stability of any part of another building.

1.2 Disproportionate Collapse

Mandatory Standard

Standard 1.2

Every building must be designed and constructed in such a way that in the event of damage occurring to any part of the structure of the building the extent of any resultant collapse will not be disproportionate to the original cause.

When considering the conversion of an existing building on behalf of the relevant person (as defined in the building standards), it is the role of the structural engineer to see that both mandatory standards are satisfied by the building on completion. A competent and appropriately experienced Engineer should be able to determine:

• how the standards are already met by the building.
• if the building is proved to be wanting in anyway, whether improvement is necessary to meet the standards.
• how the standards will be met once the conversion is complete

in a way that is sympathetic to the key characteristics of the existing building.

In short, a holistic assessment of the whole building as a structure is essential. Practical guidance on satisfying the building standards is provided in the Technical Handbooks, one each for domestic and non-domestic buildings. These Handbooks are issued by Scottish Ministers and are updated as and when required.

The Technical Handbooks provide guidance as to how the mandatory standards could be satisfied. However, it is not a requirement to follow this guidance. This chapter considers how the mandatory standards can be met in the case of existing buildings where the application of design standards for new build might not be relevant or appropriate.

1.3 Application of structural codes of practice for design

It is not a requirement of the building standards to use current codes of practice for design when appraising an existing building for conversion. It is not even a requirement of the current codes of practice to use the analytical methods set out in those codes of practice (see Clause 1.4(5) of Eurocode – Basis of structural design BS EN 1990: 2002 + A1: 2005). If a logical, carefully-defined and verifiable approach is adopted, then the requirements of the building standards – i.e. the mandatory standards – can be met.

Current codes of practice might not be appropriate because they were developed after a particular construction technique or material fell out of favour and is not therefore given consideration. A good example of this would be cast or wrought iron, or lime-based mortars. However, these materials were in use at a time when design theory was being codified, meaning that recourse to contemporary codes of practice can be useful – for example CP111 includes performance data for lime-based masonry that is directly informed by first-hand experience of such products and can therefore be applied to modern assessments of such construction.

Another reason that a current or even superseded code of practice might not be appropriate as a reference source might be because it does not deal with the particulars of that form of construction. For example, masonry codes are generally concerned with thin, cavity walls, whereas traditional stone masonry walls are thick and solid.

Whichever code of practice is adopted for the assessment of structural components, care should be taken in selecting the right materials properties for the part under consideration. If using different codes, the underlying philosophy to the different codes should be checked for compatibility between the codes – for example care should be taken in the choice of safety factors to be applied to materials properties or applied loads to ensure that an overly- or insufficiently-conservative assessment is avoided.

1.4 General considerations

1.4.1 Condition and suitability for conversion

The case of ruinous buildings is considered in chapter 1.4.9. Any building being considered for conversion must be in good condition and should be capable of tolerating the conversion without reliance on disproportionate structural intervention.

An appraisal of the building’s condition should be carried out at the start of any conversion project, irrespective of whether physical change is proposed. This appraisal should consider the various individual components including the roof(s), the floors and the walls. It should also consider the nature and condition of the different components at their various points of interaction as the structural behaviour of most (if not all) traditionally-built buildings is derived from the sum of their various parts. It is therefore essential that those parts can interact together properly.

If parts of a building require repair, these repairs should be carried out in a manner that restores the structural integrity of that part and how it interacts with the adjoining structure. In particular, the robustness of the building (i.e. its resistance to disproportionate collapse) should not be compromised by the repairs that are carried out. When a building is of greater cultural significance, careful consideration should be given to the existing form of construction, its detailing and features. There should be a presumption in favour of retaining and repairing as much as possible of the existing fabric, but there may be occasions where replicating original fabric is included as part of wider repair works.

If repair is necessary, the actual cause of deterioration should be identified and addressed to mitigate against the risk of similar deterioration happening in the future.

Consideration should be given to any deficiencies identified in the original design and construction of a building. An example is the hidden roof valley, which can be difficult to access and can incorporate routes that are inadequate to deal with modern-day rainfall levels. In these cases, the designer may have to consider if widening of the valley gutter will improve the performance and robustness of system and strengthen the building’s long-term prospects.

