Materials and Skills for Historic Building Conservation

Michael Forsyth

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118658666
• Edition: 1
• Topic: Arquitectura
• Subtopic: Arquitectura general

This book is the third in a series of volumes that combine conservation philosophy in the built environment with knowledge of traditional materials, and structural and constructional conservation techniques and technology:

• Understanding Historic Building Conservation
• Structures & Construction in Historic Building Conservation
• Materials & Skills for Historic Building Conservation

The series aims to introduce each aspect of conservation and to provide concise, basic and up-to-date knowledge for architects, surveyors and engineers as well as for commissioning client bodies, managers and advisors.

In each book, Michael Forsyth draws together chapters by leading architects, structural engineers and related professionals to reflect the interdisciplinary nature of conservation work. The books are structured to be of direct practical application, taking the reader through the process of historic building conservation and emphasising throughout the integrative teamwork involved.

The present volume - Materials & Skills for Historic Building Conservation – describes the characteristics and process of decay of traditional materials which inform the selection of appropriate repair techniques.

It provides essential information on the properties of the principal traditional external building materials. Their availability, sourcing and environmental impact is covered, as well as the causes of erosion and decay, and the skills required for their application on conservation projects.

It covers the main most commonly used materials and conservation techniques including stone, brickwork, lime products, concrete, iron and steel, timber, wattle and daub, and glass,

Thirteen chapters written by the experts present today's key issues in materials and skills for historic building conservation: Gus Astley, Patrick Baty, Charley Brentnall, Michael Bussell, Michael Forsyth, Tony Graham, Chris Harris, David McLaughlin, Brian Ridout, Mike Stock, Geoff Wallis, Ian Williams, Rory Young

1
The philosophy of repair

Michael Forsyth

Traditional or vernacular building is concerned with utilising indigenous materials and with local knowledge of climate and topography. The geology and topography of a region determine the character of its buildings, as was first consciously articulated by William Smith, the ‘father of geology’, whose pioneering geological map in the early nineteenth century ‘changed the world’.¹ Nearer to our own time, the essential and distinctive character of the English counties was captured by Sir Nikolaus Pevsner’s introductions to his county architectural guides. These always start with landscape and the earth – granite, sand, slate, chalk, clay – and the first illustrations are of hills and fields, because it is these features that give each county, and its buildings, their character. In Herefordshire ‘there is not a mile that is unrewarding or painful’. In Northumberland it is ‘rough the winds, rough the miners, rough the castles’. Gentle Hertfordshire is ‘uneventful but lovable’. Regional character is quickly eroded by unsympathetic repair and alteration using materials imported into the region and by renewal rather than repair, consolidation and effective ongoing maintenance.

The key to appropriate historic building repair is awareness of the fundamental difference between modern construction and traditional building. Modern construction is based around impermeability and relative ‘thinness’, as with cavity wall construction, known in North America as using the ‘rain screen principle’. If, through capillary action, moisture should penetrate the outer masonry leaf or the cladding, the air cavity (which may be partially filled with insulation) is wide enough to break the capillary action and surface tension of the water, which then descends by gravity and drains through weep holes. The further function of the cavity is to eliminate thermal bridging. Steel and glass may be thought of as the ultimate ‘thin’ impermeable building construction.

Traditional building by contrast is based around very different principles: thermal mass; breathability; flexibility; and, depending on the construction, the use of a protective, sacrificial skin. Thick walls provide thermal mass, sustaining warmth in winter and coolness in summer. The walls (and traditionally the floor) are breathable and admit moisture, which then evaporates freely. For masonry construction, lime mortar separating the stones or bricks is softer than the structural material and allows the building to move and settle differentially without cracking. Lime mortar is also more breathable than these materials, so the majority of evaporation is through the joints. When hard, impermeable Portland cement pointing was a introduced a century or so ago, the brick or stone became the principal conduit for evaporation, causing leaching of salts and consequent chemical corrosion in the material, and water collecting at the joints caused mechanical deterioration due to freeze–thaw action.

In limestone areas rubble construction also traditionally relies on a protective skin of lime render which is sacrificial to the structural material. The render is then coated with limewash, which may be coloured with earth-based pigments and, if the finish is smooth as opposed to roughcast, sometimes scored for ‘joints’ to produce poor man’s ashlar. The twentieth-century taste for hacking off render and plaster and revealing the stonework beneath – think of the worst pub interiors, historic plaster removed and the rubble wall beneath pointed with grey cement – began with the Victorians, and opposition to the practice by the Society for the Protection of Ancient Buildings (SPAB), founded by William Morris and others in 1877, launched the bitter war of ‘scrape versus anti-scrape’.

It is essential that traditional buildings are repaired sympathetically, and it is the stark fact that the majority of historic building repair today is required less as a result of the natural degradation of the building fabric from its original state, than of damage resulting from inappropriate repair over the last century, whether from incorrect pointing and mortar repairs, expanding rusted iron in old stone repairs causing spalling or delaminating Portland cement render.

