Guide to Conversion of Traditional Buildings

An introduction to structural behaviour in traditional buildings

Working with traditional buildings requires a different mindset to one that would ordinarily be applied to new or more recent buildings. There are a few reasons for this. Firstly, the designer is dealing with something that already exists and might have done so for an appreciable period of time. Therefore, the key questions to consider are whether the building has performed satisfactorily over time, how it has accommodated previous alterations, and whether it is showing any indications of structural distress.

Secondly, it is rare for traditional buildings to have a defined structural system in the same way as a building that has been scientifically designed and/or is the product of a formal approval process. A designer must therefore consider how the building works structurally, including the nature of its structural components and its overall structural capacity. Thirdly, consideration must be given to the implications of any proposed changes for the building’s character, fabric and performance. The very fact that the building exists can mean that an assessment of it based solely on a theoretical concept, as used for designing new buildings, is neither necessary or appropriate.

This guidance is focused on traditional buildings and the bulk of these are generally acknowledged to have been built before or around 1919. The majority of these will share similarities in the traditions of construction, essentially being built with mass masonry walls that have lath and plaster internally, and timber floor and roof construction. However, there is considerable variation in the construction of pre-1919 buildings if, for example, they are a rubble thatched cottage or an advanced factory building that might have significant amounts of early concrete or glazed brick in its construction.

Although this guidance covers the bulk of Scotland’s traditional buildings which have similarities in their construction and detail it is important that the advice is applied based on each case on its own merits, where the individual characteristic of the building informs the thinking and approach to change.

The purpose of this chapter is to set out how the requirements of the building standards can be met when change is proposed. In some respects, this does not necessarily rely on current Codes of Practice, British Standards or other theoretical building design guidance. The introduction sets out the basic process to be adopted when dealing with existing buildings, and the subsequent guidance deal with some of the more detailed aspects of the process.

The underlying presumption in this guidance on conversion is that the building is in good condition for its age and type, but this might not always be the case. A degraded or derelict historic building should always be brought back into a good state of repair prior to implementing alterations and upgrades for its conversion to a new use. These are likely to include in situ repair of historic fabric, and any enhancement works necessary for the new use. Such measures are beyond the scope of this publication, and wider advice should be sought as necessary.

From a structural point of view, traditional buildings differ from modern buildings in that they typically do not have discrete, independent, structural skeletons. A direct consequence of this is that buildings can be the collective sum of all their parts, sometimes with complex load-sharing systems. This means that certain parts may be performing structurally in a way that is not easy to predict. A good example of this is the internal partitions within individual properties of the Scottish tenement, which often act by chance as load-bearing elements. The manner by which these building types were actually built can confer a structural role on these walls that is not immediately apparent, and their removal might well have unintended and disproportionate consequences for the stability of the wider building. This is generally the result of how a building was originally procured and constructed, whereby various craftspeople employed long-standing, empirically-derived solutions to meet their own particular methods.

Aside from the larger, more prestigious buildings, it would have been unusual for an historic building project to be under the control of a single architect or other design professional in the way that we understand today. Even where a designer has been identified, they might not have had any real influence on the actual construction techniques adopted. Although the word architect is derived from the Greek for chief mason or carpenter, architects from the 18th century onwards would increasingly have seen construction details and methods as the preserve of the mason or carpenter and might not have had any formal role in the site works.

The mere survival of a building over many generations might reasonably be presented as strong evidence that all is generally well with it. Whilst such statements can be powerful, it is important that they are informed by defensible logic. It is therefore essential that a thorough understanding of the building from a structural point of view is established as far as is reasonably possible at the outset of a project and used to inform the ongoing design process. This would include establishing the following.

• The nature of the existing construction
• The quality of the existing construction
• The existence of any alterations that have been carried out to the original construction and the structural implications of those changes on the original construction.
• The form of the existing structure
• Any changes of use experienced by the building that may have changed the loads applied to its floors
• The structural role that each component is performing
• The condition of the existing construction.

Much of this may become apparent through a visual examination of the building. However, at times it will be necessary to conduct some opening-up work or other intrusive investigation to establish key hidden details in the existing structure. This work may extend to materials testing to establish the engineering properties of those materials. The need for such intrusive work can sometimes be reduced by good archival information. The benefits derived from researching a building through archival information can extend beyond the project by increasing the knowledge of building professionals in different traditional building types.

A key parameter in assessing a building is to determine its robustness – how well it has tolerated the impact of the climate, ground conditions and many generations of occupants, and how well it might tolerate them in the future. This can be the trickiest part of any building assessment as it is essentially a qualitative assessment, but judgements of this type are significantly improved through experience. That said, a logical assessment of each part of the building, from the general to the particular, is always a good basis for an assessment.

Once a comprehensive understanding of the building as a structure has been established, it is possible to assess the implications of any proposed changes to that structure. Such changes might be twofold: physical change predicated by intrusive alteration, for example, slappings (i.e. new openings) or complete removal of walls or theoretical change associated with a formal change of use (for example, increased imposed floor loadings associated with a conversion from domestic to office use).

Having established the structural implications of the proposals, physical or otherwise, the engineer can then determine how to address those implications. Dealing with proposals for physical change is made easier and results in less construction work if those changes are carried out in a manner that is compatible with the primary structure of the building. A good example of this is positioning new stairwells or lift risers within structural bays that are defined by the layout of primary floor beams, leaving those beams undisturbed.

Caution is advised against complexity. Too often, disappointing results stem from works that have been poorly executed owing to a failure to recognise the importance of correctly implementing every aspect of an intervention exactly as intended by the designer. It is also important that any intervention is legible to future building users, as documentary records describing construction works might not always survive.

The appraisal, assessment and alteration of traditional or historic buildings is best served by practitioners competent in their care and adaptation. In addition, familiarity with the building type under consideration is clearly advantageous. The Conservation Accreditation Register for Engineers (CARE) is a useful source of such practitioners. Those on the Register – often referred to as ‘Conservation Accredited Engineers’ – have gone through a rigorous procedure to confirm their range of knowledge of older and traditionally-constructed buildings and the most appropriate approaches to their care, repair and adaptation.

In addition, admittance to the Register requires understanding and empathy with conservation philosophy and practice and methods applied to heritage projects. Accredited engineers have broadened their experience to include an extensive knowledge of the use, behaviour and the degradation of the materials used in traditional construction. This gives them superior insight into the behaviour of existing buildings, allowing them to develop solutions which cause the minimum disruption to the building fabric. Whether or not a Conservation Accredited Engineer is appointed to a project, demonstrable experience of comparable work should always be sought by those procuring work on traditionally constructed or culturally significant buildings, and a sensible, sensitive approach applied throughout.

In addition to advising on building conversions, Conservation Accredited Engineers are well placed to advise dispassionately and independently on defective and dangerous buildings and structures and the planned full or partial demolition of a building. Such advice is essential when guarding against the unnecessary demolition of listed buildings as a result of unfounded or unjustifiable safety concerns or spurious advice. Further guidance on this can be found in the CARE Guidance for Engineers when assessing heritage assets that are proposed to be demolished or partially demolished in the United Kingdom and Ireland.