Such alterations may need to be limited by the structural implications of the alteration. However, any alterations to existing detailing should be carefully considered in terms of their impact on the character of the building. Alteration works which affect the character of a listed building as a building of special architectural or historic interest are likely to need consent and therefore any early discussions with the verifier may include the planning authority.

It is acknowledged by experienced practitioners that the number of changes found in a historic building are likely to increase in relation to its age, with many passing generations leaving their mark in the way of alterations and extensions. It is therefore important that the development history of a building is established as far as reasonably possible, so that the structural implications of those changes can be identified. As a guiding principle, the more a building has been changed, the harder it is to change again. This is because the original structure may have been compromised by a series of earlier works and the interaction of the various structural phases can therefore be difficult to assess and predict. In these circumstances, during the eventual execution of any changes, the possibility of uncovering unexpected alterations increases, and the risk of encountering deficient work is much higher.

It is important, therefore, that any appraisal considers whether a building can tolerate conversion proposals without disproportionate structural intervention. This is true whether physical change is proposed. Such consideration will be based on many factors but at its heart is:

• A comparison of how the building will be used against how it is known to have been used.
• A comparison of what the form of the building will be against what it was.
• A comparison of how the structural integrity is to be derived against how it was derived. This needs to be appropriately recorded and held as a record.

1.4.2 Changes to stability

No conversion should rely upon significant alterations to the original structural behaviour of the building.

It is essential to understand how a building has behaved as a structure, and how it meets basic considerations of strength and robustness. These considerations are the essence of mandatory standards 1.1 and 1.2 but are judged against the experience and performance of the building up to the point of assessment. Paraphrasing Jacques Heyman, it is not necessary to show exactly how an existing building stands up, just that it can, and there is no requirement to assess the building using current design standards. However, any significant fundamental structural flaw should of course be identified and addressed if this is found to be necessary. Consideration should also be given to improving any shortcomings if these are straightforward to achieve. An example of this might be enhancing the connectivity of floorplates with load-bearing walls to improve the building’s robustness (see also chapter 1.4.4).

It is rare for a traditional building to have a discrete structural system in the manner of a modern building. Excluding the medieval cathedral (an obvious outlier but improbable candidate for conversion) the closest comparator might be the tall warehouse which grew out of the Industrial Revolution, whereby large open spaces were achieved through the adoption of regularly positioned, isolated columns to carry the weight of the upper floors. However, the nature of the columns and the manner they are connected to each other and the floor structures typically meant that they could only deal with vertical loads. Lateral stability would be derived from the weight and solidity of the external and larger internal walls. It is therefore essential to understand the nature and composition of the various parts of a building, and their role in its structure.

Having established how an existing building might perform as a structure, the structural implications of a conversion can be identified and assessed. In general, stability is affected by physical intervention, not a change of use. Where physical intervention compromises the existing structural form, mitigation needs to be provided through the adoption of a properly-design structural alteration. Such an alteration should be compatible with the existing by maintaining original structural lines as originally conceived, for example installing a steel frame along the line of a removed wall. The alteration should align with the existing structural form, use appropriate materials that are compatible with the existing, and be executed in a way that integrates seamlessly with the existing structure.

Physical intervention to a building should be limited to the extent that the alteration only affects the existing structure locally. Any intervention that affects most if not all of the existing structure, is incompatible with the existing, or significantly changes how the existing structure works, should be avoided. Reasons for avoidance include the high risk of escalation in the complexity and cost of the works, and the increasing likelihood of unintended consequences. These risks are further exacerbated if dealing with a building that has previously been heavily changed and modified.

Care should be taken to ensure that a structural intervention is legible and not open to misinterpretation in the future. A good example is the stiffening and strengthening of an existing joisted floor through the application of carefully laid and fixed plywood floor sheathing in place of ordinary boarding. The success of this approach relies on the permanent and total integrity of the new sheathing. However, it is not normal to rely on floor sheathing in this way, and it can be vulnerable in the future to being cut through or lifted, with the good intention of renewing plumbing or electrical services.

Any new supporting structure (for a new slapping for example) must be properly designed to carry the loads that are applied to it. This must include appropriate allowance for dead loads when assessing likely deflections of the new structure and tightening up deflection limits to mitigate the risk of cracking and distortion to those parts supported by the new structure.

