The National Trust for Scotland (NTS) has recently launched the ‘Box the Hill House’ appeal to raise an additional £1.5 million for a total project budget of £4.5 million to provide conservation repairs for Charles Rennie Mackintosh’s Hill House, Helensburgh.
A wet, modern building
On completion of Hill House in 1904, harling was applied. Harling is a characteristic of many Scottish buildings (even sometimes referred to as a ‘national’ surface finish) and refers to the application of a layer of material to the exterior building surface – this is intended to be both decorative and waterproof. These rough-textured surfaces traditionally used lime and aggregate – by using lime the surface is permeable so moisture evaporates away from the solid wall (either stone or brick work). When lime is used the harling is weaker than the solid wall and therefore acts as a sacrificial layer that can be replaced and renewed on a regular basis.
In the case of Hill House, Portland Cement was used in the harling mix rather than the more traditional lime. At the time in the early 20th century Portland Cement was an expensive, innovative ‘wonder’ material that was celebrated for its beneficial uses and central to new thinking on the specification of building works and materials. Mackintosh’s use of new materials (Portland Cement is just one of many) is explored in this really interesting work by Ranald MacInnes.
Unfortunately for Hill House Portland Cement is impermeable, and by trapping moisture this has resulted in accelerated decay and erosion of the soft red sandstone, whilst water ingress has damaged the historic interiors. The problems associated with the inappropriate use of cement-based materials on traditional buildings are now very well known and much conservation work is concerned with their removal.
Further reading suggests Hill House has a very long history of repair and reactive maintenance. Analysis of early photographs by the NTS suggests that when the building was completed in 1904 the harling was already becoming discolored. Hill House has subsequently been subject to numerous repairs from the 1940s onwards with the growing awareness of the severity of the problems associated with harling. Debates have ensued over the conservation philosophy to be used on the building (specifically how retention of ‘original’ fabric could be balanced with the required remedial works) as a result Hill House was subject to a number of experimental conservation programmes.
Conservation in the box
In the case of the Hill House this is a very significant historic building and a very high profile case of accelerated decay. The NTS have most recently undertaken a condition assessment at Hill House in 2012, and it is this assessment that the current scheme of conservation works is based upon. The Hill House ‘Box’ is a temporary shelter designed by London-based architects Carmody Groarke – it will provide protection from wind and rain, and create a working space for conservation specialists to undertake their work and for the public access to see conservation work in progress.
After a high-profile launch of the NTS campaign the Hill House shelter has been named as a cage, a box, and a shield for the ‘dissolving’ architectural masterpiece. But shelters have a much longer history of use in conservation. Temporary structures are used to provide a sheltered space to enable archaeological excavation and conservation or a longer-term solution for site conservation and presentation. On archaeological sites permanent shelters can be essential conservation measures but they are controversial and can result in debate over their visual impacts on a site setting or they can change the microclimate (and therefore as an unintended consequence accelerate erosion). Of those permanent shelters on archaeological sites the most famous is the structure over the 13th century Casa Grande ruins in Arizona. The shelter here was built in 1932 and the shelter itself is also now listed on the National Register of Historic Places.
Increasingly, temporary sheltering and scaffolding of conservation works provides the opportunity for the public to engage with the conservation process through opening up access or through hard-hat site tours. For example the National Trust provides public access to Castle Drogo during its current 6 year conservation scheme (Castle Drogo is another ‘wet’ early 20th century building where a vast flat roof was made using asphalt as an ‘innovative’ material). Increasingly conservation projects makes use of so called ‘principled procurement’ to integrate public engagement within conservation project planning and financing from the outset. Just like Hill House’s Box – scaffolding has also been used for marketing construction schemes – making use of scaffold sheeting or netting (making an interpretative or marketing opportunity out of the Health and Safety in Construction requirements for containment of work in open or vulnerable locations).
Chichester Cathedral’s temporary enclosed scaffolding, Spring 201