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Theme: conservation

The combination of John McAslan + Partners' Modernist principles and deep-rooted interest in how buildings are put together has produced many interesting context-based conservation projects, successfully sitting the old and the new side by side Over a period of 15 years John McAslan + Partners has completed a number of projects that incorporate historic buildings. In date these range from the Royal Naval Hospital in Greenwich, dating predominantly from 1650, to Swiss Cottage Library, which was opened by the Queen in 1972. In scale they range from a house of no more than 50m 2in Northampton to a college of 20,000m 2. The challenge in this context is to develop a design approach that is thoroughly context-based and takes its clues from the historic character of the existing buildings, while developing a portfolio of work with a sense of its own consistency and material quality. New glass and timber sit next to rubble stone and horsehair plaster in one instance, and against a black basalt precast concrete in the other.

The practice has no hard-and-fast rules with regard to materials and detailing. Beyond a deep-rooted interest in how buildings are put together and the basic functional need to make things robust and watertight, each project starts from first principles. The design process always begins with a thorough appraisal of what exists and a distillation of the key characteristics of a historic site. This rarely leads to a process of historic reconstruction - though there are situations where existing parts of a building need to be 'stitched together' seamlessly. More often the response is to enhance the legibility of the best of the old by giving it breathing space, peeling back the layers and removing distracting additions. In this context, new interventions can develop their own language without compromising the historic character of the place. The practice also has a view that new interventions should be specifically focused and offer as much technical and operational benefit as possible.

Much of this relates to the nature of the existing building fabric. Where this sits hand in hand with historic detailing that matches our own Modernist principles - at Peter Jones and Swiss Cottage - new components often use the same materials and principles of construction as the existing buildings.

Where these are less consistent - at the Royal Society of Arts and the Royal Academy of Music - the practice's approach is to create visual separation between the new and old and to make sure that the new elements, while being legible in their own right, also enhance the historic quality of the site.

While the practice has a strong association with 20th-century buildings, it has also completed a number of projects that deal with historic buildings from the 16th to the 18th century. The first group of case studies deal with either Georgian or Victorian buildings.

Royal Society of Arts The practice's first completed project of this type was the remodelling of the Royal Society of Arts' Great Room in John Adam Street, London W1, which was originally designed by Robert Adam in 1750. The commission was to upgrade an important historic space to provide a high-quality debating and conference facility for the society, its members and commercial clients. The most significant design decision was to confine all new interventions to two key elements: a new roof lantern and a new raked floor. The space had originally included a circular lantern. This had been replaced in the 1920s but, due to concerns about the conservation of Charles Barry's allegorical paintings that line the walls of the space, the glazing had remained obscured for most of the 20th century.

Research concluded that the existing rooflight was of no great historic or architectural quality and would require substantial remodelling to allow appropriate daylight control. Rather than working with the existing lantern, we agreed the principle of a new lantern with English Heritage. This lantern allowed daylight to be moderated to conserve the paintings, but it also incorporated artificial lighting and air extract from the space, obviating the need for any additional interventions at ceiling level. The lantern itself was constructed from a frame of stainless steel flat plates that were cut to radius. It was then lined with curved laminated glass that was white on its vertical faces and clear on top. The top of the lantern is essentially flat (sloping by 3°). All of the metalwork and glazing was carried out by F A Firman, which commissioned engineer Techniker to develop constructional detailing.

Lighting around the perimeter of the lantern was inset into a deep recess to provide visual separation between the lantern and the space's historic ceiling finishes. Erco provided the light fittings. Around the perimeter of the lantern, all the adjacent materials were stitched together to match the historic materials of Code 8 lead, Welsh slate, oak timbers and fibrous plaster. The new component is overtly contemporary but its impact on the adjacent structures has been kept to a minimum. It works as a carefully considered intervention that enhances the character and the functionality of the space.

For the floor, a new steel substructure was constructed (by Costain) with plywood and white oak forming the wearing surfaces.