1.4.3 Research and investigation

No alteration should be made to a building that is not properly understood. As has already been seen, it is important to understand a building as far as possible before the detailed design of alteration starts. This understanding should extend to the previous uses of the building (see also 1.4.5) as well as its cultural significance, development history, physical nature and condition. Any decisions made about the future of the building should be informed by all of this knowledge.

A full desktop study of the building should be carried out. Whilst there is obvious benefit in retrieving drawings describing the original construction of the building and subsequent alterations, valuable information can be derived from specifications, bills of quantities, contracts and other construction records. The study should also establish the history of the site before the building existed, as this knowledge may improve the understanding of the building’s behaviour.

Inevitably, some degree of intrusive investigation will be needed to understand key hidden details of the building’s structure, especially if it is in poor condition. This need becomes more pronounced in buildings of increasing complexity which may be due to the amount of change they have undergone since construction. The extent of investigation can be tempered by an engineer’s experience of similar buildings and reduced further through the availability of good archive information on the building.

It is not usually necessary to physically investigate every part of a building. Any requirement for opening up should be informed by the holistic assessment of the building and can often be limited to those parts that will be directly affected by changes arising from the conversion. Typical areas of interest include:

a) At the outset, the following should be investigated in advance of the preparation of detailed proposals:

• Cased primary structural members
• Locations where different phases of construction meet
• Foundations
• Unusual structural features

b) And once proposals are further developed the following should be investigated:

• Locations of historical structural intervention. This will ensure that new works can be designed and detailed properly, whilst ensuring that deficiencies in the existing fabric locally can be identified and dealt with.

Care should be taken when relying solely upon archive information as many drawings which survive do not represent the building as actually built. Sufficient investigation should be carried out to confirm that it is representative of what was ultimately carried out on site.

If investigation is necessary, non-destructive techniques (NDTs) should be explored in the first instance. An example of this is Ground Penetrating Radar, which is viable on a vertical surface as well as the horizontal, can be used to detect flues and other voids embedded within thick masonry walls. Specially trained dogs, often referred to as rothounds, can also be used to detect timber decay.

Care should be taken when using NDTs as some form of calibration is normally necessary to improve interpretation of the results. This calibration generally means localised opening to reveal an example of the hidden features being investigated. A high level of operator skill in the use of this equipment is essential. If wider intrusive investigation proves to be necessary, the use of endoscopes should be considered to limit the extent of intrusion. Any investigations that are disruptive to the existing building fabric should be carried out by a skilled contractor who is familiar with the type of building and can employ a method that minimises disturbance. If a building is listed, the designer should confirm with the planning authority whether the investigation works are sufficiently disruptive so that they require listed building consent before the works can start.

It is worth noting here that much useful information about common historic construction practices can be found in print. The eighteenth century saw a boom in publications on construction matters that have continued right through to the present day, many of which were written by practitioners. Eighteenth or nineteenth century treatises that focus on topics like carpentry, joinery, brickwork or the like are just as relevant today as they were when first printed. The techniques described in these texts are unlikely to have changed significantly, and they commonly show a preference for illustration over written description. Familiarity with what contemporary writers considered normal is invaluable in detecting what is not normal, and what therefore needs further consideration, be it through investigation or structural appraisal. This may vary geographically and by building typology.

The performance and integrity of certain parts of a building may have been compromised over time by the cumulative effect of smaller interventions. This becomes increasingly true in buildings with a higher turnover in ownership and occupancy, as each handover normally results in a renewal of services and décor. It is therefore important to verify that any apparent shortcoming has not been caused by ill-judged intervention which can easily be reversed. A good example of this would be the notching of floor joists associated with the installation of central heating plumbing or electric cabling. The existence of cut, damaged and irregular floorboards is a good indicator that this has happened. Widespread notching of a joisted floor will lead to excessive deflection and a lack of rigidity, but this can be easily reversed: redundant services can be removed and the notches filled, thereby restoring the capacities of the affected joists.

