Thursday, 26 February 2015

Minimum intervention: or the art of doing less

As a conservation professional and member of the Institute of Historic Building Conservation, I am guided by the fundamental principles of minimum intervention, reversibility and like-for-like repair. This post considers specifically what is meant by minimum intervention and how it can be applied to building projects in order to retain historical significance during works of repair, maintenance and adaption.

Conservation principles are based on an unequivocal respect for historic building fabric. All work that has the potential to cause harm to that fabric requires a cautious approach of "changing as much as necessary but as little as possible" (Burra Charter, ICOMOS, 2013).

Conservation principles are established in UK law through British Standard 7913 - Guide to the conservation of historic buildings and the National Planning Policy Framework (NPPF).

BS 7913 and the NPPF recognise that interventions (physical changes to existing materials) are not only necessary for historic buildings in order to maintain the asset in a good state of repair, but are often desirable to ensure a building is kept fit for purpose and in regular use. Adaptive re-use of buildings requires careful management of change based on a detailed understanding of the fabric and its historical significance. Often there is a desire to improve the performance of historic buildings to enable economic development for new use or to respond to changes in building legislation. Minimum intervention design is an essential tool for all professionals involved in the design and specification of repair and maintenance works, development and change of use of historic buildings.

"Conservation is based on a respect for the existing fabric, use, associations 
and meanings. It requires a cautious approach of changing as much as 
necessary but as little as possible."
- Burra Charter Article 3.1

In practical terms, the default starting position for all works to historic buildings should be to prioritise the retention of historic fabric, in its original location, and only carry out alterations or removal of fabric if there is "clear and convincing justification" for doing so. The option of doing nothing at all should always be a consideration. Sound conservation solutions require the designer to adopt a deliberate conservation-minded approach to design and specification in place of a more traditional cost-driven approach, whilst also recognising the commercial needs of your client.

Where existing fabric is damaged or decayed, the minimum amount should be removed as is practically possible to allow a repair to be undertaken. This means for example, that where timber decay is evident, only that part of the timber that is decayed should be removed, with new timber spliced to the existing using traditional carpentry repair methods. Wherever possible, repairs and strengthening works should use traditional skills and materials rather than modern interventions using new materials and resulting in changes of load path.

The principal of minimum intervention can be applied not only to historic buildings but for all works to existing buildings. Often it can be more economic, quicker and safer to reduce interventions in to existing fabric in order to retain sound and stable conditions. It is often necessary to go the extra mile to prove existing building fabric is sound or suitable for re-use but the benefits to the building and its stakeholders means the extra time spent is seldom time wasted.

For another example of minimum intervention design, have a look on the case studies tab at the Memorial Hall case study where I designed a new steelwork floor within an existing Grade II* building without the need to strengthen or alter existing walls, beams, columns or foundations.

Have you used a minimum intervention approach to your historic building projects and retained historic fabric that might otherwise have been lost? I'd love to hear more from you.