Historic building repair embraces a spectrum of interventions from routine maintenance and the ‘do nothing’ option, through a comprehensive repair programme, to restoration, the replacing of lost features or entire rebuilding (as with the National Trust’s Uppark, West Sussex, almost destroyed by fire in 1989 and rebuilt), provided there is precise evidence of what was there. Replacement is never acceptable when it is conjectural. Sir Bernard Feilden lists this spectrum as consisting of seven degrees of intervention: (1) prevention of deterioration; (2) preservation of the existing state; (3) consolidation of the fabric; (4) restoration; (5) rehabilitation; (6) reproduction; (7) reconstruction.²

The preferred option is always minimal intervention, and the general principle is to use traditional materials and techniques wherever possible. In the case of ruined monuments, minimal intervention may extend to retaining ivy on the basis that it may actually protect the structure that it covers – a kind of managed ‘picturesque decay’. However, the basic well-known golden rules of conservation – minimal intervention, conserve as found, ‘like for like’ repairs, and reversibility – are not always compatible with these principles, or with each other. For example, when repairing a timber roof structure, discrete insertion of steelwork – far from a ‘like for like’ repair – may result in minimal or no loss of historic fabric compared with cutting back to sound material for a ‘like for like’ repair with a scarfed joint using new, similar timber; indeed, iron has been used for strengthening timber structures for centuries. The ‘conserve as found’ principle, meanwhile, may fly in the face of a philosophical decision to wind the clock back to the original architect’s intention, while some repairs, such as grouting a rubble stone wall, are intrinsically non-reversible.³

These are but imperfect guidelines and each situation must be assessed. A philosophy or policy for the building fabric and its repair must be adopted, not only for major projects where this might form part of a conservation plan, but also for localised repairs, such as a small repair to a lime render (Chapter 4) or to wattle and daub (Chapter 10). Once conservation work is under way, recording at all stages is essential. It has always been a tenet of SPAB that repairs should be identifiable, and in the early days masonry repairs would be carried out with tiles, though today more subtle means would usually be used such as writing a date on new timber in a roof space.

The manifesto which William Morris and the other SPAB founder members issued in 1877 was written in reaction to the over-zealous, over-confident church and cathedral restoration work of the eighteenth and nineteenth centuries where the aim was to return the buildings to a uniform style and to make them look smooth and crisp:⁴

It is for all these buildings … of all times and styles, that we plead, and call upon those who have to deal with them, to put Protection in the place of Restoration, to stave off decay by daily care, to prop a perilous wall or mend a leaky roof by such means as are obviously meant for support or covering, and show no pretence of other art, and otherwise to resist all tampering with either the fabric or ornament of the building as it stands; if it has become inconvenient for its present use, to raise another building rather than alter or enlarge the old one; in fine to treat our ancient buildings as monuments of a bygone art, created by bygone manners, that modern art cannot meddle with without destroying.

The manifesto may predate the concept of adaptive reuse, but it laid the ground rules of modern building conservation practice and still forms the basis of the SPAB’s philosophy. Another influential publication that is still available was Repair of Ancient Buildings by the architect A.R. Powys, Secretary of the SPAB before and after World War I.⁵

An interesting monitor of the continuing evolution of conservation philosophy today is the presentation of country houses by the National Trust and English Heritage. The sanitising of country houses in the early days of the National Trust, involving the rather lifeless restoration of their interiors to a given, original period, was advanced at Kingston Lacy, Dorset, from 1982, towards an approach of retaining the history of the building with its nineteenth-century alterations. The ‘conserve as found’ option had more radical expression at Brodsworth Hall, South Yorkshire. Here, English Heritage carried out a full conservation programme for the building fabric from 1988, but carefully retaining – and, where necessary, removing then later reinstating – water-stained wallpaper, faded fittings and everyday objects that had been left in the house, as if the owners had simply gone out for the day. Newhailes House, near Edinburgh, was perhaps the extreme swing of the conservation pendulum – more ‘conserve as left’ than ‘conserve as found’. After conservation had taken place, the furniture was carefully heaped back into the corner of the library as it was when the property was acquired by the National Trust for Scotland. The last occupant’s sitting room was reinstated with television and electric fire, and the ironwork to the steps up to the front door consolidated but left rusty.

Endnotes

1. Simon Winchester, The Map that Changed the World: A tale of rocks, ruin and redemption (Penguin Books Ltd, London, 2002).2. Bernard M. Feilden, Conservation of Historic Buildings (Butterworth Heinemann, London, 2003), p. 8.3. See also Understanding historic building conservation, Chapter 1, and Structures & construction in historic building conservation, Chapters 1 and 2.4. The best account of this era is Gerald Cobb, English Cathedrals: The forgotten centuries: restoration and change from 1530 to the present day (Thames and Hudson, London, 1980).5. A.R. Powys, Repair of Ancient Buildings (J.M. Dent & Sons Ltd, London, 1929; Society for the Protection of Ancient Buildings, 1996).

2
Stone

TYPES OF WALL CONSTRUCTION

Ian Williams

Stone construction in traditional building can be initially divided into two types: rubble and ashlar. These two methods of construction are subject to further division. In the last century or so stone has also been used as cladding. Repairs must follow carefully previous methods of preparation and setting.

Rubble

Rubble walls are either random, the stones being used more or less as they come to hand, or squared, with straightened edges. These two types further subdivide. ...

Table of contents

• Cover
• Table of Contents
• Series
• Title
• Copyright
• Preface
• Contributors
• 1: The philosophy of repair
• 2: Stone
• 3: Brickwork
• 4: Lime-based plasters, renders and washes
• 5: Concrete and reinforced concrete
• 6: Stone slate, clay tile and metamorphic slate
• 7: Cast iron, wrought iron and steel
• 8: Understanding decay in building timbers
• 9: Timber
• 10: Wattle and daub
• 11: Sash windows
• 12: Window glass
• 13: Exterior colour on the smaller town house
• Index
• End User License Agreement