Pre-contract site investigations can be disruptive to building occupants, time-consuming, and (particularly in the case of non-destructive testing) are often perceived as costly. However, when research is carefully planned and strategically executed, it can significantly reduce the need for intrusive physical work. This proactive approach not only streamlines the scope and complexity of the contract work but also shortens their duration and lowers overall costs. Further wider benefits are the reduction of carbon emissions by potentially reducing the amount of unnecessary construction works.

1.4.4 Disproportionate collapse

New structural elements inserted into an existing building should comply with the disproportionate collapse requirements as set out in the Eurocodes and any other relevant Codes.

In existing buildings where the Disproportional Collapse Risk Class will rise following conversion, a clear written strategy should be prepared that details the existing potential for disproportionate collapse, what any new strategy is and why that new strategy is sufficient for the intended use. This strategy should include, but may not be limited to, discussion on condition, betterments, areas of higher risk and why the level of intervention proposed is appropriate. Calculations may be necessary to justify the existing condition or the proposed betterments. A holistic appraisal should be undertaken. Shortcomings in the original building arrangement may need to be addressed to provide robustness appropriate to the scheme proposed depending on the degree of risk associated with failure.

In buildings where the Disproportionate Collapse Risk Class reduces or remains the same, it is normally sufficient to limit intervention to bringing its condition back to a good state of repair and addressing any significant shortcomings in the existing structure.

The risk of disproportionate collapse is essentially a measure of the robustness of a building, being its ability to tolerate a failure of part of its structure without incurring disproportionate results or a progressive collapse.

In the building standards, the extent to which the risk of disproportionate collapse must be managed is defined by the use of the building. Each of the different building uses are assigned to a certain Risk Group, which sets out the measures that should be taken to guard against disproportionate collapse for that use.

In the case of a conversion, there is no formal requirement to manage disproportionate collapse contained within the building standards beyond the stipulation to “meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion.” This is true even of conversions that result in the converted building being reassigned to a higher Risk Group such as a single dwelling being converted to a hotel.

From all this, it might reasonably be concluded that any new structure inserted into or built against an existing structure should be detailed in a robust manner, whilst those parts not directly affected by alteration can be left alone (providing there are no other reasons for improvement).

However, as has been discussed above, traditional buildings do not always contain a discrete, sensible, structural system and are often the collective structural sum of their parts. In situations like this, the interaction of the various parts might be found to be convoluted or rely on friction between components. Such interaction can be susceptible to deterioration, misuse or ill-judged interference. It is sensible to address such shortcomings wherever possible, as improving the individual parts will naturally improve the robustness of the whole.

Fortunately, significant improvement can often be achieved through the adoption of low-key works that are simple to carry out and that will have a low impact on a building’s fabric or character. A primary goal can be to introduce direct connectivity between key parts of the building, thereby simplifying but improving the interaction between them. Improving these interactions will greatly increase the robustness of the structure both locally and globally. Examples of this sort of intervention include tying floor plates to walls (especially gables), introducing restraint to tops of slender internal partitions, and positively connecting beams to beams, or beams to columns. The latter two examples may be of particular benefit in the conversion of a warehouse building.

In every case, it is imperative that the designer, certifier and verifier have relevant expertise and competence in buildings and projects of the same size and type.

A strategy for dealing with disproportionate collapse should be agreed at the outset of the project with the verifier, whether the SER Certification approval route is being adopted. SER Certification is discussed in more detail in chapter 1.4.8.

1.4.5 Increase in loading

Building loads comprise two elements – the self-weight of the materials from which a building is built (dead load), and the loads that are applied to the building when in use (live loads or imposed loads). These live loads can be further sub-divided into the environmental loads experienced by the exterior of the building – for example, wind or snow load – and the floor loads associated with the various uses of the different spaces within the building.

These live loads are either identified through a site analysis (i.e. wind and snow load that might be termed as environmental loads) or are defined in codes of practice according to the formal occupancy class of the space in question. Safety factors are normally applied to these loads when designing a new structure or appraising existing structure. This is to account for uncertainty in defining absolute values to the loads being considered.

It should be possible to identify with some certainty the dead loads of an existing building. This being the case, there will be certain occasions that the associated safety factors can be reduced from those normally allowed for. This matter is considered in detail in the Institution of Structural Engineers publication ‘Appraising factors of safety in existing engineered structures.’

