Sevenoaks Performing Arts Centre, Sevenoaks, Kent

Introduction

The new Performing Arts Centre for Sevenoaks School sits on a beautiful sloping site overlooking Knole Park. It contains a concert hall large enough for a symphony orchestra, choir and an audience of 450, a recital room for 100, and a music school with 23 teaching spaces of various sizes, plus a new drama studio, with drama teaching and technical spaces.

A successful independent co-educational school, Sevenoaks occupies a 40-hectare site, much of it an informal backlands campus of existing properties and land. In 2005, as the school had outgrown some of these facilities, Tim Ronalds Architects was commissioned to devise a development plan, part of which proposed the sequential redevelopment of three 1960-80s buildings around a new landscaped space. The performing arts centre, set between an existing theatre and sports hall, is the first of these new developments.

The building is built of warm, solid, natural materials – grey brick, timber and zinc on the outside, a soft red brick and timber on the inside. Tim Ronalds Architects decided at the outset that natural timber should be used throughout the building interior, particularly in the performance spaces where it would offer the warmth and acoustic response conducive to music. Douglas Fir was chosen for its colour and vitality; it was used throughout - for the roof structures, windows and doors, joinery, panelling, handrails and fitted furniture – except for the flooring where oak was specified.

Building description

The concept was determined by the site, a two-storey slope towards east-facing views, with the approach from the west. To make the most of the view and to harmonise with the rest of the school in scale and complexity, the building is designed as an aggregation of smaller forms rather than a single entity. It has three distinct volumes – the concert hall, the recital room, and the teaching block; the sloping site was used to set these elements into the landscape, minimising their bulk.

The main entrance leads into the foyer, an internal public space on the intermediate level, which connects naturally with the performance spaces on the level below and the music/drama classrooms above. The concert hall is a magnificent space, finely tuned to its function. It is rectangular in shape, 18 metres wide and 25 metres long, with raked seating facing the platform at the eastern end. It has a steeply pitched roof which is exposed on the inside and lined with Douglas fir boarding; the roof structure, also exposed, is a series of timber and steel trusses which rest on concrete columns.

The shape of the hall, together with the timber structure and boarded finish, provides the volume and detail needed for the acoustics of a high quality concert hall. The high roof with its low eaves gives the space scale yet intimacy.

The smaller recital hall, used for smaller and more intimate performances, has a pyramid-shaped roof with an off-centre pitch and is also lined with Douglas fir.

The roof structure and acoustic strategy of the concert hall

As Tim Ronalds explains: ‘We had the chance to create a music venue of great acoustic quality. There are two acoustically proven forms for music halls of this scale; rectangular, such as The Wigmore Hall, or with a big roof, such as Snape Maltings. We settled on the latter model, partly because it could provide the large acoustic volume required and still have intimacy, and partly because we wanted to avoid a box-like form on the site. To retain views from the school out over Knole Park, volumes needed to be at right angles to the contours and we liked the drama of a roof ridge projecting out into the space of the landscape’.

It was important to the architect that the acoustic quality was achieved by using the shape and geometry of the hall and by exposing the timber construction and finishes, rather than by the addition of adaptive or remedial elements. The 50 degree roof slope gives a volume of about 4,700 cubic metres to provide sufficient reverberation and to control the loudness of a full orchestra.

The volume of the concert hall is clearly visible due to the delicacy of the roof structure. Each truss is a double chord of 320 x 97mm Douglas fir members set 75mm apart, connected to 145 x 145mm Douglas fir struts and 28/39mm diameter steel tie bars with forked connectors and steel connection plates. Despite their delicacy, the trusses support a two-storey natural ventilation plenum at the apex of the roof, and a particularly solid roof construction made up of 13 layers to exclude external noise (the hall is below the flight path to Gatwick and lawn mower noise is a summer problem). This dead load, in combination with bottom chords which were not horizontal, all bearing on relatively thin columns, led structural engineer Price & Myers to pre-tension the steel tie bars to pull the columns inwards so that the subsequent roof spread resulted in the walls becoming vertical.

The roof soffit, the walls and the balcony fronts are all lined with Douglas fir boards. As the architect records: ‘People love the warmth, the colour and variety in the grain of the timber. Our hope is that it will stand up to the rigours of life in a busy arts building, and age beautifully’.

Environmental and services design

These play a larger part in the design than might be apparent. The building is comprehensively daylit, generally by windows with fixed timber-framed glazing and solid timber ventilator panels. Services were integrated in what is a complex and irregular building, with few suspended ceilings.

The height of the concert hall allowed natural ventilation to work by stack effect. It is ventilated by a plenum that feeds outlets below the raked seats. Air passes through a series of acoustic baffles, to reduce external noise, and out through a long ridge cowl. Supply air is cooled by water from boreholes. Four Okasolar 4 x 4 metre rooflights have been integrated into the north and south sides of the pitched roof of the hall to provide daylight. They are double-glazed and incorporate aluminium fins between the glass layers to restrict unwanted rays of sunlight beaming into the hall.

The classrooms and foyers are also naturally ventilated; air-handling units are used in the recital room and the drama studio has chilled beams. Heavily serviced raised floors allow concrete soffits to be exposed, regulating the temperature. Because the fabric is highly insulated, small radiators fed by gas-fired boilers provide sufficient heating.

The recital room

The recital room structure consists of steel columns and a ring beam supporting timber rafters with steel flitch plates. Interior surfaces are made entirely of Douglas fir, with a scale suited to smaller music ensembles. For acoustic reasons the room was designed to have a non-orthogonal geometry and a timber-lined asymmetric pyramid roof. The rhythm of rafters and purlins in the exposed Douglas fir roof structure, together with the Douglas fir batten wall linings, naturally reflect and diffuse sound to provide optimal acoustics.