Wednesday02 September 2015

Tim Ronalds’ Sevenoaks School centre raises the roof

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A high pitched roof of honey-coloured Douglas fir on this new performing arts centre meet both visual and acoustic requirements

Architect Tim Ronalds Architects
Structural engineer Price & Myers
Location Kent

Why, I teasingly ask architect Tim Ronalds, has he used Douglas fir and not oak for the roof of the new Performing Arts Centre, given that it is for Sevenoaks school, a co-educational private school founded in 1432.

“You know I never thought of that,” confesses Ronalds, adding almost to excuse the oversight: “But we are using oak for the floors.”

However, seeing the buildings’ new auditorium, I understand why Douglas fir was specified. The timber in the vast barn-like roof space has a wonderful rich honey colour, one of the main reasons why the timber was chosen.

Isometric of the centre’s three main buildings

Isometric of the centre’s three main buildings

The main auditorium is the large rectangular building on the right and shows the series of 18m span timber and steel trusses that support the unusual 12-layer roof buildup.

To the left of the auditorium, is the smaller Recital Hall and features a pyramid roof with an off-centre pitch.

It also weathers well, and is a good joinery material, a fact that was evident when seeing the beautifully crafted ventilation shutters to the windows and the high-quality finish to the internal doors.

The centre, with a construction cost of £8.8 million, is conceived as a group of three separate buildings rather than one large block. These comprise the auditorium or concert hall, a recital room and a three-storey teaching block that incorporates practice rooms, classrooms, offices, a drama studio and a split-level foyer which connects all the levels of the building.

The school’s continuing expansion has meant that it has outgrown many of the facilities that are dotted around its 40ha grounds. In 2005, it asked Tim Ronalds Architects to prepare a masterplan for the school’s future development. The study highlighted the need for a new performing arts centre as the first priority and the school subsequently commissioned the practice to design the cluster of buildings.

In an area called the Flat, which is located in the southernmost part of the school grounds, the architects identified a suitable sloped spot between the existing Sackville Theatre and Marley Sports Hall, to position the new buildings.

Given the auditorium’s use as a space for orchestral and choral music and assemblies, the roof’s design has been largely influenced by acoustic requirements but equally its design has been influenced by the natural ventilation strategy developed by engineer Max Fordham Partnership.

Auditorium and Recital Room Roof Structure

The buildup of the roofs and their shape and choice of materials may have been influenced largely by their acoustic and thermal requirements, but the scale of the buildings has been influenced by their surroundings and sloping site. The slope of the site has been fully exploited, allowing the buildings to be set into the landscape to minimise its bulk.

The auditorium is a rectangular concrete framed hall, 18m wide x 25m long, with most of the raked seating facing the platform at the eastern end and featuring a big trussed roof structure.

The roof is not particularly innovative, being a modern interpretation of a traditional timber barn. But what is unusual is its 12-layer buildup and the two additional storeys above the main roof to accommodate natural ventilation requirements and musical equipment to meet the acoustic requirements.

The natural ventilation strategy, developed by engineer Max Fordham, involves air entering the hall from a low level on the building’s north facade and being drawn from the building through a rooftop ventilator.

The roof features a series of 18m span timber and steel trusses that support the 12-layer buildup. Four 4m x 4m rooflights have been integrated into the north and south sides of the pitched roof drawing in daylight. The double glazed rooflights, by Okalux called Okasolar, incorporate aluminium fins sandwiched between the layers of glass so that sunlight is restricted into the hall. The roof is zinc clad.

The smaller Recital Hall, which connects to the eastern end of the auditorium, has a pyramid shaped roof with an off-centre pitch. As the recital hall will be used for smaller and more intimate performances the acoustic and thermal qualities for the room are still important and has a similar 12-layer buildup as the auditorium.

Auditorium pitched roof, buildup from top

Auditorium pitched roof, buildup from top

0.7mm VM Zinc+ standing seam roofing separating layer
18mm OSB
18mm OSB
50mm ventilation gap 2in x2in battens @ 600mm breather membrane
50mm Rockwool Rockfall overlay board
150mm Rockwool Rockfall underlay board — 2in x 6in battens @ 600mm vapour barrier
18mm Cement-fibre boarding
18mm OSB
22mm Douglas fir boarding

Acoustic Qualities of the Auditorium

Tim Ronalds initially favoured a high pitched roof for the auditorium. It took a trip to the concert hall at Snape Maltings in Suffolk, still considered to be one of the best concert halls in the country for its excellent acoustic qualities, to convince him that a Snape-style roof might be better.

But rather than building a roof with a 45-degree pitch as in Snape’s concert hall, a pitch of about 50 degrees was incorporated into the design, the high pitch providing the necessary acoustic volume and the shape enhancing the acoustics.

Perfect pitch: a high pitched roof has volume for good acoustics.

Perfect pitch: a high pitched roof has volume for good acoustics.

Raf Orlowski, now at Ramboll but formerly an associate director at Arup Acoustics which advised on the Performing Arts Centre, says a horizontal plastered ceiling could perhaps produce sound of a similar quality, but a high pitched roof also provides the necessary volume for good acoustics.

The three key elements in the design of an auditorium for music are volume, shape and surface finishes. A volume of around 4,700cu m has been set for the auditorium in order to provide sufficient reverberation and control the loudness of a full orchestra. Timber was also the architect’s preferred material, favoured, both for its warm finish and because musicians like to perform in a space with timber finishes.

Project team

Architect Tim Ronalds Architects, Structural engineer Price & Myers, Services engineer Max Fordham, Acoustic consultant Arup Acoustics, Theatre consultant Carr & Angier, Main contractor R Durtnell & Sons, Roofing subcontractor Albany Brent, Roofing material VM Zinc, Trusses Constructional Timber, Carpentry MWM Carpentry


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