If there is no change in formal occupancy or use of a space, then there is generally no need to show that its existing floor structure can carry the imposed load associated with the continued use of the space. However, it is unsafe to assume that a space’s designation within a formal classification system guarantees that the existing floor structure can support the imposed loads typically associated with that classification.

Each floor structure must be judged on its own merit, including an appraisal of how it has performed historically. In doing so, it can be helpful to bear in mind that what might be viewed as unacceptable nowadays may not have been so originally. A good example of this is the deflection of a timber floor structure where it is rare for them to comply with the deflection criteria in current Codes of Practice, even for the design of domestic floors. However, if there are no signs of structural distress in the floor, and the associated finishes are not suffering in any way, users may be willing to accept a floor that is slightly bouncy or may have a perceptible slope. The key is to ensure that the proposed actual use of the space following conversion is commensurate with what the floor has accommodated in the past. This is a principle that will inform the assessment of a building’s suitability for conversion (see 1.4.1).

When making a comparison of future total loads a converted building will be subjected to against the past total loads it experienced, care must be exercised when determining historic imposed loads, because it is difficult to determine with any certainty what magnitude those loads ever reached.

However, reliable and verifiable archive information can provide a very strong justification for accepting a higher allowable imposed than a theoretical appraisal might otherwise suggest. This approach can be particularly useful when dealing with structural materials like wrought or cast iron, materials that currently are not widely served by recent analysis techniques or materials assessments.

Design load data for defined uses became available as publications detailing construction techniques became more widespread in the late eighteenth and early nineteenth centuries (see 1.4.3). Much of this information originated from the emerging steel industry, where growing standardisation in iron and steelwork manufacturing coincided with the fact that responsibility for the design of structural steel often rested with the manufacturers themselves. These design loads would be adopted by early codes of practice like the London County Council (General Powers) Act of 1909 (applicable in Scotland) and therefore form the genesis of current Eurocodes.

Where relevant, this information may provide further justification for accepting a higher imposed floor loading than might otherwise be considered appropriate. Examples of buildings relevant to this situation are likely to include mill buildings, warehouses and bonded storage buildings, as well as municipal buildings like schools, public theatres and assembly halls.

It is not necessary to adopt the imposed floor loads identified in current Codes of Practice when assessing an existing structure. Limiting loads can reduce the impact on a historic or traditional building because of the reduced necessity to implement improvement. The adoption of an imposed load allowance lower than that which is ordinarily applied does not necessarily restrict the use of a space, particularly if that space can be carefully planned and managed. This matter is considered in some detail in the English Heritage publication ‘Office floor loading in historic buildings’ (1996).

However, relaxation of such loads should be handled with care, and the full and formal agreement of a relaxation should be obtained from the project team and the Developer (i.e. the project sponsor – typically taken to be the same as the building owner). This should take place at the outset of the design process (see 1.4.1). Any agreed relaxation must be clearly documented and included in the Health and Safety File. These relaxations may not be appropriate for converted buildings that will end up in multiple ownership owing to the difficulties in ensuring the long-term building management necessary for ensuring continued adherence to unusual design constraints.

Increases in dead loads can arise through the adoption of new floor finishes or inter-floor insulation (both sound and thermal).

Any increase in load to a floor or part of a structure will generally lead to an increase in the deflection of that part of the structure. In the case of increased dead or sustained imposed loads, deflection will normally become permanent. The implications of this increased deflection on the wider fabric of the building should be assessed. For example, consideration should be given to whether plaster ceilings attached to the underside of a floor subject to increased loads will be affected by this change.

The load-carrying capacities and stiffnesses of timber floor structures can be significantly compromised by ill-considered interventions like notching of joists and beams. It can be straightforward to address this damage and restore much if not all of the original potential of the structure. Care should therefore be taken to avoid drawing conclusions about potential performance that are unduly pessimistic.

Environmental design-imposed loads are continually being refined, particularly in response to the effects of Climate Change. Although the issue of most concern to the built environment is the steady rise in quantity and intensity of rainfall, this is not directly of structural concern. The main structural considerations associated with Climate Change are increased wind and snow loads.