Between the original oak floor structure and the new raking floor, a complex mechanical and electrical system was installed in the shallow void (1.3m) to accomodate air distribution and emergency lighting. Battle McCarthy with Rybka designed the system and particular attention was paid to the acoustic performance of the plant that sits immediately below the room's raked seating, designed by Robin Day for Hille. Purpose-designed carpet runners (by Roger Oates) were installed to soften the room's acoustics. The perimeter of the floor incorporated a broad shadow gap to allow the new floor to disengage from the fibrous plaster and plywood wall paneling.

Both interventions are unashamedly contemporary. There are links back to Adam's architecture - the floors are of the same material as Adam's (removed at the end of the 18th century) and the lantern's form is Classical in geometry, but these connections are quite indirect and the materials and detailing are clearly rooted in the Modernist tradition.

Royal Academy of Music Of a similar nature but on a far larger scale, the new Recital Hall at the Royal Academy of Music created a clearly contemporary new intervention among a group of varied historic buildings. Located in a semi-redundant courtyard used in recent years for parking and storage, the new hall now sits quietly between Nash's white stucco York Gate building (1822) and Sir Earnest George's banded brickwork of the academy's main building (1977). As well as providing a new and valuable facility in its own right, the new hall also forms new connections, below ground, between the two historic buildings to improve their use and secure their future as part of the college's teaching space.

The most visible element at street level is the barrel-vaulted form of the Recital Hall roof. This is constructed of white precast concrete by Docomo. It sits on a structural frame of in-situ concrete and is clad in standingseam zinc with a fully glazed elevation onto Marylebone Road. The roof build-up comprises insulation bonded directly to the concrete shell and the standing-seam zinc is bonded to the insulation in turn to provide a fully sealed sandwich. The roof itself oversails the supporting walls and the zinc is turned around the edges of the shell. Externally, the walls are lined with insulated render. The glazing of the hall's end wall is constructed from standard MAG Hansen sections, but multi-layered to provide important acoustic isolation from Euston Road. Two pairs of double-glazed, steel-framed units are spaced 800mm apart to provide a fully sealed acoustic envelope.

Internally, the concrete frame of the new hall structure is lined with oak and painted timber panels, and the flooring is of oak.

The spaces around the new hall, which abut Nash's painted stucco elevations to the west and George's brick elevations to the east, are surfaced in white precast stone paving slabs with under-slab drainage.

As with the Royal Society of Arts, none of these materials is deliberately historic. The simplicity of form takes its clues from Nash's Neoclassical elevations, but the detailing is of the early 21st century. The tonal quality of the white concrete sits comfortably with Nash's painted exteriors. The standing-seam zinc resembles the more traditional leadwork on the roofs of the adjacent buildings.

Peter Jones The practice's first major phase of work at Peter Jones was completed in the summer of last year, and the second major phase is due to open shortly. Having spent many years working with Mendelsohn's construction methods at the De La Warr Pavilion, the practice quickly got to grips with the materials and detailing of Crabtree, Slater and Moberly's store, which was clearly influenced by Mendelsohn's German pre-war department stores. The building is constructed from a steel frame with concrete floors and a steel and glass Crittall curtain walling system. While the building's perimeter along Sloane Square, King's Road and Cadogan Gardens appeared unified and consistent, the interior planning of the store was far less consistent. Two deep lightwells had been formed in the central section of the building, and over time these had been partially infilled. High-level areas meant for recreational use by staff - squash courts and a small theatre - had become part of the selling space but were poorly connected and difficult to access.

Our approach was to rationalise and extend the central lightwell to allow it to rise through seven floors, and to re-plan the spaces around the perimeter of the lightwell to provide more consistent and useable space. In functional terms, the addition of two new floors in the central section of the store has allowed space for a new coffee shop and partners' dining room with panoramic views over Kensington and Hyde Park.

Much of the central section of the store was reconstructed using a steel structural frame with concrete floor slabs keyed in to the existing structure. One of the principal design challenges was to install a new cooling system within very limited floor-to-floor heights.

Rather than use conventional fan-coil units within a suspended ceiling, the ventilation approach developed with Troup Bywaters and Anders was to incorporate chilled ceilings within the coffers formed by the steel frame, to draw air in through the external curtain wall, and to extract air through the central atrium.