There is no requirement in the building standards to prove that an existing building can tolerate wind loads derived from current codes of practice. However, it is necessary to demonstrate that an alteration or extension to an existing building will not generate increased imposed loads experienced by the building. This type of check should be carried out using current codes of practice for calculating the live load under consideration such as wind or snow. Early assessment techniques were particularly crude but the way of calculating these types of loadings has become increasingly accurate over time.

When this type of check is being carried out, care should be taken to avoid reaching a conclusion that is unnecessarily conservative and therefore results in unrealistically high loadings. For example, proper allowances should be made for wind-direction factors, topography, altitude and distance to the sea amongst other things. Careful attention should be paid to how a structure is constructed, and therefore how it will behave. It is essential to understand the likely failure mechanisms, and whether a higher-than-normal deflection in a roof structure genuinely presents a problem. In addition, careful attention should be paid to how a structure is constructed, as this will influence its behaviour under load and therefore how it will behave. Features that are most vulnerable to short-term loads like wind or snow are structural connections that have a low tolerance for repeated applications of sudden (being wind) or medium-term (i.e. snow) loads. These features include:

• Lightweight connections within roof structures (For example – nailed connections between collar ties and rafters)
• Valley constructions
• Fixings for tiled cladding (to walls or roofs)
• Informally constructed roof structures
• The roofs to open-sided buildings

In many cases, acceptable improvement can be achieved by enhancing the affected part.

If a cladding type is changed, then it is necessary to demonstrate that its fixings, and the structure to which the new cladding is fixed, can resist the environmental loads they might experience. Such a check should be carried out using loads derived from current codes of practice.

1.4.6 Mechanical and electrical plant (MEP)

A structural strategy for MEP should be considered from the beginning, and the impacts should not be underestimated.

Mechanical and Electrical Plant (MEP) installations are likely to result in significant and onerous structural implications for an existing building if a servicing strategy is not carefully worked out at the outset of a conversion project. Any strategy should cover installation, access for servicing, and removal. If the servicing strategy is not informed by and compatible with the existing structure, there is a high risk of escalation in the scale of the associated structural works, their complexity and cost, and the increasing likelihood of unintended consequences.

Issues to consider include:

• Will the layout of distribution routes and openings compromise the structural stability of the existing building, either locally or more widely?
• Have the structural elements of builders’ work in connection (BWIC) with the proposals been properly designed?
• Are heavy items of plant and equipment sited sensibly?
• Are there likely to be any issues with vibration arising from the installation of new plant and equipment?
• What enabling works associated with the installation of heavy items of plant or equipment are required?
• How will the items of plant or equipment be transported through the building to its new position and will the existing structure tolerate this route?
• Many MEP installations carry hidden implications for the existing structure that need to be understood and addressed during the design phase, an example being the installation of an underfloor heating system.

1.4.7 Materials

New materials should be compatible with the existing fabric of the building.

The materials found in traditional buildings can be very diverse in nature, ranging from basic earth through to carefully produced, highly refined cast and wrought iron. Some later 19th century buildings, such as factories and warehouses, may contain a wide range of these materials. For this reason, practitioners dealing with the conversion of traditional buildings should be familiar with all types of construction materials and the different ways those materials would be used.

There is a wide and increasing range of research published on the structural performance of building materials that are commonly identified as being traditional. This research ranges from earth-based and lime-based mortars, through to timber and cast and wrought iron. This research includes the determination of material properties for inclusion within current codes of practice for design. Full reference to this research should be made when assessing the capabilities of individual parts of an existing structure.

It is wrong to assume that a traditional building is made of low-quality, low-performance materials. The quality and structural performance of materials used will vary but will largely be dictated by ease of supply. This is why coastal urban areas can be well supplied by high-quality timber brought in through trade with the Baltic states and North America. Conversely, isolated areas may be poorly supplied with good quality timber or refined products like lime owing to the difficulty of transport.

As a rule of thumb, pre 1850 buildings would usually be constructed using locally sourced materials, but this changed with the advent of the railway. Materials could be transported large distances from quarries or docks, and this coincided with the increased use of manufactured materials like corrugated iron. For earlier or more rural buildings, the commonly referenced “four-hundred-yard rule” can be said to apply, where everything would have been obtained within a given radius of the site.