These infill panels also contain all of the display lighting and allow the building frame to remain legible, as intended by Crabtree, Slater and Moberly.

The roof of the extended atrium is constructed from flat glass with a partial frit to reduce solar gain. This is supported on a steel substructure designed by Techniker. Sharing similarities with the Royal Society of Arts rooflight - though of a much larger scale - this component does more than just provide daylight. It also provides the primary ventilation extract route and incorporates the high-level smoke extract, which allowed the atrium to be extended without the need for fire partitions around its perimeter.

To the north of the store, new elevations are clad in painted aluminium-framed curtain walling. At high level this is predominantly clear to afford long views, while at mid-level the glazing is translucent to allow daylight into back-of-house areas, where racking and storage sit immediately against the curtain wall.

Within the new atrium space, the new slabs are lined in painted plaster to match the original detailing - the wells are 'D' shaped in plan and have radiused edges. The original zincplated handrails have been retained but, to resolve current health and safety requirements, these have been set inboard of new clear, curved and toughened glass panels that provide edge protection around the wells. Within the wells a suite of new escalators with glazed side panels (by Otis) provides access between the floors and reflects Crabtree's unexecuted intention of escalators between floors.

At Peter Jones, the distinction between old and new is not so clearly discernible. The building's Modernist language shares much in common with John McAslan + Partners' construction principles and the junctions between historic fabric and new components are predominantly seamless. What is of importance is that the quality of the interior spaces and the environmental character of the store have been enhanced significantly, but in the light and voluminous spirit of the original building.

Swiss Cottage Library The practice has recently completed a major redevelopment project at Swiss Cottage Library (Sir Basil Spence, 1964). Here, Spence's rigorous and modern construction is closer in character to John McAslan + Partners' own constructional language than in any of the three projects described before. The distinction between original fabric and new materiality is quite subtle and in some instances barely perceptible. This was a deliberate approach, reflecting the parity between Spence's pared-back and entirely rational building and our own interest in the principles of Modernism. Unlike both the Royal Society of Arts and the Royal College of Music, where interventions were quite distinct, at Swiss Cottage the entire building has been represented. From reconstructing the in-situ concrete foundations and slabs at the southern end of the building to repair subsidence, to re-glazing the central atrium, the intention has been to enhance the simple and elegant logic of the building, rather than to make obvious interventions.

Treatment of the building envelope was primarily a conservation exercise. Concrete fins and their infill panels of black basalt aggregate-faced concrete were repaired with Sika mortar and cleaned. Most of the aluminium-framed single glazing of the external elevations remains unaltered but the treatment of spaces around the building perimeter has been re-planned to allow this glazing to be unobscured by book shelving and office fittings. The original mastic asphalt of the roof was replaced with a new elastomeric bitumen membrane and specific attention was paid to the dressings around the very shallow roof upstands to maintain the original roofline while ensuring watertightness at the parapet junctions. At low level much of the original Portland stone walling had cracked badly and had to be reconstructed, but was replaced with the original material salvaged from the adjacent swimming baths. The approach here was not to make bold statements about new and old, but to reconstruct and repair the envelope and enhance its simplicity and rigour.

The primary new interventions relate to the building's interiors. Here, frameless glazed openings have been introduced at atrium level (glazed doors are by F A Firman) to increase the sense of accessibility and transparency. Georgian-wired glass within the painted steel perimeter glazing has been replaced with Pyran clear glass from Schott Glas to enhance the sense of cross-visibility between the perimeter library spaces and the central atrium. In particular, the original linking cross-bridges have been stripped of their cluttered offices and now provide informal seating areas and a place to drink coffee and read periodicals.

In this instance, many of the new materials are entirely consistent with the original methods of construction - floors are predominantly of white terrazzo (W B Simpson and Sons), joinery is in American walnut (Humphrey and Stretton) and ceilings are of perforated aluminium (Hunter Douglas). They are new but are constructed from the materials used by Spence in the 1970s.

Adam Brown is a director at John McAslan + Partners

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