Typically, such considerations are applied to existing components like timberwork. Materials like masonry are less affected because they are massive forms of construction that generally carry low levels of stress, although care must clearly be taken when applying new point loads or creating large openings or isolated piers.

If there is uncertainty about the properties of an existing material, then it should be tested. Any materials testing should be conducted by a testing house that is UKAS (or EU equivalent) accredited and is familiar with the sort of material in question. However, the results of testing alone should not be seen as providing an answer. It may be that the factor governing the potential performance of any component is not its strength or stiffness but how it has been used.

It should be remembered that in the past, there was a greater emphasis and understanding of the need for ongoing renewal of certain materials. This means that any investigation should look beyond the visible surface, to ensure that the correct structural material has been identified. A good example of this is the nature of mortar within masonry where it is visible at the joints (usually referred to as ‘pointing’). The pointing might not bear any relation at all to the mortar at the heart of the wall because the ‘pointing’ is not original, has been renewed over time, possibly on numerous occasions.

A separate but related matter extends to the possible original (and therefore still extant) variation in mortar across the thickness of a wall. The bedding mortar at the heart of a thick solid wall didn’t need to be as robust (weatherproof) as that at its external face, meaning that it is not uncommon to find relatively soft, earth-based bedding mortars behind a more robust lime/Portland Cement-pointed or harled wall.

As previously outlined in this chapter, traditional buildings are a sum of their various parts. It is therefore also important to understand the wider characteristics of the various component materials and their importance to the successful performance of the building as a whole. Prime examples of this are the earth or lime-based mortars used in solid-masonry wall construction. The ability of the solid wall to release moisture is essential in the regulation of the internal environment of the building. The adoption of Portland Cement (PC) based mortars for pointing earth or lime-based masonry should not be permitted because they have very low moisture capillarity properties. This means that their application, even in small quantities, can be particularly and disproportionately destructive. This is the reason why conservation professionals normally resist the specification of PC-based materials in traditionally built walls.

Any new materials used in the alteration of structural components within a building should be compatible with the parent material to limit the extent to which the alteration might change the original behaviour of the parent component (structural or otherwise).

1.4.8 SER certification process

In the case of the conversion and rehabilitation of existing and historic buildings, if the SER Certification route is adopted, it is essential that the Certifier has demonstrable experience of comparable projects and is competent in dealing with the building type being dealt with.

Structural Engineers Registration Ltd (SER) was appointed by the Scottish Government's Building Standards Division (BSD) to administer a scheme for Certification of Design (Building Structures). The Scheme was established as a result of a joint initiative by the Institution of Structural Engineers (IStructE) and the Institution of Civil Engineers (ICE). This initiative was driven by the introduction of the Building (Scotland) Act 2003.

The Act encourages a holistic overview of the design of any building project by enabling an engineer who is a member of the scheme, as an Approved Certifier of Design (Building Structures), to certify the structural design of buildings in Scotland and to include a certificate with an application to local authorities for building warrant. This certificate is typically referred to as the ‘SER Certificate’, the certification process as ‘SER Certification’, and the certifying engineer as the ‘Certifier’.

The certifying engineer is responsible for ensuring that all aspects of the design of the structure of a project satisfy the requirements of the Building (Scotland) Regulations 2004. In this way, the requirements of the building warrant approval process are met from a structural point of view.

The legislation also makes engineers who are responsible for building structures specifically aware of their responsibilities and duties in the design, thus improving assurance of structural safety.

The SER Certification process is the preferred approval process for design and has gradually superseded previous procedures. This ties in with wider Scottish Government work to promote the use of Certification of Design and Construction.

1.4.9 The conversion of ruins

Chapter 5.6 of Part 1 of provides guidance on the reuse of ruins. It is recommended that practitioners engage with the verifier directly to establish the best way to apply the building regulations and whether the work should be treated as a conversion or alteration.

From a structural perspective, a ruin should not necessarily be assessed differently from an intact building. A clear understanding of the cause of ruination is essential, including whether it resulted from a single catastrophic event, such as fire, or from prolonged disuse and neglect.

The key to dealing with ruins is to assess the surviving parts of the building for what they are, taking into consideration all the matters considered in the preceding chapters.

Questions that should be considered when dealing with ruins include:

a) How has a single destructive event affected the surviving parts of the building?

b) Has long-term exposure of the surviving parts affected the material integrity of those parts? For example – has mortar been lost from masonry? Has there been degradation of mortar within the masonry?

c) Are the surviving parts capable of being consolidated and repaired to bring them back into good order?

d) Are the surviving parts capable of serving a structural role?

Removal or reconstruction of parts that are considered incapable of acting in a structural capacity does not preclude their retention in a non-structural role. In the case of listed buildings, there is usually a presumption in favour of retaining existing fabric. In these cases, it would be necessary to provide an alternative structural system, but the provision of such a system should not be detrimental to the existing fabric and character of the building.

a) Is the conversion simply the reinstatement of what has been lost, i.e. the restoration of the original building, or will its form be different?

b) Can the causes of deterioration be stopped?

1.4.10 Temporary works

Temporary works are those physical works necessary to facilitate the safe execution of the permanent alterations. They are normally removed once the alteration is complete.

The design and installation of temporary works should be handled with the same care given to permanent works. There are, however, additional considerations for temporary installations which are listed as follows:

a) Increasing the extent and complexity is likely to increase the level of physical interventions to the building. This should be avoided for obvious reasons, and in the case of an historic asset, owing to the increased risk of loss of existing historic fabric.

b) Managing deflections and movements within the existing fabric of a building becomes increasingly difficult with complicated temporary works. This is especially true in situations of multiple load transfers between load-carrying systems. Deflection and movement should be minimised as far as possible to mitigate the risk of damage to existing fabric.

c) Where possible, temporary works should be fully reversible. This extends to scaffold-restraint systems.

d) Stabilising access scaffolds by fixing into the existing fabric of the building should be avoided if possible. This is especially true for historic or listed buildings.

This is to avoid the permanent physical intervention associated with installing fixings capable of resisting tensile forces. The damage from drilling and any associated repairs is likely to have a significant impact on the character of traditional buildings, particularly if scaffolding installations are repeated over time. This is particularly important in substrates like ashlar masonry.

If physically tying into the building is unavoidable, the fixing components should be entirely removed and the substrate fully repaired. The ability of the entire receiving substrate to resist the applied loads should be checked – not just the capacity of individual fixings.

e) The actual design responsibility for temporary works normally lies with a temporary works designer who is separate to the permanent works designer. However, the permanent works designer must satisfy themselves that the temporary works designs are compatible with the existing structure and the proposed alterations. This review process must happen before any work starts on site.

In some instances, structural alterations may be complicated, unusual in nature, or located in a sensitive historic environment. In these cases, the permanent works designer should produce sufficient information that describes the sequence of operations and associated temporary works that have been assumed in the design of those alterations. This information should be captured in the SER Certification or building standards sign off.

1.5 Further reading

• Bates, W. (1991) Historical structural steelwork handbook. 4th edn. British Constructional Steelwork Association Ltd
• British Standards Institution (1970) BS CP 111:1970 Structural recommendations for loadbearing walls
• British Standards Institution (2005) BS EN 1990:2002+A1:2005 Eurocode: Basis of structural design
• Building (Scotland) Act 2003.
• Building (Scotland) Regulations 2004
• CARE (2026) Assessing historic assets that are proposed to be demolished or partially demolished
• Conservation Compendium articles as published in the magazine of the Institution of Structural Engineers ‘The Structural Engineer’.
• Davey, A. (1995). The Care and Conservation of Georgian Houses. Architectural Press.
• English Heritage publication (1996) Office floor loading in historic buildings
• Gilbert, J. (2025) The tenement revealed: History, design and construction. Whittles Publishing.
• Heyman, J. (2025). The Stone Skeleton. Cambridge University Press.
• Mainstone, R. (1975) Developments in Structural Form Allen Lane/Viking
• Smith, A & Kalorkoti, D. (2025) ‘Appraising factors of safety in existing engineered structures.’ IStructE Ltd.
• Structural Engineers Registration Ltd (2022) Procedures for Auditing the Activities of Approved Bodies and Approved Certifiers. Appendix B: Criteria for the Assessment of Projects March 2022